NEWS & REVIEW

1. Editorial


This edition contains our second review article. Very different from the first (Kai Griebenow et al, Int. J. Vib Spect,[www.ijvs.com], 3, 1, 8), this one covers a very broad and highly topical area - the structure of skin and hair and the use of vibrational spectroscopy in analysing them. The article, written by Kathleen Martin who works in the cosmetic field, is very detailed, easy to read and well worth reading.

Prof. Tom Klapötke at the University of Munchen is always supportive of IJVS. IN this edition, he and his colleagues have supplied a paper on a series of organic salts containing heavy elements. The consequence, is that the frequency of vibration is low. Tom's paper includes spectra recorded in the Raman shifts as low as 80cm^-1. This points up an advantage of Raman over infrared - it is easy, with the right equipment to read the complete spectrum 3500 nearly Zero cm^-1. [OK, I know I'm an old Raman specialist - on the other hand I have, in the fact, struggled with far infrared (400Zero cm-¹)].

Last time I wrote a very lightweight introduction to the use of polarised light in vibrational spectroscopy with particular reference to Raman measurements. The article has obviously created some interest because as you will see in the Dear Reader section, we have some queries and replies. Neil Everall from ICI Technology has supplied a follow up article on the use of polarised light in studying polymers. Neil describes the piece as a tutorial and again I recommend it highly to you. I thin it is the most useful source on this subject I have read. Neil is to be thanked.

Two editions ago (see Int. J. Vib. Spect, [www.ijvs.com], 2, 4, 4), Robert Alexander described to us how microscopic measurements mainly in the mid i.r. but also to some extent in Raman scattering are of value in Combinatorial Chemistry. Dr. Christophe Fromont, a young post-doctural fellow from my own department here at Southampton has written a detailed survey for us on "Solid Phase organic reactions". In reality, Christophe's piece is so comprehensive it counts as a review rather than a feature article and is a "good read" to anyone with an interest in organic chemistry.

Dr Andy Brookes, at what was the UK Office of the Government Chemist, LGC has been building up a Raman service available to the UK and other customers. As a result he needs to survey Raman users and potential users and has asked us to carry out a survey for him. Internet Journals must be the ultimate way of doing this job so we agreed - another first for us. Andy's particular need is to try to assess the level of interest there is in developing large computer searchable databases of Raman spectra.

Personally, I am very keen to see large and expanding databases appear and have been involved in trying to push the matter along since 1988. At that time, the F-T machines were being developed and it was clear that Raman would have a real future in routine analysis. I remember that under the auspices of Sadtler we held a meeting and tried to get agreement from all the manufacturers to generate a series or library of spectra in a format acceptable to them all but somehow nothing happened. Since then each manufacturer has offered a small database, Aldrych have generated a library and Sadtler have been active but the problem is that the market is too small for several competing libraries to be viable. Unless sales are reasonable, no publisher is going to invest the huge sums involved to develop a usefully large library. I'm a bit 'long in the tooth' to flog away at this one any more, so with relief and best wishes I hope Andy Brookes will get something permanent on the rails.

Always on the look out for innovation we start this edition with a regular feature - Manufacturer's Announcements. Hardly innovative I hear you say. Our announcements are much more comprehensive than the 50-100 words plus mini picture typical of most periodicals. We will publish formal and fairly detailed descriptions of length up to around 1000 words and appropriate figures and also application notes. Obviously, I will check the copy for errors and/or pornographic content but these announcements are the responsibility of the manufacturers and cannot be covered by my seal of approval.

Bill George at the University of Glamorgan in South Wales has sent us an article on the vibrational spectra of alcohol dimers trimers and larger conglomerates. You might feel the monomer of ethanol is the only interesting homologue of this lot and you might further feel that spectroscopy seems to be a little over the top as a tool in its enjoyment. However, this would be a mistake because Bill uses fundamental mode calculations to assist him in his work. When originally submitted, Bill's article was a fine description of an interesting piece of science but the style made its understandable only to folks already familiar with methods of calculating, from first principles, the frequencies of fundamental modes. I therefore, asked Bill George to rewrite the piece carefully taking the reader with him and assuming you folks are new to this area.

I learned a lot from this piece so 'give it a go'. If you find any points hard to follow - send an email to Bill George (copy please to the Editorial Office).

A few weeks ago, I was invited to give a short talk on the use of the internet/web in publishing as part of the UK's Infrared and Raman Discussion Group Spring meeting held, as it turned out at the University of Glamorgan.

I described how IJVS works and then went on to think about the future. My view is that within 25 years we won't be using scientific libraries as we know them now. Papers will be available on the web and all references will be linked. As a result, one will be able to read a paper and follow a chain of references from one's desk. The huge lengths of shelves supporting hundreds of dusty tomes will be redundant. The snag with all of this is cost and profits and the role of the journal publishers.

The cost of generating a journal solely on the web is around 10-15% of the cost of doing the same job in a paper format. Further, a web journal can offer colour, photographs, and shortly in our case movement and short film clips at a trivial cost increase. As a result, there really is no need to charge for journals published on the web. Sponsors (Perkin Elmer in our case), inclusion of advertisements (Applied Spectroscopy and many of the free journals are already supported in this way, in paper format) or a revised form of page charge would cover the cost easily. A modest charge paid by the authors of each paper would be sufficient to cover the cost of production.

Where does this leave the publishing houses like Elsevier or Wiley or the learned Societies? All of these organisations are profit making. The learned societies hope to earn cash from their journals to support their other activities. And this is the rub. The principle titles are owned by organisations unlikely to see free publication as anything other than a threat. If they won't agree to link references, the concept of removing the need for a scientific library remains a pipe dream. Seems sad! But then King Canute was clearly pretty miserable when he got his feet wet!

Have you any thoughts on this very important matter?

2. Using Polarised Vibrational
Spectroscopy to Characterise
Molecular Orientation in Polymers:
An Introduction - A Tutorial

Neil Everall
ICI Technology
Wilton Research Centre
PO Box 90
Wilton, Middlesbrough
Cleveland
TS90 8JE
UK

Email Neil_Everall@ici.com

The physical properties of polymeric articles such as films, fibres and bottles can depend strongly upon the degree of alignment of the polymer chains. For example, properties such as tensile strength, Young’s Modulus, toughness and barrier performance all correlate with the degree of orientation. Likewise, measurements of orientation are important for testing theories of how polymer networks deform. Therefore, it is important for the polymer scientist to have access to methods for characterising orientation. There are several methods available, including X-ray diffraction, nuclear magnetic resonance (NMR), birefringence, polarised fluorescence, ultrasonic measurements, and polarised vibrational spectroscopies (IR and Raman). Of all these techniques, IR dichroism is one of the most frequently applied, since it is applicable to many polymer systems, it can be used to make measurements on the microscopic and macroscopic scales, and it can specifically measure orientation of different phases (i.e. amorphous or crystalline) in the same sample.

Given the importance of the field, this article is intended as a tutorial to introduce new practitioners to the basics of quantifying molecular orientation with vibrational spectroscopy. For the sake of simplicity we concentrate mainly on IR measurements of uniaxial (one-way drawn) systems. After discussing the basic principles, an example is given where polarised IR microscopy was used to quantify crystalline and amorphous phase orientation in drawn poly(ethylene). The theory outlined here is rigorous for uniaxial systems, but demands only basic trigonometry and calculus; hence the physical principles are not obscured by the maths which can intrude when more sophisticated treatments are applied. In a future article the treatment will be extended to include biaxially oriented systems, Raman measurements, and the experimental options for obtaining polarised IR data from real world (i.e. thick, strongly absorbing samples). For the purposes of the current article we will not dwell on how the IR spectra are obtained in cases where simple transmission measurements are not possible.

This tutorial can only serve as a brief introduction to orientation measurements. There are a number of excellent references in the literature, which cover the subject in much more detail. I would particularly recommend the books by Samuels [1] (general discussion of orientation and its measurement by a variety of techniques) and Zbinden [2] (concentrates on IR measurements, but has a large section on the different types of orientation function), although neither work discusses Raman spectroscopy. Two more modern works by acknowledged leaders in the field cover both IR and Raman measurements in detail, and also cover the rigorous analysis of biaxial orientation [3,4].

To see how we might quantify molecular orientation in a polymer article, first lets consider a distribution of rod-shaped molecules in the co-ordinate system defined in Figure 1. (Usually one or more of the (x,y,z) axes will be chosen to coincide with a relevant macroscopic axis such as the draw-direction of a fibre, or the in-plane optical axes of a film or bottle). Figure 1 shows the alignment of a single molecule - the orientation distribution function N(q,f,y) defines the probability of the molecule having the spherical-polar co-ordinates q,f and y. Obviously, to fully define the orientation one would need to measure N for all possible values of q, f and y. This is in fact impossible, the best we can do in real life is to measure certain averages of the distribution function. Fortunately, these averages give useful insight into the mechanisms by which polymer chains orient on drawing, and can be correlated with end-use properties such as tensile strength, toughness, propensity to creep and so on.

In this article we largely restrict ourselves to the case of general uniaxial orientation, i.e. systems in which only angle q takes non-random values. This restriction allows us to fully explore the physical basis of measuring orientation using infrared spectroscopy, but removes some of the complexity introduced with biaxial systems (where all angles q, f and y must be characterised). The most obvious uniaxial example is that of a fibre, but one-way drawn films can sometimes approximate to this symmetry. To describe uniaxial orientation we only need to characterise N(q). We will describe two ways of doing this, one rigorous and one not, but both of which yield the same numerical result.

Orientation Averages: - P₂, P₄
and Fraser’s Orientation Fraction "f"


The simplest description of orientation, due to Fraser [5], makes the (unrealistic) assumption that in a uniaxially-drawn polymer article, a fraction "f" of the molecules are perfectly aligned along the draw axis (q=0), whereas the remainder (1-f) are isotropically oriented (Figure 2).

Although this description is physically unrealistic, we find that "f" is numerically identical to the orientation parameter obtained by a rigorous analysis. One such treatment expresses the orientation distribution function (ODF) as a series expansion of Legendre polynomials [3,4]. For uniaxial orientation, with no preferred orientation about the chain director axis (i.e. y random), the ODF is given by

.

Here P_n(cosq) is the n^th order Legendre polynomial in cosq, the angular brackets denote the average value, and only terms involving even "n" are nonzero. It turns out that by using infrared spectroscopy (or any effect relying on linear dichroism) we can only measure the value of P₂ for the distribution, where P₂ is defined by Equation 1.

Equation 1.

P₂ has the property that it varies from -0.5 for perfect alignment perpendicular to z (q=90), through zero for random orientation (<cos²q> = 1/3 for random values of q in spherical polar co-ordinates), to unity for perfect alignment along z (q=0). It is identical to Hermans orientation function [6].

If one employs a two-photon technique such as polarised Raman spectroscopy or fluorescence, a second parameter, P₄, can be measured (Equation 2).

Equation 2.

Figure 3 compares the variation of P₂ and P₄ as a function of q. One immediate advantage of P₄ is apparent: - for highly oriented systems (q~0), it is more sensitive than P₂ to small changes in orientation. In all cases P₀ =1 so its measurement is not necessary .

Even using Raman spectroscopy we can only actually measure the first two coefficients in the ODF (i.e. terms with n=2 and 4). To fully characterise the ODF we would need to measure P_n up to n=„ - a time-consuming proposition even if a technique could be found! It is important to realise this fundamental limitation: - we do not measure the complete ODF using optical spectroscopy - at best we measure certain averages, up to fourth power in cosq, and we must accept that several different ODF’s could give rise to these average values. By measuring P₂ we obtain the minimum level of information – with P₄ we increase slightly the information content. These limitations are exemplified at the end of this article.

Why can polarised IR spectroscopy be used to measure orientation? It is well known that molecular vibrations only give rise to infrared absorption if the molecular dipole moment changes during the vibration (in the language of quantum mechanics we say that the vibration has a nonzero transition moment m). This is a fundamental selection rule, but in itself does not guarantee that IR absorption will occur; in fact, the transition moment must have a nonzero component parallel to the electric field of the incident IR beam. The band absorbance is proportional to the square of the scalar product of the transition moment and the electric field vector, which in turn is proportional to cos²(W) (see Figure 4). Obviously, the absorbance is maximised at W=0 but is zero if W = 90^o. This simple discussion illustrates two key points: - (a) the absorption band intensity depends fundamentally on the square cosine of an angle, and (b) we measure the orientation of oscillating dipole moments, not of molecules themselves. In a fluid, or any sample with no preferred orientation (isotropic), the band intensity will be the same irrespective of the angle of polarisation of the infrared light. If molecules are preferentially aligned in a certain direction, the band intensity will be maximised when the IR light is polarised in the same direction as the oscillating dipoles.

To illustrate the principle of these measurements let's first show how we can calculate Fraser’s orientation fraction f using a polarised IR experiment [1,5]. Remember that we assume the sample to consist of a fraction f of vibrational dipoles perfectly aligned parallel to the z-axis, and (1-f) randomly aligned dipoles (Figure 2). We measure two IR spectra, the first using IR light polarised parallel to the z-axis, the second with light polarised perpendicular to the axis. (For a uniaxial sample the orientation of this second axis is immaterial provided it is orthogonal to z, but for convenience lets assume we choose the x-axis). We select an absorption band, measure its peak or integrated absorbance for the two configurations (A_z and A_x) and compute the dichroic ratio (D), which is defined as D = A_z/A_x.

Assuming there are N vibrational dipoles in the sample, each with a transition moment of magnitude m, then the observed band intensities are given by Equations 3 and 4, where k is a constant: -

Equation 3.

Equation 4.

The first term in equation (3) is the absorbance due to the oscillating dipoles aligned parallel to z. The second term is the z contribution from the isotropically aligned dipoles - from symmetry, the projection along a given axis must be equal to 1/3 of the total transition moment for all the randomly arranged species.

The dichroic ratio is then given by: -

Equation 5.

Equation 5 can be rearranged to yield an expression for f in terms of D.

Equation 6.

Therefore, we can easily compute what nominal fraction of perfectly aligned dipoles would give rise to the observed dichroic ratio. Note, if the dichroic ratio is unity we obtain f=0, hence the sample is isotropic.

So far, so good, but we are really interested in the orientation of the molecules in the sample, not the orientation of the transition dipoles. In general, the transition dipole will not lie along the extended polymer axis but will be oriented at an angle b to it (Figure 5). Even if the molecules are perfectly aligned along z, the transition dipoles will have a component along x and y, reducing the observed dichroic ratio. We have to allow for this factor to compute the true chain orientation. In fact this is easily done using methods described by Fraser [5] and Samuels [1]. We will not repeat the derivation here but simply quote the result, which is: -

Equation 7.

Here D₀ is the dichroic ratio which would be observed if the all chains were parallel to z; it can be shown that D₀ = 2cot²(b).

A rigorous analysis of orientation has to allow for the fact that a real system does not consist of perfectly aligned molecules plus randomly aligned ones - in fact, all orientations will be observed to a greater or lesser extent depending on N(q,f,y). Returning to Figure 1, (i.e. assuming for the time being that b=0), we wish to calculate the observed dichroic ratio for an ensemble of molecules with non-random orientations. We have already seen that the absorbance observed using IR light polarised along a given axis is proportional to the square of the projected component of the transition dipole along that axis, and this in turn depends solely on q and f when b=0. (For uniaxial orientation it depends solely on q, since f is random for the molecular ensemble). In spherical polar co-ordinates, the z component of the transition moment m is mcos(q), whereas the x component is given by msin(q)cos(f). The dichroic ratio is then given by Equation 8.

Equation 8.

The angular brackets denote the average over all molecules in the ensemble. Now, since f is random for uniaxial systems, it is independent of q, and in spherical polar co-ordinates, so we have

Equation 9.

Note, for random alignment, D=1 and <cos²q> = 1/3. In this case we cannot distinguish the situation where all molecules are in fact randomly aligned, or all molecules are aligned at ~55^o to the z-axis! Both situations give the same <cos²q> (although the latter is unlikely)!

Combining Equations 1 and 9 we now have the required expression for P₂: -

Equation 10.

Comparing this expression with Equation 6, we see that P₂ and Fraser’s f are numerically equal and are easily calculated from the observed dichroic ratio.

The case of nonzero b was illustrated in Figure 5. Clearly, in order to calculate the dichroic ratio, we need to compute the magnitude of the z-component of m, and either the x or y components. This is no longer straightforward since the projection depends not only on q and f but also on b and y, the rotation about the chain axis. Papers usually quote the result (Equation 7) without showing the derivation. The result can be obtained elegantly by using the addition properties of Legendre polynomials [3,4], but this requires mathematical sophistication beyond the level of this tutorial. Fortunately, one can obtain the result by "brute force" using a straightforward, if tedious, series of co-ordinate transforms (Figure 6). They are outlined below, but the reader interested only in the practical application can skip this section.

First, we start with the chain aligned parallel to the z-axis. The (x,y,z) components of the dipole are: -

Equation 11.

We then rotate this axis system anticlockwise through an angle q about the y₁ axis (Fig. 6a® 6b), followed by a rotation anticlockwise through f about the z₂ axis (6b® 6c). Each rotation is characterised by a co-ordinate transformation matrix R(q) and R(f), the forms of which are derived in elementary texts [7]. The dipole co-ordinates in the resultant system are then given by Equation 12.

Equation 12.

The absorbance observed along the axes in the new co-ordinate system will then be proportional to the square of the x,y or z component, taking the appropriate angular averages. The reader can verify that the resultant dichroic ratio A_z/A_y is given by Equation 13.

Equation 13.

To obtain this result, one needs to remember that <cosy> = <siny> = 0 and <cos²y> = <sin²y> = 0.5. By substituting b=0 the reader can verify that it reduces to Equation 9 as required. Rearranging Equation 13 to express P₂ (for the chain) in terms of D we obtain Equation 14:

Equation 14.

Equation 14 is identical to Equation 7, which was derived by non-rigorous means. It tells us that the P₂ value for the chain director (q) is given simply by the P₂ value for the dipole orientation ((D-1)/(D+2)) divided by the P₂ value for the dipole moment angle b. Therefore, to interpret a measured dichroic ratio we must know the value of b=0. The extreme cases of b=0 (parallel band, dipole parallel to chain director) and b=90 (perpendicular band, dipole at 90 degrees to chain director) are considered in the box below.

Extreme Cases.

Its important to note if the dipole angle is such that cos²b=1/3, (i.e. b ~ 55^o) then P₂(cosb) = 0, and (D-1)/(D+2) = 0. This can only be the case if D=1 irrespective of the state of orientation of the sample. Vibrational bands with dipole angles close to 55⁰ are very insensitive to orientation, the most sensitive bands have b=0 or 90^o degrees.

There are basically three methods of obtaining b for a particular vibration. Firstly, if we know the atomic displacements for the vibration, either through measurement (Normal Coordinate Analysis) or quantum-mechanical computation, we can calculate the direction for the dipole transition. A more common route is to measure dichroic ratios for a sample for which the orientation is already known (for example from X-ray data), and then compute b from Equation 14. Finally, values of b are sometimes estimated by comparison with molecules containing similar structural units, or even by "guessing" the form of a particular vibration with the aid of Group Theory. If measurements are used for comparing the same molecules under different states of orientation, errors in the value of b are not too important. It is only when absolute values of P₂ are needed that b must be determined accurately.

Unfortunately, even for uniaxial orientation, calculating P₂ and P₄ values from polarised Raman data is a much more complex affair than the analogous IR calculations. This is because polarised Raman scattering is described by a tensor with up to six independent elements, as opposed to the single angle b needed to describe the orientation of the vibrational dipole relative to the molecular axes. Even for an isotropic sample some interesting polarisation effects can arise in Raman spectroscopy, as discussed in a previous issue of IJVS [8]. In oriented systems we must calculate the transformation properties of all of the nonzero tensor elements as a function of molecular orientation - and this is a complex job [9]. It can be a major task simply to obtain the values of the tensor components themselves! For these reasons, a detailed discussion of calculating P₂ and P₄ from polarised Raman intensities is beyond the scope of this tutorial, and the interested reader is guided to other examples and references on the topic [3,4,10-11].

Its worth noting that in the extreme case of a single tensor component dominating the scattering, the "Fraser Fraction" approach can be used to interpret (uniaxial) Raman data. It has been shown [12] that in this case "f" is given by Equation 15

Equation 15.

Here R is the ratio of the Raman intensities I_zz/I_yy , where the subscripts ij denote the macroscopic polarisation directions of the laser field (i) and the polarisation analyser (j). This is the Raman analogue of the dichroic ratio - its the ratio of the Raman intensities obtained with both laser and Raman scatter polarised first parallel, and then perpendicular to the z-axis. For an isotropic sample R=1 and f=0. For a perfectly aligned sample, R tends to infinity and f=1. However, in this case the value of "f" does not equate to the P₂ calculated from a rigorous analysis, it slightly overestimates the degree of orientation. Furthermore, a proper analysis also makes use of the "cross-polarisation" term I_zz/I_zy and so yields P₄ as well as P₂. Thus if all the relevant tensor components are available, a rigorous analysis should be carried out. A comparison of using the simplified and rigorous analytical approaches to study orientation of a conjugated polyene has been given elsewhere [12].

In the biaxial case, neither q nor f are randomly aligned, so we must calculate average orientation parameters based on both of these angles. This entails measuring two IR dichroic ratios, namely A_z/A_y and A_z/A_x to characterise orientation in the x, y and z directions. If the angle y is random (Figure. 5), just two dichroic ratios from a single band are sufficient to characterise the distribution. However, if b is nonzero and chains adopt preferential rotational angles about the director, it turns out that we must measure A_z/A_y and A_z/A_x for each of two bands with differing values of b (ideally, a perpendicular and a parallel band) [10]. If this is done we can compute the values of <cos²e_x>, <cos²e_y> and <cos²e_z>, where e_x, e_y and e_z are the angles between the chain director and the x, y and z axes respectively. The details of this analysis are beyond the scope of this article but are given in detail elsewhere [10].

Now let’s apply the tools developed above to the analysis of an actual oriented system. The literature contains hundreds of such examples - here we’ll discuss a very simple case, namely poly(ethylene) (PE) which has been drawn to different extents and subjected to thermal relaxation. The aim of the work was to compare orientation of the polymer under different draw and relaxation conditions in order to optimise the production process [12]. It’s a useful example since it shows how the orientation of different phases can be studied by careful choice of IR band.

The samples themselves were tubes with ca 1mm wall thickness which were cold-drawn with draw ratios 4:1 through to 7:1, and then relaxed by annealing at ~80 ^oC for ~30 seconds. Each tube was cut in half lengthways and an FTIR microscope (NicPlan) coupled to a Nicolet 850 FTIR bench was used to obtain spectra in transmission through the wall using IR light polarised either parallel or perpendicular to the long axis (Figure 7). Because the bands used in this study were fundamentally weak, spectra could be obtained in transmission without saturating the spectra. Strongly absorbing samples cannot be studied easily in transmission - they absorb too much radiation - and so a variety of IR techniques have been developed for such cases, including specular reflectance [13] and attenuated total reflectance [14]. These will be considered in a future article.

The IR bands used for this study were located at 1894, 1078, and 2016 cm^-1. The first two vibrations have been assigned to molecular conformations specific to the crystalline and amorphous phases respectively. The conformers responsible for the third band occur in both the crystalline and amorphous phases. It is therefore possible to compare orientation of the crystalline and amorphous species simultaneously - this is a major advantage over X-ray diffraction, which is sensitive primarily to crystalline-phase orientation. Figure 7 shows part of typical IR spectra obtained with the polariser aligned parallel and perpendicular to the tube long axis. It is apparent that the two bands shown here have opposite dichroism, and, following Wedgewood and Seferis [15] we assign values of b=0 and 90^o to the 2016 and 1894 cm^-1 bands respectively. The dichroic ratio A_z/A_x was measured for all three bands and each tube, and Equation 14 used to compute the P₂ values. Figure 8 shows the results - curves (a) and (b) are the P₂ values for each phase, before and after annealing, as a function of draw. These data illustrate two key points. Firstly, the crystalline phase (C) orients much more strongly than the amorphous phase (A) as a function of draw ratio. Secondly, the amorphous orientation relaxes to a much greater extent than the crystalline phase (as a proportion of the pre-annealed value). The behaviour of the conformer present in both phases (C+A) is intermediate. Data such as these are very useful in quantifying the influence of draw conditions on polymer orientation, and hence in optimising production to tailor the final properties of a polymer article.

The mechanical properties of a polymer article will be governed by both amorphous and crystalline phase orientations (perhaps to different extents), so the value of a technique that can measure both parameters is obvious. Infrared and Raman spectroscopies are not the only such options (NMR measurements are another example), but they are the most common approach. Most techniques can only measure either crystalline phase orientation (e.g. X-ray diffraction), or measure the overall orientation (crystalline plus amorphous).

At this stage its important to stress the limitations of these measurements in terms of characterising the ODF. The great problem inherent in our approach is that, at best, we measure orientation averages up to fourth power in cosq , we do not measure the shape of the distribution. Figure 9 illustrates this graphically - each of the distributions has the same value of <cos²q>, although their shapes are completely different. We cannot even calculate <q> without prior knowledge of the functional form of the ODF – the two distributions in Figure 9 both have <cos²q> ~0.88, but the "exponential" curve has <q> of ~16^o while the "delta-function" distribution has <q> of ~20^o. This point is sometimes overlooked – a knowledge of <cos²q> or P₂ does not in itself give us the average angle itself, unless the shape of the ODF is known. Simultaneous measurements of P₄ would narrow the range of possible distributions, but to completely specify the ODF we would need to measure order parameters up to P„ . While we expect physical properties to correlate with P₂, this correlation could break down for samples with grossly different ODF’s (e.g. comparing uniaxially and biaxially oriented systems). However, P₂ and P₄ values are very useful for comparing and optimising orientation in samples subjected to differing draw conditions, and can be used to investigate theoretical descriptions of polymer deformation mechanisms, provided that the overall shape of the ODF remains similar through a sample set.

In this tutorial I have tried to give the reader sufficient grounding to enable polarised IR data from uniaxially aligned polymers to be converted into quantitative orientation results. The approach outlined is rigorous for a uniaxially aligned system yet is mathematically straightforward; a proper consideration of biaxial systems requires a more complex treatment than is appropriate here. The same is true for Raman measurements of either uniaxial or biaxial systems. Furthermore, we have not considered how one might generally obtain IR data from polymer articles - many will be too thick to analyse in transmission. For these reasons, a future tutorial will discuss experimental approaches for obtaining IR data from such articles, and the mechanics of calculating biaxial orientation functions from such data. This is important since most real-world articles (e.g. bottles, films, and plaques…) all have biaxial orientation - only fibres and tubes usually show uniaxial alignment.

The author would like to thank ICI plc for permission to publish this work. He would also like to thank John Chalmers and Peter Mills, who collaborated on the poly(ethylene) orientation work described above.

References

1. R J Samuels, "Structured Polymer Properties", J Wiley & Sons, New York, (1974).
2. R Zbinden, "Infrared Spectroscopy of High Polymers", Academic Press, New York, (1964).
3. D I Bower, "Infrared dichroism, polarised fluorescence and Raman spectroscopy" in "Structure and Properties of Oriented Polymers", Ed I M Ward, Chapman and Hall, London, (1997) pp 181-233.
4. I M Ward,"Determination of Molecular Orientation by Spectroscopic Techniques" in "Advances in Polymer Science" 66, Springer-Verlag, Berlin, (1985) pp81-115.
5. R D B Fraser, J Chem. Phys. 21, 1511 (1953).
6. J J Hermans, P H Hermans, D Vermaas and A Weidinger, Rec. Trav. Chim. 65, 427 (1946).
7. See any elementary text covering Group Theory or matrix algebra for derivation of the rotation transformation matrices. For example, J B Dence, "Mathematical Techniques in Chemistry", Wiley Interscience, New York, (1975) pp288-293.
8. A de Paepe and P J Hendra, Internet J. Vib. Spect., [www.ijvs.com], 3, 1 (1998)
9. D I Bower, J. Polym. Sci., Polym. Phys. Edn. 10, 2135 (1972).
10. D A Jarvis, I J Hutchinson, D I Bower and I M Ward, Polymer 21, 41 (1980).
11. N J Everall, Appl. Spectrosc. 52, 1498 (1998).
12. N Everall, J Chalmers and P Mills, Appl. Spectrosc. 50, 1229 (1996).
13. N Everall, J Chalmers, A Local and P Mills, Vib. Spectrosc. 10, 253 (1996).
14. N Everall and A Bibby, Appl. Spectrosc. 51, 1083 (1997).
15. A  R Wedgewood and J C Seferis, Pure and Appl. Chem. 55, 873 (1983)

Christophe Fromont
Department of Chemistry
University of Southampton
Southampton
SO17 1BJ
U.K.

Although solid phase chemistry has been around for a long time [1], it has seen a real expansion since the early 1990’s with the introduction of combinatorial chemistry. Several reviews [2] have been devoted to combinatorial chemistry, as well as web sites (5z.com., Combinatorial.com., warr.com.). Rapid progress in molecular biology and automated screening techniques (robots can now evaluated several hundred thousand of compounds per year) has lead to a lack of candidates for screening. Traditionally, candidates were provided either by nature [3] or by design and production by organic chemists. Products assembled by classical solution phase chemistry are often not very diverse, while on the other hand, production by nature is much more diversified but requires lengthy and sometimes difficult purifications, and finally, can lead to a complex total synthesis. The demands of high throughput screening within the pharmaceutical industry have therefore boosted recent expansions in the field of combinatorial chemistry [4].

In traditional solution phase chemistry, two reagents A and B are reacted together to give a compound A-B. This technology is limited to the production of one compound or a small mixture of compounds per flask and purification is compulsory if an excess of reagent is used (Figure1).

The power of combinatorial chemistry lies in the use of solid supports and the so called "split and mix" strategy [5] to allow the production of several thousand of compounds in one flask, the ultimate reactor being the resin bead itself (one bead one compound)[6] (Figure 2).

Mixing and splitting ensures the statistical presence of each compound in a library. Since the reaction rate for a given compound depends on the resin [7] and on the reactants, each reaction step is carried out in different batches.

The initial batch of resin, for example, is divided into 3 batches corresponding to 3 reagents (A₁, A₂, A₃) used for the first coupling. In each batch, the reaction is driven to completion by using an excess of reagent (easily removed by simple washing of the support), and possibly several coupling cycles. To ensure the overall representation, the batches are mixed together and split again into 3 batches for the second coupling with 3 other reagents (B₁, B₂, B₃). The resulting mixed batch now contains 9 compounds. The sequence is repeated until the end of the solid phase synthesis.

The size of a library (a library is the set of all the compounds produce in a combinatorial synthesis) obtained will depend on the number of reactants or building blocks (linear relation) and the number of reaction steps (exponential relation). In this case, a library of 3³ compounds equally represented is obtained (Figure 2).

Although solid phase synthesis has a number of advantages, new problems have risen:

a) whereas the creation of a whole library takes only a few days, the development of a new reaction on resin can take several months,

b) the product must be released from the resin by an extra step of cleavage,

c) last but not least, since the resin bound intermediates can not be purified, it is crucial that the efficiency of every synthetic step must be confirmed. However, conventional analytical techniques in solution are no longer applicable. The need for reliable analytical techniques on solid phase was felt in the early seventies [8], since most organic reactions don’t go to completion. Unlike solution phase synthesis, unreacted materials bound on the resin can not be removed after each reaction step. They will accumulate or react during other steps and so will cause purity problems and decrease the yield of the final product.

We will focus in this review on "on bead" analysis of the product, since if the reaction is not complete on the resin, lower yield and lower purity will result. In the first part, we will briefly summarise the different methods to monitor reactions on solid phase and methods to characterise the final product. In the second part, we will concentrate on infrared techniques adapted to the solid phase.

Some other recent reviews has been published [9,10,11,12,13] , and present a good overview of different methods used to monitor the synthesis or analyse a compound resulting from solid phase synthesis.

Colorimetric tests:
The Ninhydrin (or Kaiser’s) test is a useful primary amine colour test, which has been used to quantify the loading of aminomethyl resin [14]. The Fmoc test, which consists of coupling a Fmoc protected amino acid or Fmoc chloroformate and measuring the UV absorbence of the fulvene derivative after deprotection using 20% piperidine in DMF, is also used to calculate the loading on solid phase [15]. For couplings to proline or secondary amines, bromophenol blue is used as indicator [16]. Ellman’s reagent bis (3-carboxyl-4-nitrophenyl) disulphide has been used to monitor reactions with sulfhydryl group [17].

NMR spectroscopy:
The first technique used was the "gel phase NMR". This consists in taking a slurry of polymer bound organic substrate swelled in an appropriate solvent [18,19] and running a standard NMR. NMR Spectra of ¹⁹F, ¹⁵N, ³¹P, ¹H and ¹³C have been recorded and used to monitor solid phase reactions. The main drawback of this method is the low sensitivity because of the relatively low amount of compound on the resin (usually, 100mg of resin (1.2 mmol/g) are used). For ¹H NMR the broadness results in loss of information and poor structure determination. Long accumulation times or labelled carbons [20] are useful to obtain suitable signals. To circumvent this low resolution, MAS NMR (Magic Angle Spinning) has been adapted from the solid state NMR technique. In this method, a special probe is needed and the spectra is recorded from the sample still in his solid state (in fact, swollen in a minimum of solvent)[21,22] . MAS NMR leads to NMR spectra approaching the quality of solution phase NMR [23,24,25,26], and make it truly available as an analytical tool for solid phase synthesis. The technique is now sensitive enough that it has been used for the study of the asymmetric dihydroxylation of polymer bound olefin [27]. Due to the high quality of the spectra available now, the authors could determine directly the enantiomeric excess directly on the resin. These enantiomeric excesses were in good agreement (<1%) with whose determined by more time consuming procedure: cleavage, derivatisation with the carbonyldiimidazole, and analysis by chiral HPLC. The assignment of almost all of the resonances was also ascertained by the combination of several 2D NMR experiments.

NMR has a number of advantages over IR. It does not require undisturbed spectral regions or suitable functional groups, and is truly a non-destructive method. However, it is time consuming and special probes are needed. The weakness of signal has been circumvented by use of expensive enriched ¹³C tagging groups.

Mass spectrometry:
Recent reviews have been published on the application of mass spectrometry in combinatorial chemistry [28,29]. Our group has demonstrated the efficiency of the direct analysis on the resin by matrix-assisted laser desorption/ionisation time of flight techniques (MALDI TOFMS) [30,31], with a diverse array of linkers [32].

Other Methods:
Gravimetric analysis is based on the weight gain of the resin after reaction. However, the weight gain in each step of the synthesis is too small compared to the resin backbone to be accurate. Moreover, some solvent and impurities can be trapped into the resin.

Electron Spin Resonance (ESR): Katritzki et al. have reported the special preparation of spin labelled styrene divinylbenzene copolymer to quantify accurately resin loading [33].  ESR has also been used to show the mobility of bound substrate is dependent on the extent of swelling [34].

Combustion elemental analysis can suffer from the same drawbacks as gravimetric analysis, but Yan et al. have made a systematic investigation on the role of combustion elemental analysis in the quantitative monitoring solid phase synthesis. First they applied the method to the direct analysis of 9 common resins and 8 resin bound compounds [35]. The study showed that on all the samples analysed, the method was remarkably reproducible (variation <3%) and the relative error from the expected value was under 5% for most of the samples, which compares well with other established photometric method (Kaiser’test [14]), release of HOBt, dye coupling method [36,37]).

Yan et al. have contributed to the development of IR techniques, especially single bead microspectroscopy and ATR, applied to solid phase synthesis [9, 11].

Many IR methods have been developed by different groups, to monitor or analyse reactions on the solid support. IR spectroscopy was first used in a qualitative way. Monitoring either the appearance or disappearance of an IR chromophore in a molecule can provide a straightforward method for following a solid phase reaction. Disappearance is easier to monitor since intensity of an absorption band is difficult to quantify.

A single bead extracted randomly from the batch is to be considered as representative of the entire population as demonstrated by Yan et al. on a systematic study [38].

Some difficulties lie in the weakness of the IR signals of the compound of interest compare to the strong background signal due to the resin backbone. IR signals to be reliable must lie outside these regions. For example functions such as C-D stretching (2300-2200 cm^-1), imide (1730 cm^-1), nitro groups (1360 cm^-1), azide (2108 cm^-1), cyanide (2225 cm^-1) are all clearly visible.

The KBr method:
The KBr pellet method was naturally first applied to the analysis of resin bound compounds. Indeed, the compound on the resin is in a solid state and a simple IR apparatus recording the transmitance is needed [8,39,40] . Nowadays, despite improved techniques, the KBr method is still widely used [41,42,43,44,45] since it provides a rapid qualitative answer to follow derivatisation of the resin [44] or the progression of a reaction [42, 45].

Recently, Jung et al have realised a map [41] of a library synthesised by the "split and mix" strategy. For this, they took advantage of the immobilisation of the beads in the crystalline matrix. By choosing particular IR chromophores and scanning at their absorption wavelength, they were able to assign the structure of all resin-bound compounds, creating a 2D map of their small library.

However, the KBr method has some limitations. The quality of the spectra depends on resin: better spectra are obtained with polystyrene resin rather than with the TentaGel matrix [43]. In some cases, the size of particles (generally 80µm instead of 0.5µm ideally) can cause light scattering. The preparation of the sample for analysis consumes several milligrams of resin, which can disturb the repartition of compound in the library. Finally, the presence of water in the KBr disc restricts the spectral region. Loss of informations for acids, alcohol and amides is common. When indications about these functions are needed DRIFT-IR is used [46].

The DRIFT and PAS methods:
DRIFT (Diffuse Reflectance Infrared Fourrier Transform Spectroscopy) [47] and PAS (Photoacoustic spectroscopy)[48]. DRIFT is a good method together with IR microspectroscopy to assess the presence of hydroxyl groups on resin. A comparison between IR techniques has been published earlier in this journal [49].

a) DRIFT is based on the analysis of diffuse reflectance radiation. When infrared radiation is flashed onto a solid surface, depending on the characteristics and environment of the surface, this incident ray may be absorbed, specularly reflected (without modification, as on a mirror), internally reflected or diffusely scattered over a wide area. The detector is specially designed to maximise the collection of this latter energy, while minimising the other components (Figure 3).

DRIFT enables IR spectra to be recorded on diffusely scattering solids without suffering from the difficulty of sample preparation. It allows the direct observation of chemical modification on the surface and therefore is particularly suitable for monitoring reactions on pins or crowns [50]. DRIFT was used to analyse polymer films grown on platinum electrodes to prove that functional groups such as esters or nitro groups were not damaged during the oxidative polymerisation [51]. It has proved to be useful to follow derivatisation of polyester polymers (trans esterifications and amidations).

Technically, beads are placed in a metallic couple at the focal point of the diffuse reflectance accessory, under an inert atmosphere. This analysis can be automated, and has been applied to observe the disappearance of an azido stretch (2108 cm^-1) in the synthesis of azasugars [52]. This method (and functionality) were also used to monitor the synthesis of a library of aspartic acid protease inhibitors [53].

b) PAS is a non-destructive method to analyse a gas, liquid or solid. It has been mainly used to study inorganic surfaces, but its applicability to solid phase has been demonstrated [45]. The principle of PAS relies on heat transfer. The modulated infrared radiation is directed towards the sample. Absorbed radiation is transformed into heat (analogous to the green house effect). This thermal wave is then transmitted to the surrounding inert gas resulting in local pressure variations, which are detected by a sensitive microphone. The technique is less sensitive to interference such as light scattering or reflection since it does not record the resulting infrared wave, but is dependent on thermal diffusion. The main inconvenience of both methods is that the analysis chamber must be flushed with an inert gas to avoid interferences with atmospheric pollutants. Although it is really a non-destructive method, a large amount of resin is needed. A DRIFT micro version has been developed, using a smaller sample compartment. Despite a lower resolution, no significant loss of information compared to normal DRIFT was demonstrated [54].

FTIR microspectroscopy:
FTIR microspectroscopy is the first IR method developed for single bead analysis. It requires an IR microscope equiped with a very sensitive detector (this kind of apparatus is quite specialised and not widely available). A major improvement in the resolution of the IR spectra has been noticed by Yan et al. when the bead is flattened, especially on Merrifield based resins [55]. It is the most sensitive IR method and high-resolution spectra are obtained. Yan et al. were able to study the hydrogen bonding resulting from site-site interactions of hydroxyl groups in the resin. This factor is of special importance since the ability to insulate molecules from each other by attaching them to an inert and rigid matrix would have been a good alternative to the high dilution effect. Nevertheless, it has been shown that site-site interactions occur even in highly cross-linked (rigid) resins [39].

Usually, these studies are difficult to achieve because the absorption band of the hydroxyl group is very broad. However, different IR spectras, taken from single flattened bead, for Wang resin, tritylhydroxyl resin and ethylene glycol trityl resin, were recorded and showed two bands: a sharp one at 3580 cm^-1 and a broad one at 3420 cm^-1. They were attributed respectively to the vibration of the free hydroxyl group and to the hydrogen bonded hydroxyl group. The difference of intensity between these two bands was in accordance with the fact that a bulky surrounding (such as the trityl group) favoured the non-bonded hydroxyl [56].

When site-site interactions occur, using a large excess of reagent with respect to the number of functional groups on the bead is crucial to minimise cross coupling. Using a diacid chloride for the esterification of an alcohol onto resin it was found that 10-fold excess suppressed the cross-linking observed when only two equivalents are used. The effect of the diacid length being marginal, it is thought that site-site interactions occur because of the dynamic structure of the polymer backbone (when it is more rigid less interactions occur)[39].

The same conclusion was reached by studying site availability. In an oxidation experiment, as the reaction progresses, the population of non bonded hydroxyl groups increases since the previously bonded hydroxyl groups became free as some of its neighbours were oxidised to aldehyde [56,57].

Using the same system, Yan and Li also studied for the first time quantitatively seven reaction kinetics on the common polystyrene and TentaGel resins. Their interesting results contradict the popular presumption that TentaGel resin always allows faster reactions compared to polystyrene support. For example, the oxidation of an alcohol to an aldehyde is faster on TentaGel, but opening of oxazolidinones is 18 time faster on polystyrene resin than on TentaGel [58,59] .

Esterifications on Merrifield and TentaGel resin also gave interesting results [60]. The SN₂ reaction was monitored by removing a drop of the suspension solution every 30 min., and after washing, an IR spectrum of the flattened beads recorded. The determination of reaction rates has shown that the reaction was faster on Merrifield resin than on the TentaGel and even more rapid than the one in solution. This could be attributed to the high local concentration of reagent in the polystyrene bead.

In the same publication a comparison of kinetics between the surface and the interior of the bead was also undertaken using a combination of IR flattened bead in the transmission mode and the ATR method (Attenuated Total reflection) or Internal Reflection Spectroscopy.

ATR method (Attenuated Total reflection) or Internal Reflection Spectroscopy

This technique uses the fact that the beam penetrates only 2 µm, before reflection and collection of the beam. The principle and some applications have already been reported in this journal [61]. As it is a surface sensitive technique, quality of ATR spectrum is dependent on sample contact rather than sample thickness. When applied to polymer bound-molecules, the detection limits has been estimated to 130 femtomoles [60]. ATR has proved to be useful for other applications. As it was normally assumed that bead chemistry was sensitive to diffusion control, a more rapid reaction should occur on the surface bead. The transmission mode will indicate the progression on the whole bead, whereas the ATR objective will furnish indication from the reaction taking place on the surface. Three studies (2 esterifications and 1 nucleophilic substitution) were compared using both methods, and the results demonstrate that the reactions proceed at a comparable rate [60].

Jung et al. used FT IR ATR spectroscopy to study the kinetics of cyclo addition between a polymer bound nitrone and an olefin in solution, following the imide absorption (1715 cm^-1) normalised to the amide absorption of the linker (1682 cm^-1) [62].

By its specificity, this method is particularly useful to study the reaction taking place on planar support. For example, the Fmoc deprotection of rink amide linker and the synthesis of a small peptide on a crown were monitored in this way [47, 50].

Examples from the laboratory
In our laboratory researchers routinely use the 'Golden Gate diamond ATR accessory' [For a description see IJVS (www.ijvs.com) 2, 2, 3 ]to check, or follow transformation on the resin. For example, the following figure represents the IR spectra of a diester derived polystyrene resin (green spectra). The second spectra (purple) show the result of reduction of the ester. As the ATR is however less sensitive for alcohol (as already demonstrated) the main clue is to follow the disappearance of the ester absorption at 1731 cm^-1 (Figure 4).

Another example shows the transformation of an ester bond into an amide bond during the solid phase synthesis of dendrimers. The absorption band at 1735 cm^-1 disappears as the ester is treated with a diamine. The amide is characterised by the appearance of the strong absorption band at 1663 cm^-1 and the broad band at 3297 cm^-1 attributed to the stretching of the N-H bond of both the free amine and the primary amide (Figure 5).

RAMAN Spectroscopy

This method is less used for routine structure determination than Infrared spectroscopy but is often used for the detection of certain functional groups as a complement to IR, if the functional groups are well chosen. Raman spectroscopy is particularly advantageous to the combinatorial chemist for analysis of a mixture. This time, the sample is irradiated with monochromic IR beam, and the scattered beam (composed from the parent component and the attenuated one) is then examined. The difference of frequency between the parent line and the Raman line is the frequency of the corresponding vibration. Faurskov Nielsen et al. have used Near Infrared Raman spectroscopy to follow the solid phase peptide synthesis on TentaGel of a decapeptide using the Fmoc strategy. By the observation of the spectral change in the region of the amide, the authors were able to follow the formation of b-sheets, as the peptide was growing (after 6 residues). The occurrence of this b-sheet was also responsible for the partial removal of the Fmoc protecting group upon standard treatment with piperidine in DMF [63].

By choosing carefully IR and Raman tags (nitrile, phenol, alkyne) Rahman et al. were able to unambiguously identify the different template chosen randomly by a combination of IR and Raman microspectrometry [64]. This strategy seems promising since it can be used for code reading directly from the resin. However, the strategy is still limited to a small number of molecules and increasing the number of coding elements means using a combination of other analytical methods to read the code (see also [41]).

Infrared spectroscopy proves to be a method of choice for quickly monitoring reactions occurring in the solid phase, as the method demonstrates several advantages over it's competitors:

Among the techniques presented, ATR is a bit less sensitive than DRIFT or microspectroscopy for the detection of hydrogen bonding functions, but is much cheaper and based on good quality spectra. The results are very rapidly acquired (spectra can be recorded in less than one minute) and easy to get from a diamond apparatus. However, chemists will be unable to monitor a reaction on solid phase with IR spectroscopies if no change of spectral absorption in a functional group occurs. Thus, IR will be best exploited when used in combination with other methods (NMR, mass spectrometry, HPLC…).

References

1. Merrifield R. B., (1963): Solid phase peptide synthesis. I. The synthesis of a tetrapeptide, J. Am. Chem. Soc., 85, 2149-2154
2. a) Fruchtel J. S., Jung G. (1996): Organic chemistry on solid support, Angew. Chem. Int. Ed. Eng., 35, 17-42
   b) Balkenholl F., Von dem Bussche-Hunnefeld C., Lansky A., Zechel C. (1996): Combinatorial Synthesis of small molecules, Angew. Chem. Int. Ed. Eng., 35, 2288-2337
   c) Special issue of Acc. Chem. Res., 1996, 29, 3, 112-170
3. a)Koshla C. (1998):Combinatorial biosynthesis: New tools for the medicinal chemist, Chemtracts-Organic chemistry, 11, 1-15
   b) Liu D. R., Schultz P. G. (1999): Generating new molecular function: A lesson from nature, Angew. Chem. Int. Ed. Eng., 38, 36-54 
4. a) Gallop M. A., Barrett R. W., Dower W. J., Fodor S. P. A., Gordon E. M. (1994): Application of combinatorial technologies to Drug discovery. 1. Background and peptide combinatorial libraries, J. Med. Chem., 37, 9, 1233-1251
   b) Gordon E. M., Barrett R. W., Dower W. J., Fodor S. P. A., Gallop M. A. (1994): Application of combinatorial technologies to Drug discovery. 2. Combinatorial organic synthesis, library screening strategies, and future directions, J. Med. Chem., 37, 9, 1386-1401.
   c) Terrett N. K., Gardner M., Gordon D. W., Kobylecki R. J., Steele J. (1995): Combinatorial synthesis – The design of compound libraries and their application to drug discovery, Tetrahedron, 51, 30, 8135-8173
5. Furka A., Sebestyen F., Asgedom M., Dibo G. (1991): General method for rapid synthesis of multicomponents peptide mixtures, Int. J. Protein Res., 37, 487-493
6. Lam K. S., Lebl M., Krchnak V. (1997): The "One-bead-one-compound" combinatorial library method, Chem. Rev., 97, 411-448.
7. Czarnik A. W. (1998): Solid phase supports are like solvents, Biotechnol. Bioeng. (Comb. Chem.), 61, 77-79
8. Frechet J. M., Schuerch C. (1971): solid phase synthesis of oligosaccharides. I. Preparation of the solid support. Poly[p-(1-propen-3-ol-1-yl)styrene], J. Am. Chem. Soc., 93, 2, 492-496
9. Yan B. (1998): monitoring the progress and the yield of solid phase organic reaction directly on resin support, Acc. Chem. Res., 31, 621-630
10. Gallop M. A., Fitch W. L. (1997): New methods for analysing compounds on polymeric supports, Curr. Opinion in Chem. Bio., 1,1 94-100
11. Yan B., Gremlich H-U., Moss S., Coppola G. M., Sun Q., Liu L. (1999): A comparison of various FTIR and FT Raman methods: application in the reaction optimisation stage of combinatorial chemistry, J. Comb. Chem., 1, 46-54
12. Egner B. J., Bradley M. (1997): Analytical techniques for solid phase organic synthesis and combinatorial synthesis, Drug Discovery Today, 2, 102-110
13. Swali V., Bradley M. (1997): Analytical techniques for single bead analysis in combinatorial chemistry, Anal. Comm., 34, 15H-18H
14. Sarin V. K., Kent S. B. H., Tam J. P., Merrifield R. B. (1981): Quantitative monitoring of solid phase peptide synthesis by the ninhydrin reaction, Anal. Biochem., 117, 147-157
15. Wells N. J., Davies M., Bradley M. (1998): cleavage and analysis of material from single resin beads, J. Org. Chem., 63, 643-6431
16. Krchnark V., Vagner J., Safar P., Lebl M. (1988): Non-invasive continuous monitoring of solid phase peptide synthesis by acid-base indicator, Coll. Czech. Chem. Comm., 53, 2542-2548.
17. Virgilio A. A., Ellman J. A. (1994): Simultaneous solid phase synthesis of ?-turn mimetics incorporating side-chain functionality, J. Am. Chem. Soc., 116, 11580-11581
18. Blossey E. C., Cannon R. G., ford W. T. , Periyasamy M., Mohanraj S. (1990) : synthesis reactions, and 13C FT NMR spectroscopy of polymer-bound steroids, J. Org. Chem., 55, 4664-4668
19. Look G. C., Holmes C. P., Chinn J. P., Gallop M. A. (1994): Method for combinatorial organic synthesis: the use of fast 13C NMR analysis for gel phase reaction monitoring, J. Org. Chem. 59, 7588-7590
20. Kanemitsu T., Kanie O., Wong C-H. (1998): quantitative monitoring of solid phase synthesis using gated decoupling 13C NMR spectroscopy with a 13C-enriched group and an internal standard in the synthesis of sialyl Lewis tetrasaccharide, Angew. Chem. Int. Ed. Eng., 37, 24, 3415-3418
21. Anderson R. C., Jarema M. A., Shapiro M. J., Stokes J. P., Ziliox M. (1995): Analytical technique in combinatorial chemistry: MAS CH correlation in solvent swollen resin, J. Org. Chem., 60, 2650-2651
22. Fitch W. L., Detre G., Shoolery J. N., Keifer P. A. (1994): High resolution 1H NMR in solid phase organic synthesis, J. Org. Chem., 59, 7995
23. Anderson R. C., Stokes J. P., Shapiro M. J. (1995): Structure determination in combinatorial chemistry : utilisation of Magic Angle Spinning HMQC and TOCSY NMR spectra in the structure determination of Wang-bound Lysine, Tet. Lett., 36, 30, 5311-5314
24. Pursch M., Schlotterbeck G., Tseng L-H., Albert K., Rapp W. (1996): Monitoring the reaction progress in combinatorial chemistry: 1H MAS NMR investigations on single macro beads in the suspended state, Angew. Chem. Int. Ed. Eng., 35, 23-24, 2867-2869
25. Chin J., Fell B., Pochapsky S., Shapiro M. J., Wareing J. R. (1998): 2D SECSY NMR for combinatorial chemistry. High resolution MAS Spectra for resin-bound molecules, J. Org. Chem., 63, 1309-1311
26. Ruhland T., Andersen K., Pedersen H. (1998): Selenium-linking strategy for traceless solid phase synthesis: direct lading , aliphatic C-H bond formation upon cleavage and reaction monitoring by gradient MAS NMR spectroscopy, J. Org. Chem., 63, 9204-9211
27. Riedl R., Tappe R., Berkessel A. (1998): Probing the scope of the asymmetric dihydroxylation of polymer-bound olefins. Monitoring by HRMAS NMR allows for reaction control and on-bead measurement of enantiomeric excess, J. Am. Chem. Soc., 120, 8994-9000
28. Siuzdak G., Lewis J. K. (1998): Application of mass spectroscopy in combinatorial chemistry, Biotechnology and bioengineering (combinatorial chemistry), 61, 2, 127-134
29. Swali V., Langley G. J., Bradley M. (1999): Mass spectrometric Analysis in combinatorial chemistry, Current Opinion in Chemical Biology, in press.
30. Egner B. J., Langley J. G., Bradley M. (1995): SPC: Direct monitoring by matrix–assisted laser desorption/ionisation time of flight mass spectrometry. A tool for combinatorial chemistry, J. Org. Chem., 60, 2652-2653.
31. Egner B. J., Bradley M. (1997): Monitoring the solid phase synthesis of analogues of Lysobactin and katanosins using in situ MALDI-TOF-MS, Tetrahedron, 53, 41, 14021-14030
32. Egner B. J., Cardno M., Bradley M. (1995): Linkers for combinatorial chemistry and reaction analysis using solid phase in situ mass spectrometry, J. Chem. Soc., chem. Commun., 2163-2164
33. Katritzki A. R., Belayakov S. A., Strah S., Cage B., Dalal N. S. (1999): Preparation of spin labelled styrene divinylbenzene copolymers and a new approach for the quantitative determination of the resin loading using ESR spectroscopy, Tet. Lett., 40, 407-410
34. Regen S. L. (1975), Macromolecules, 8, 689
35. Yan B., Jewell C. F., Meyers S. W. (1998): Quantitatively monitoring of solid phase organic synthesis by combustion elemental analysis, Tetrahedron, 54, 11755-11766.
36. Yan B., Li W. (1997): Rapid fluorescence determination of the absolute amount of aldehyde and ketone groups on resins supports, J. Org. Chem., 62, 9354-9357.
37. Li W., Yan B. (1997): A direct comparison of the mixing efficiency in solid phase organic synthesis by single bead IR and fluorescence spectroscopy, Tet. Lett., 38, 37, 6485-6488
38. Yan B., Kumaravel G., Anjaria H., Wu A., Petter R. C., Jewell C. F. JR., Wareing J. R. (1995): Infrared spectrum of a single resin bead for real-time monitoring of solid phase reaction, J. Org. Chem., 60, 5736-5738
39. Scott L. t., Rebek J., Ovsyanko L., Sims C. L. (1977): Organic chemistry on the solid phase. Site-site Interactions on functionalized polystyrene, J. Am. Chem. Soc., 99, 2, 625-626
40. Farrall M. J., Frechet M. J. (1976): Bromination and lithiation: two important steps in the functionalization of polystyrene resins, J. Org. Chem., 41, 24, 3877-3882
41. Haap W., Walk T. B., Jung G. (1998): FTIR mapping-A new tool for spatially resolved characterisation of polymer-bound combinatorial compound libraries with IR microscopy, Angew. Chem. Int. Ed., 37, 23, 3311-3314
42. Gordeev M. F., Patel D. V., Gordon E. M. (1996): Approaches to combinatorial synthesis of heterocycles: a solid phase synthesis of 1,4-dihydropyridines, J. Org. Chem., 61, 924-928
43. Hauske J. R., Dorff P. (1995): A solid phase Cbz chloride equivalent–a new matrix specific linker, Tet. Lett., 36, 10, 1589-1592
44. Stranix B. R., Ping Gao J., Barghi R., Salha J., Darling G. D. (1997): Functional polymers from (vinyl)polystyrene. Short routes to binding functional groups to polystyrene resin through a dimethylene spacer: bromine, sulphur, phosphorus, silicon, hydrogen, boron and oxygen, J. Org. Chem., 62, 8987-8993
45. Russell K., Cole D. C., McLaren F. M., Pivonka D. E. (1996): Analytical techniques for combinatorial chemistry: qualitative infrared spectroscopic measurement of deuterium labelled protecting group, J. Am. Chem. Soc., 118, 7941-7945
46. Deben I.E.A., Van Wijk T.H.M., Van Doornum E. M., Hermkens P. H. H., Kellenbach E.R. (1997): Characterisation of reaction products on solid support by FT-IR. Mikrochim. Acta, 14, 245-246
47. Gremlich H-U (1998): Infrared and Raman spectroscopy in combinatorial chemistry. American Laboratory 33-39
48. Gosselin F., Di Renzo M., Ellis T. H., Lubell W. D. (1996): Photoacoustic FTIR spectroscopy, a non destructive method for sensitive analysis of solid phase organic chemistry, J. Org. Chem., 61, 7880-7981
49. Alexander R. (1999): FT-IR analysis of combinatorial beads and crowns http://www.ijvs.com/volume2/edition4/section1.htm
50. Gremlich H-U., Berets S. L. (1996): Use of FTIR internal reflection spectroscopy in combinatorial chemistry, Appl. Spect., 50, 4, 532-536
51. Passos M. S., Queiros M. A., Le Gall T., ibrahim S. K., Pickett C. J.(1997): Solid phase chemistry of electropolymers, J. Electroanalitycal Chemistry, 435, 189-203
52. Chan T. Y., Chen R., Sofia M. J., Smith B. C., Glennon D. (1997): High throughput on-bead monitoring of solid phase reaction by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Tet. Lett., 38, 16, 2821-2824
53. Kick E.K., Ellman J. A. (1995): Expedient method for the solid phase synthesis of aspartic acid protease inhibitors directed toward the generation of libraries, J. Med. Chem., 38, 1427-1430
54. Deben I., Goorden J., Van doornum E., Ovaa H., Kellenbach E. (1998): MicroDRIFT: rapid and efficient IR analysis of SPOCR, Eur. J. Org. Chem., 63, 697-700
55. Yan B., Kumaravel G. (1996): Probing solid phase reaction by monitoring the IR bands of compounds on a single "Flattened" resin bead, Tetrahedron, 52, 3, 843-848
56. Yan B., Sun Q. (1998): Crucial factors regulating site interactions in resin supports determined by single bead IR, J. Org. Chem., 63, 55-58
57. Yan B., Sun Q., Kumaravel G., Jewell C. F. JR., Wareing J. R (1996): Real-time monitoring of the catalytic oxidation of alcohol to aldehyde and ketoses on resin support by single-bead Fourier transform infrared microspectroscopy, J. Org. Chem., 61, 8765-8770
58. Li W., Yan B. (1998): Effect of polymer support on kinetics of solid phase organic reaction: a comparison of polystyrene- and TentaGel- based resins, J. Org. Chem., 63, 4092-4097.
59. Marti R. E., Yan B., Jarosinsky M. A. (1997): solid phase synthesis via 5-oxazolidinones. Ring-opening reactions with amines and reaction monitoring by single bead FTIR microspectroscopy, J. Org. Chem., 62, 5615-5618.
60. Yan B., Fell J. B., Kumaravel G. (1996): Progression of organic reactions on solid supports monitored by single bead FTIR microspectroscopy, J. Org. Chem., 61, 7467-7472
61. Combs D. (1998): The use of diamond as an ATR material, http://www.ijvs.com/volume2/edition2/section1.htm
62. Haap W. J., Kaiser D., Walk T. B., Jung G. (1998): solid phase synthesis of diverse isoxazolidines via 1,3-dipolar cycloaddition, Tetrahedron, 54, 3705-3724.
63. Ryttersguard J., Due Larsen B., Holm A., Christensen D. H., Faurskov Nielsen O. (1997): Monitoring of a solid phase peptide synthesis by NIR-FT-Raman spectroscopy, Spectrachimica Acta part A-molecular and Biomolecular spectroscopy, 53, 1, 91-98
64. Rahman S. S., Busby D. J., Lee D. C. (1998): Infrared and Raman spectra of a single resin bead for analysis of solid phase reaction and use in encoding combinatorial libraries, J. Org. Chem., 63, 6196

[IJVS Home]  [Hot Sources]  [Links]  [Bookshelf]  [Contents]
[Publication Details]  [Help]  [Back Issues]  [Feedback]

Copyright © 2004 John Wiley & Sons, Ltd