Volume_2/Edition_1 THE INTERNET JOURNAL OF VIBRATIONAL SPECTROSCOPY


NEWS & REVIEW



Editorial



1 Over the years I have published many papers in American Journals and have been honoured so to do. My only problem is not with the science but rather with the desk Editorial practice. These people have invariably imposed "American useage and spelling". I have even had a referee comment that my English is unacceptable - from a Colonial!
This Journal will appear in Her Majesty's Useage. Those of you unfamiliar with Her Majesty Queen Elizabeth II are advised to pick up http://www.royal.gov.uk. Thus 'sulfur' is 'sulphur' or 'fiber' is correctly spelt 'fibre'. We realise that some of you will find this difficult, but in the end the English Language has to be defined within the covers of the Oxford English Dictionary. I'm the first to admit it doesn't really matter - but it's fun!

2 To be more serious - several authors have raised the question of copyright but more seriously is our habit of publishing spectra in GRAMS. Their problem - and it is a problem - is that the data we offer is of newly recorded quality. Hence readers can manipulate the author's data as they will. AND authors are quite rightly nervous. The problem here at IJVS is scanning. We would ideally like to scan submitted diagrams and spectra and hence produce figures that support the author's story but do not provide the reader with a definitive data stream.

The problem we find is that scanners, designed normally to produce acceptable reproductions of photographs in colour do so by losing resolution. Spectra need sharpness to look impressive and the scanned output looks dreadful. If we use ultra high resolution the files become enormous and in a spectrum most of the page is blank!

One solution would be to offer degraded data in GRAMS but this is a bit pointless. Not everyone has GRAMS and some people have to go to some lengths to view the spectra through it. If the output they then get lacks detail why bother.

Do any of the readers have any suggestions? Remember, we must continue to use a format that is available ANYWHERE and we must keep the file lengths as short as possible.

3 Turning to this edition - we are featuring Innelastic Neutron Scattering. "What on earth is that?" I hear you shout. Read and all will be revealed. Its a very specialised technique but it is really worthwhile. Stewart Parker's and Hervé Jobic are real experts in the field and they have sent us very readable accounts. The number of submitted articles continues to increase. We have a very mixed bag for you this time. Tom Klapötke's group work on unbelievably unstable materials. Rumour has it Tom regularly blows up his instruments! Their paper on N2O5 is well worth reading. SERS using optical fibres - a very interesting account on technique from Jiaying Ma and Ying Sing Li - well worth your attention.

Gary Ellis can always be relied upon to help as he is a great web surfer - do look at his piece on spectroscopy links.

That's enough from me - good reading.

Patrick Hendra



4 An Interesting Conference



We have received the following announcement of a Conference in 2 years time in Xiamen, China. The organiser Prof Tian is an old friend.

On behalf of the Committee of Light Scattering, Chinese Physical Society and the State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Prof Tian is planning to organise an international symposium on 'Progress in Surface Raman Spectroscopy: Theory, Technique and Application' to be held in Xiamen on August 17-19th, 2000.

This symposium is organised as a satellite symposium for the 17th International Conference on Raman Spectroscopy (ICORS '2000') to be held in Beijing in China, from August 20-25th, 2000.

The symposium is devised to survey the latest developments in surface Raman Spectroscopy and will last for three days, including a half-day excursion. It will consist of invited lectures, oral and poster contributions and a round table discussion. As you know, the ICORS is a large International Conference having more than 20 sections and normally arranges 3-5 speakers for each section. It is hoped that our symposium will bring together both experimentalists and theoreticians to present concepts, methods and discussion on common problems whose understanding can benefit from complementary approaches from only apparently different starting points, and outline future coordinated directions of research capable of bringing new insight into the general field of surface, material and life sciences.

Xiamen (also known as Amoy) is a well-known historic seaport city located on the southeast coast of China. As one of the four Special Economic Zones in China, Xiamen has developed as a modern city. It is also one of the top ten tourist cities in China, that boasts a beautiful landscape and the so called Garden Island. Xiamen can be reached easily from Hong Kong (3 one-hour flights a day). Participants can also take flights to Xiamen via Beijing (3 two and a half hour flights a day), Shanghai (5-7 one and a half hour flights a day) and Guangzhou (4 one hour flights a day), which are connected with about 40 major cities in the World by direct flights. Direct international flights from Singapore, Manila, Jakarta and Penang to Xiamen are also available. The temperature in August will be between 21-33°C and all the hotel and meeting rooms are air-conditioned.

The symposium will take place at Xiamen University, one of the Key Universities in China and having newly built facilities with numerous modern lecture theatres on a very attractive campus. The Committee has experience of organising many international conferences and symposia. For example, the 46th Annual Meeting of the International Society of Electrochemistry (ISE46) was successfully organised between August 27th and September 1st, 1995, when the Committee hosted 812 participants including 508 foreign participants from 46 countries. The Committee is sure that a visit to Xiamen will be a very pleasant and memorable one.

The main research field of the State Key Laboratory for Physical Chemistry of Solid Surfaces at Xiamen University includes electrochemistry, catalysis, structural chemistry and cluster science. The Laboratory is a State Key Laboratory. Our Raman Laboratory is the largest in China with four Raman spectrometers including a Ramalog 6 (Spex), U1000 and S3000 (J-Y), LabRam I (Dilor) and three lasers from Coherent and SP. Much attention is paid to the development of new techniques including surface Raman spectroscopy. A technical tour of the State Key Laboratory can be arranged one evening during the symposium.

Prof. Tian is an old Southamptonian, as is his lady wife Dr. Bing Wei Mao. I have had the pleasure of visiting Xiamen - fantastic - a lovely region with many sites well worthy of a visit. The science - superb! Z.Q. Tian has contributed papers to IJVS. See Volume 1 Edition 2.

Editor



Copyright



5 When the Journal was established, we accepted that a proportion of our most valuable output would be to feature good work recently published elsewhere, so the question of copyright arises.

Strictly, the manuscripts and diagrams of a paper are copyright but trivial changes are enough to make a manuscript a new creation. I am sure we all agree that this legalistic approach is dishonest.

All of us make presentations at conferences talking about our latest work. I am sure you, like me, present alternative diagrams to the ones published in papers. Your verbal or poster contributions are very different from your papers.

Could this approach be adopted when presenting material to us? It is always worth remembering that our manuscripts tend to be shorter and less rigorous than those offered in a formal Journal, because our readership is non-specialist so I do not think there should be a problem. Please use diagrams you have NOT published elsewhere and if you need to republish please use re-drafted diagrams.

I hope that in this way authors, colleagues and the publishing industry will not feel we are doing anything suspect.

Patrick Hendra





Feature Article



6 Neutron Vibrational Spectroscopy

Stewart F.Parker
ISIS Facility
Rutherford Appleton Laboratory,
Chilton, Didcot, Oxfordshire OX11 0QX,
United Kingdom.
E-mail: SFP@ISISE.RL.AC.UK



Introduction

Vibrational spectroscopy is commonly used in both industry and academia to provide both quantitative and qualitative information on molecular species and the functional groups present in them. The vast majority of such measurements are carried out using infrared spectroscopy in absorption to directly measure the energies of the vibrational transitions. The other form of vibrational spectroscopy that is commonly encountered is Raman spectroscopy and this is an example of inelastic scattering. With this technique the vibrational spectrum is obtained by measuring the energy difference (gain or loss) between the incident radiation and the small fraction of the radiation that is inelastically scattered. The basic idea when looking for inelastic scattering as a method of measuring the vibrational spectrum is to study the comparison of the energy of incident and scattered photons (Raman spectroscopy), atoms (inelastic helium atom scattering), electrons (electron energy loss spectroscopy and inelastic electron tunnelling spectroscopy) and neutrons (inelastic neutron scattering). It is this latter technique that is the focus of this article. The aim is to give an overview of the practice and uses of inelastic neutron scattering (INS) spectroscopy with examples drawn from research carried out within the last few years with applications to polymers, inorganic chemistry and biology. A very important field for INS spectroscopy is catalysis and this is discussed in the following piece by Hervé Jobic.



7 Neutron properties

The neutron is an atomic particle with a mass of 1.008 amu, almost exactly the same as that of a hydrogen ion (a proton). The neutron is uncharged and only interacts weakly with matter. This means that neutrons are highly penetrating and readily pass through aluminium and steel so suitable cells to allow experiments at extremes of temperature and/or pressure are relatively straightforward to devise. Neutrons are quantum particles and behave simultaneously like particles and waves. Neutrons with an energy equivalent to room temperature (200 cm-1) have an associated wavelength of 1.8 Ĺ i.e. on the order of typical interatomic distances in a compound. Probably the most familiar use of neutrons is in neutron diffraction (an elastic scattering process) for structure determination, particularly the precise location of hydrogen atoms. This expolits the wave-like properties; inelastic scattering uses the particle-like properties since the scattering is analogous to the cueball impacting on the pack of reds in snooker. However, the two types of behaviour are inseparable and we make use of diffraction in order to analyse the energy of inelastically scattered neutrons. Neutrons also have a magnetic moment (like a tiny bar magnet) and can be scattered both elastically and inelastically from magnetic centres. This forms the basis for a huge amount of work (primarily by physicists) that is carried out on magnetic materials.



8 Inelastic neutron scattering spectrometers

The prime requirement for INS spectroscopy is a source of neutrons. The two methods of generating neutrons are by shattering nucleii with a high energy proton beam (spallation) or by fission in a nuclear reactor. There are a number of reactor sources around the world [1], the leading one of which is the Institut Laue Langevin (ILL) at Grenoble in France. The world's most powerful spallation neutron source is the ISIS Facility at the Rutherford Appleton Laboratory (Chilton, UK) Access to such instrumentation requires some planning and of course you have to do the work remotely from your own laboratory and this can be a disadvantage of INS [2].

The two types of source usually operate in different ways; the spallation sources are pulsed while the reactor sources are continuous. This is not an absolute divide: spallation sources can be run continuously and reactor sources can use choppers to run in a pulsed mode. The advantage of using the pulsed mode of operation is that energy analysis can be carried out using time-of-flight techniques (explained later).

9 However they are initially generated, the neutrons are very energetic. They are brought to usable energies by multiple inelastic collisions with a hydrogenous material (a moderator). This provides a means of tailoring the neutron energy to the range of interest since a partial thermal equilibrium is set-up between the neutrons and the moderator. At ISIS the moderators are water (300K), liquid methane (108K) and liquid hydrogen (20K). The type of spectrometer that is used depends on whether the source is pulsed or continuous. With a continuous source, the spectrometer is analogous to a single-beam dispersive infrared spectrometer: the source is a polychromatic beam from which a 'single' wavelength is selected by a monochromator, in this case the wavelength dispersion is by Bragg reflection off a single crystal rather than by using a prism or grating, the beam is then scattered by the sample, the scattered neutrons are energy selected by transmission through a beryllium filter and then detected. Beryllium only transmits neutrons of energy less than 32 cm-1, the higher energy neutrons are scattered out of the beam and do not reach the detector. The energy scan is accomplished by varying the angle of the crystal in the beam. Now Bragg's law states:



(1)



since the crystal interplanar distance, d, is constant, the wavelength (and hence energy) varies as the angle of incidence changes. The energy transferred to the sample, Etrans, is:



(2)



10 Since the incident and final energies are both known, it is straightforward to calculate the energy transfer. To cover the full range 0 - 4000 cm-1 range a number of different crystal planes need to be used. The instrument IN1BeF at the ILL typifies this type of instrument.

With a pulsed source, the same principle can be used but it is inefficient since most of the neutrons are wasted. An alternative method is to use a broad spectrum beam of neutrons and to use the time-of-flight technique for the energy analysis and it is this method that is employed on the TFXA (and its successor TOSCA) spectrometer at the ISIS pulsed spallation neutron source at the Rutherford Appleton Laboratory. A schematic of the spectrometer is shown in Figure 1. A small fraction of the incident neutrons are inelastically scattered by the sample; those that are backscattered through an angle of 135°and impinge on a graphite crystal. From equation (1), since both d and are constant only one wavelength (and its harmonics) will be Bragg scattered by the crystal, the remainder will pass through the graphite crystal to be absorbed by the shielding. The neutrons at multiples of the fundamental wavelength are scattered out of the beam by the beryllium filter which acts as a longpass filter and the remaining neutrons are then detected by the 3He filled detector tubes. The net effect of the combination of the graphite crystal and beryllium filter is to act as a narrow bandpass filter.



Fig.1 Schematic diagram of the inelastic neutron scattering spectrometer, TXFA


The kinetic energy, E, of a neutron is given by:



(3)



where m is the mass of the neutron and v is its velocity. Rearranging (3) gives:



(4)



and since



(5)



It follows that the time of arrival at the detector, T, is the sum of the time from the moderator to the sample, ti, and the time around the analyser tf, thus:



(6)



Now since the final energy, Ef, the distance round the analyser system, l, and the length of the flight path from the moderator to the sample, L, are all known, it follows that the time of arrival at the detector uniquely defines the incident energy, Ei. and hence the energy transfer at the sample, Etrans. Thus it is a simple matter to convert from time-of-flight to energy. The result is a spectrometer with no moving parts than can record spectra from 0 to 8000 cm-1, although, for reasons that will be discussed later, the best results are obtained below 2000 cm-1. The resolution of the spectrometer is determined by a number of factors but for practical purposes can be taken to be 2 - 3% of the energy transfer.



11 Applications of INS

Perhaps the first question that should be asked is: "why use neutrons?" Part of the answer is given in Figure 2. This shows the infrared, 2a, Raman, 2b and INS spectra, 2c of N-phenylmaleimide, a model compound for bismaleimide composites that are used in aerospace applications. It can be seen that all three spectra are very different. There are two main reasons for this. Firstly, whether a mode is infrared and/or Raman active (or is inactive in both) is determined by the symmetry of the molecule. N-phenylmaleimide has 54 normal modes, all of which are Raman allowed, however, seven of the modes are forbidden in the infrared. Note that even when a mode is allowed it may still have little or no intensity in one spectra or the other. In contrast, there are no selection rules for INS spectroscopy and as a result all modes are allowed.



Fig.2 (a) Infrared, (b) Raman, and (c) INs spectra of N-phenylmaleimide. (d) INS spectrum of N-(perdeuterophenyl)maleimide.



Secondly, from studies on related systems, it would be expected that the totally symmetric modes (those where there is no change in dipole moment) would be strong in the Raman and weak in the infrared, vice versa for the asymmetric stretch and deformation modes. These predictions are largely borne out; the strongest band in the infrared spectrum is the asymmetric (out-of-phase) carbonyl stretch at 1707 cm-1, in the Raman spectrum the strongest band is the symmetric (in-phase) carbonyl stretch at 1770 cm-1. However, a number of the modes cannot be located with any degree of certainty in either spectrum. By contrast, the carbonyl bands are completely absent in the INS spectrum, but there are several strong bands that do not have any obvious counterparts in the infrared and Raman spectra and these are the 'missing' bands.

The differences arise because the intensity of the ith INS band is proportional to:



(7)



12 Since neutrons have a mass approximately equal to that of the hydrogen atom, an inelastic collision results in a significant transfer of momentum, Q (ʼn-1), as well as energy, to the molecule. On TFXA (and IN1BeF), the design is such that there is only one value of Q for each energy, (ETrans . 16Q2). (Other instruments at the ISIS Facility and the ILL allow both the energy and the momentum transfer to be varied, but they constitute a different story). Ui is the amplitude of vibration of the atoms undergoing the particular mode. The exponential term in equation (7) is known as the Debye-Waller factor, UTotal is the mean square displacement of the molecule and its magnitude is in part determined by the thermal motion of the molecule. This can be reduced by cooling the sample and so spectra are typically recorded below 50K.

is the inelastic neutron scattering cross-section of all the atoms involved in the mode. The scattering cross-sections are a characteristic of each element and do not depend on the chemical environment. The cross-section for hydrogen is 80 barns while that for virtually all other elements is less than 5 barns. This means that modes that involve significant hydrogen displacement will dominate the spectrum. This dependence on the cross-section is why the INS spectrum is so different from the optical spectroscopies. There, the intensity derives from changes in the electronic properties of the molecule that occur as the vibration is executed, (the dipole moment and the polarisability for infrared and Raman spectroscopy respectively).

The cross-section is not only element dependent it is also isotope dependent and that of deuterium is 5 barns. This can be exploited to 'eliminate' parts of the molecule from the spectrum. Figure 2d is again N-phenylmaleimide but with the phenyl ring deuterated. This results in a considerable simplification of the spectrum and all of the modes can be assigned to vibrations of the maleimide ring with the phenyl group treated as a point mass.

Infrared and Raman spectroscopies are frequently called complementary forms of vibrational spectroscopy. Figure 2 strongly suggests that INS should be considered the third leg of the 'piano stool' of vibrational spectroscopy.

A very wide range of systems have been studied by INS spectroscopy including: hydrogen in metals [3], catalysts [4], polymers [5], biological samples [6], fullerenes [7] and battery materials [8]. Three examples will be given that illustrate what can be achieved with modern instrumentation.



13 The nine modes of complexed water

When a water molecule is coordinated to a metal atom the three translations and the three rotations of the free molecule become 'frustrated' and give rise to six new modes in the complex. Thus a coordinated water molecule has nine vibrational modes associated with it as illustrated in Figure 3 for the [FeCl5(H2O)]2- ion.


Fig.3 The nine modes of complexed water. Arrows refer to motion in the Fe-OH2 plane, and to motion perpendicular to the plane (above and below respectively) str stretch, asym asymmetric, sym symmetric, ip in-plane, oop out-of-plane.


Figure 4 show the FT-Raman, infrared and INS spectra of K2[FeCl5(H2O)]. The frequencies and assignments are given in Table 1. Clearly it is the combination of techniques that allows the various modes of coordinated water to be observed since no single spectroscopy shows all of the modes. The internal water stretch modes are best seen in the infrared, since the FT-Raman spectrum shows strong thermal emission in this region+ and the INS spectrum is dominated by the phonon wings. A major differences between INS and optical spectroscopies is that in the latter, overtones and combinations are typically 1% of the intensity of the fundamental, in INS the intensity can be up to 75% of the intensity! Furthermore, combinations between the internal modes and the external (lattice) modes occur, these are known as phonon wings. As the energy transfer increases, so does the momentum transfer and one of the effects of this is to redistribute the intensity from the fundamental into the phonon wings, shifting the apparent position and considerably broadening it. Both the Debye-Waller factor and this redistribution depend on Q2 and hence Etrans. The resolution is also energy transfer dependent and the combined effect of the three factors is to broaden and attenuate the high frequency features.

+ Editors note - The iron complex absorbs the near infrared laser source and hence heats up. The Raman instrument detects the emmision.



Fig. 4 Comparison of (a) infrared, (b) FT-Raman and (c) INS spectra of K2[FeCl5(H2O)] in the 0 - 1800 cm-1 region.


14 The H-O-H bending mode is observed in both the FT-Raman and the infrared and a weak feature is also apparent in the INS spectrum.

Of the three librational modes, none are visible in the FT-Raman spectra, this region is dominated by the Fe-Cl stretch and bending vibrations. The infrared spectrum shows a weak band at 600 cm-1, in the INS spectrum three modes are visible at 604, 449 and 438 cm-1.These are assigned as the wag, rock and twist respectively.

The translational modes are expected to occur below the librational modes. The Fe-O stretch is clearly seen in all three forms of spectroscopy near 400 cm-1. The remaining two modes correspond to Fe-O bending modes. Since the Fe-O bend is likely to produce a larger amplitude of vibration than the Fe-O stretch (since it is an angular displacement rather than a linear one), the intensity should be at least comparable to the stretch in the INS spectra. The intensity of the band at 222 cm-1 in the INS spectrum would suggest proton motion. There is no obvious candidate for the remaining mode and it is believed that both bending modes are coincident. Note that for a C4v complex (i.e. no hydrogen atoms) the two bending modes are degenerate.



15 Advanced composites

Advanced composites are engineering materials that offer similar mechanical properties to metal alloys but are lighter. The materials consist of fibres embedded in a polymer matrix and there is a need for a spectroscopic technique that can examine the cured resins in the presence of the fibres to aid the understanding of the cure chemistry. The ability to study the reaction(s) is greatly hampered by the nature of the products: they are often highly cross-linked and thus insoluble, and the presence of the fibre matrix makes them difficult to study spectroscopically. Inelastic neutron scattering (INS) has considerable potential in this regard since two common fibre types, glass and carbon, are (almost) invisible to neutrons.

A wide variety of polymers have been used including epoxies, bismaleimides and polyimides. One of the most common polyimides is PMR-15. The chemistry is complex, but consists essentially of two stages; polymerisation to give a norbornene end-capped oligomer followed by reaction of the norbornene group to give the cross-linked polymer. The temperature at which the cross-linking reaction is carried out has a major effect on the mechanical properties of the finished product, particularly its susceptibility to microcracking.

INS spectra of the composites cured at 270 and 330°C are shown in Figure 5a and 5b respectively. There are clearly differences between the two spectra; bands at 1031 and 1114 cm-1 have diminished in intensity and there are indications of changes in the region 200 - 400 cm-1 and at 638, 720 and 1273 cm-1.



Fig.5 INS spectra of PMR-15 resin carbon fibre composites cured at (a) 270 and (b) 330°C.


16 N-phenylnadimide (see Figure 6 for the structure) has been used very successfully as a model compound for the norbornene endcap and the cross-linking reaction. By deuterating the phenyl ring it is possible to eliminate the spectral features of the phenyl ring and just leave the norbornene endcap. This provides an excellent model compound for the endcap and the spectrum is shown in Figure 6. Comparison of Figure 5 and Figure 6 suggests that the decrease in the 1114 cm-1 and the changes in the 200 - 400 and 600 - 800 cm-1 regions can reasonably be assigned to loss of the endcap. The 1031 cm-1 band does not fit this pattern and may be assigned to the cross-linking.



Fig.6 INS spectrum of N-(perdeuterophenyl)nadimide.


17 The examples studied here are particularly difficult samples because as a result of the cure temperatures employed, a significant proportion of the end-groups have already reacted. In addition the composites are only 30% by weight resin, of which only 2/7 of the molecules are the end-cap. Nonetheless, as Figure 5 shows, differences between the two samples are apparent. Furthermore, the spectra are characteristic of the bulk of the polymer, in contrast to the infrared methods that only sample the top few microns of the cured composite.



18 Imidazole and zinc tetraimidazole

The metal centres in biological metalloenzymes are frequently found to be coordinated to the amino acid histidine via the unprotected nitrogen of the attached imidazole ring. In carbonic anhydrase, a zinc atom is bonded to three histidine groups and a hydroxyl ligand. An excellent model compound for this is the zinc tetraimidazole complex in conjunction with a non-coordinating counterion. (The structures are shown in Figure 7).



Fig.7 The structures of (a) histidine, (b) imidazole and (c) zinc tetraimidazole. In (c) filled circles are nitrogen atoms, open circles are carbon atoms. One hydrogen atom is attached to each carbon atom and each nitrogen atom that is not bonded to zinc.


19 With INS spectra, the frequencies are determined by the normal modes of vibration, exactly as for infrared and Raman spectra, but the intensities are dependent only on the scattering cross-section, equation (7), and the amplitude of vibration. The latter quantity is calculated as part of the standard Wilson GF matrix method and thus the intensities can be used as an additional constraint in the fitting. This almost doubles the information available without recourse to isotopic substitution. This is in contrast to the optical spectroscopies where the intensities are determined by the electronic properties of the molecule. These are extremely difficult to calculate and require a full ab initio calculation to estimate them. At present this is only practical for small molecules.

The result of a simultaneous fit to all the vibrational frequencies (obtained from infrared, Raman and INS spectra) and the INS intensities of imidazole [9] is shown in Figure 8. It can be seen that the quality of the fit is excellent and this is a direct result of the additional information provided by the INS intensities. The frequencies are obtained with a mean deviation of only 3 cm-1, four times better than that achieved in previous work.



Fig.8 INS spectrum of imidazole (solid line) and the calculated spectrum (dashed line). The top panel shows the frequencies of the fundamentals (solid lines) and the overtones and combinations (dashed lines). Reproduced with permission from: P.W.Loeffen, R.F.Pettifer, F.Fillaux and G.J.Kearley, "Vibrational force field of solid imidazole from inelastic neutron scattering", Journal of Chemical Physics, 103 (1995) 8444.


20 The force field for imidazole was then used in the analysis of the zinc tetraimidazole [10] complex. Figure 9 shows the evolution of the model used for the low frequency region that contains the skeletal motions of the molecule. These may be roughly categorised as Zn-N stretches, in-plane wags of the imidazole rings, out-of-plane bends and torsions of the imidazole rings about the Zn-N bond. The analysis is complicated by the overlap of the internal and external modes in this region. Nonetheless, the final fit is impressive.



Fig.9 Evolution of the model used to calculate the low frequency spectrum of zinc tetraimidazole. The solid line is the experimental spectrum and the dashed lines are the calculated spectrum using the geometry shown in the right-hand side of each panel. Reproduced with permission from: P.W.Loeffen, R.F.Pettifer and J.Tomkinson, "Vibrational force field of zinc tetraimidazole from inelastic neutron scattering", Chemical Physics 208 (1996) 403.




21 Future Directions

Crystal ball gazing is always an activity fraught with peril! However, it is possible to highlight a few trends that are becoming apparent. For anybody interested in developing molecular force fields, there is already a strong case to be made that INS spectra are essential for such work. This derives both from the ability to detect vibrations that are either weak or forbidden in optical spectroscopies and the constraint imposed by the need to fit the INS intensities.

For catalyst research, the use of INS to investigate the states of hydrogen on catalysts is already established and will grow. The need to record spectra at low temperature is probably the biggest disadvantage in this area, but, with suitable cell design, there are exciting possibilities for obtaining a 'snapshot' of a working catalyst or battery by quenching the process and recording the spectrum, without the need to remove the sample from the reactor.

The work shown here in this article all used TFXA at ISIS. TFXA has just been replaced by a new instrument TOSCA. TOSCA will be installed in two phases, the first phase is identical to TFXA except that where TFXA had two detector banks and a total of 28 detectors, TOSCA has ten detector banks and 150 detectors. This means that there will be major increase in sensitivity. In the year 2000, the second phase of TOSCA will be installed and this will offer a further increase in sensitivity as well as three times better resolution. These advances will drastically reduce the sample measurement time (at present 6 - 12 hours is typical) and so increase the availability of the instrument. There will be greater sensitivity allowing the use of smaller samples, this will be important in biology and biochemistry were it is often difficult to obtain the gram or so of sample needed at present and also for newly developed materials that are only initially available in small quantities. Non-hydrogenous samples can be studied at present but large specimens (5 - 20 gm) are needed. These will become more straightforward in the future. It will also mean that oxide supported metal catalysts with industrially relevant metal loadings (1 - 5%) will be routinely studied.



22 Conclusions

Inelastic neutron scattering spectroscopy is a technique that is still evolving, but has already found widespread applications in physics, chemistry, biology and geology. It is a technique that will always be complementary to other forms of spectroscopy (not least because of the scarcity of spectrometers) and the very high cost per spectrum but the information gained is frequently difficult or impossible to obtain by any other means and add unique insight into the structure and dynamics of the system under study.



References

1  The most up-to-date and useful source of information on neutron scattering centres and their instrumentation is the World Wide Web. The site: http://www.neutron.anl.gov/Neutronf.htm provides an extensive list of websites.
2  Further information about how to obtain access can be found at the ISIS and ILL websites. ISIS is at: http://www.nd.rl.ac.uk/isishome.html and it can contacted by e-mail on: ULS@ISISE.RL.AC.UK. Similarly for the ILL, the website is at: http://www.ill.fr/ and it can contacted by e-mail on: SCO@ILL.FR.
3  T. Springer and D. Richter, "Hydrogen in metals" in Methods of Experimental Physics (D. L. Price and K. SkÖld, eds.), 23B, Academic Press, San Diego, 1987.
4  J M Nicol, "Chemisorbed hydrogen and hydrogenous molecules", Spectrochimica Acta 48A (1992) 313.
5  B. Gabrys, D. Hong, F. Nardi, D. G. Peiffer and J. Tomkinson, "Dynamics of sulfonated ionomers in the solid state: inelastic neutron scattering studies", Macromolecules 26 (1993) 2007.
6  H. D. Middendorf, R. L. Hayward, S. F. Parker, J. Bradshaw and A. Millar, "Vibrational neutron spectroscopy of collagen and model polypeptides", Biophysics Journal 69 (1995) 660.
7  K. Prassides, H. W. Kroto, R. Taylor, D. R. M. Walton, W. I. F. David, J. Tomkinson, R. C. Haddon, M. J. Rosseinsky, and D. W. Murphy, "Fullerenes and fullerides in the solid state: neutron scattering studies", Carbon, 30 (1992) 1277.
8  F. Fillaux, C. Cachet, H. Ouboumour, J. Tomkinson, C. Lévy-Clément and L. T. Yu, "Inelastic neutron scattering study of the proton dynamics in manganese oxides", Chemical Physics 164 (1992) 311.
9  S. F. Parker, K. Shankland, J. C. Sprunt and U. A. Jayasooriya, 'The nine modes of complexed water', Spectrochimica Acta A, 53 (1997) 2245.
10  P.W.Loeffen, R.F.Pettifer, F.Fillaux and G.J.Kearley, "Vibrational force field of solid imidazole from inelastic neutron scattering", Journal of Chemical Physics, 103 (1995) 8444.
10  P.W.Loeffen, R.F.Pettifer and J.Tomkinson, "Vibrational force field of zinc tetraimidazole from inelastic neutron scattering", Chemical Physics 208 (1996) 403.





Feature Article



23 Neutron Spectroscopy in Catalysis

H.Jobic
Institut de Recherches sur la Catalyse,
CNRS
2 Avenue Albert Einstein,
69626 Villeurbanne
France



Introduction


Because of its penetration power, the neutron does not act as a surface probe. However it is possible to obtain spectra of surface species provided that the adsorbate has a considerably larger neutron scattering cross section than the substrate. Since the hydrogen atom has the largest neutron scattering cross section, (see the paper by S.F.Parker), it is this that is the preferred probe for adsorption studies.

INS is one of several vibrational techniques available for us to probe surface phenomena. Each technique has its own particular advantages for a given system in terms of spectral domain, resolution, sensitivity and experimental conditions. For example, infrared spectroscopy is a very efficient method to detect adsorbed CO but it is much less sensitive to adsorbed hydrogen.

The applications of INS to catalysis have mainly focused on systems which are either difficult or impossible to study by other spectroscopies. This arises where the sample is almost opaque or even completely black so that it may have only limited frequency windows in infrared similarly the sample may decompose or fluoresce in the laser beam in Raman. Comparisons have been made in a few cases between INS and the results obtained by electron energy loss spectroscopy (EELS).

Real catalysts, supported or unsupported, can be tackled by neutron scattering. These catalysts have generally inhomogeneous surfaces, e.g. oxides, sulphides and metals, although zeolites, which are well-crystallised materials, are well suited to the method. These substrates can be almost transparent to neutrons if they contain only a small quantity of hydrogen, in which case the neutron spectrum will be fairly flat and it will be possible to observe all the vibrational modes of the adsorbate.

24 Examples

As applications of INS to catalysis, one can mention studies concerning hydrogen chemisorbed on metals, sulphides and oxides, or hydroxyl groups; organometallic compounds; hydrocarbons and water on different catalysts and zeolitic systems. Some recent examples will now be described. The spectra were recorded with the IN1BeF spectrometer at the Institut Laue-Langevin (Grenoble, France), or with the TFXA spectrometer at ISIS (Rutherford Appleton Laboratory, UK).



25 1. Hydrogen chemisorption

Hydrogen chemisorbed on high surface area materials is difficult to characterise by optical spectroscopies. EELS data have been obtained on single crystals and since only a limited number of sites are available on well-defined crystal planes, the results have turned out to be useful for assigning the spectra obtained from polycrystalline materials.

With INS, all the local modes of hydrogen can be observed because there are no selection rules, which is clearly an advantage. However, if there are different planes exposed on the surface or if there are several species adsorbed, the assignment can be more difficult.


1.1. Hydrogen on nickel

If we consider for example hydrogen on nickel powders, all the recent INS studies indicate that hydrogen is predominantly multiply bonded, under (3-H) form [1-3]. Hydrogen is indeed predominantly multiply bonded to metal surfaces, this has been found by many experimental and theoretical methods. Intuitively, hydrogen tends to maximise its interaction with the surface. Here is a design of the 2 modes:



26 However, some authors still claim that after dissociation one hydrogen atom is only bonded to a single nickel atom (e.g. ref. 4), in complete disagreement with theoretical and experimental results, including the work on single crystals.

In the first INS studies on Raney nickel [1-2], the catalyst was outgassed at a sample temperature of 573 K, and it was clear from INS that no hydrogen was left on the surface. However, sintering occurs under these conditions and the surface area was found to decrease from 130 down to 40m2g-1.


Figure 1: INS spectra of hydrogen adsorbed on a doped Raney nickel [3]:
(a) residual hydrogen, after desorption at 373 K.
(b) spectrum obtained after adsorption under 13 mbar of H2 (the spectrum of residual hydrogen has been subtracted).
(c) spectrum obtained after adsorption of H2 at atmospheric pressure (spectra a and b have been subtracted).

27 The spectrum shown in Fig. 1(a) was obtained after outgassing at only 373 K. The signal produced by the aluminium container and the phonons due to the nickel sample have been subtracted. INS intensities show that a large quantity of hydrogen is present on this catalyst, about 30% of the amount originally adsorbed (this quantity depends on the pumping rate). At this temperature (100°C), no structural modification is induced but water has been completely removed. The spectrum of residual hydrogen consists of two bands at 800 and 1100 cm-1 sitting on a large background. The background is due to the broad distribution of sites for hydrogen. Even if the surface is mainly made of low index planes, the surface is far from being perfect: defects, steps, and kinks are known to occur. The two modes at 800 and 1100 cm-1 were assigned to the antisymmetric (E) and symmetric (A) stretching modes of hydrogen atoms adsorbed on sites of nearly C3v symmetry, located on the (110) faces. The weaker contribution observed at 940 cm-1 was assigned to hydrogen adsorbed on C3v sites, on (111) faces (See above).

28 The two modes of this species are more clearly observed in Fig. 1(b) which corresponds to the spectrum obtained after adsorption under an equilibrium pressure of 13 mbar of H2. The contribution from residual hydrogen has been subtracted and hence the base line is close to zero. The band at 940 cm-1 is assigned to the E mode of hydrogen adsorbed on C3v sites, while the shoulder at 1130 cm-1 corresponds to the A stretching mode. The overtones and combination of these two modes produce a broad contribution around 2070 cm-1. The band centred at 1800 cm-1 is assigned to the stretching mode of on the top hydrogen (a hydrogen bonded to a single Ni atom).

After hydrogen adsorption under atmospheric pressure, and subtraction of all the previous contributions (spectra a and b), the spectrum shown in Fig. 1(c) is obtained. The main species we see is still the one located on C3v sites, but the relative intensity of the band at 1800 cm-1 has increased.

In conclusion, after evacuation of the Raney nickel at 373 K, 30% of the total hydrogen uptake remains on the surface. This residual hydrogen is adsorbed on (110) and (111) faces, and also on defects. The top layered hydrogen is only detected upon readsorption of hydrogen, at higher pressure. This shows that multiply bonded hydrogen is more strongly adsorbed than linear hydrogen. This species appears close to the saturation of the surface. Even if the proportion of linear hydrogen is small (about 15%), it could be the active species in hydrogenation reactions since it has been found from kinetic studies and pulse experiments that only weakly adsorbed hydrogen is active for hydrogenation of acetonitrile [3].


29 1.2. Hydrogen on ruthenium sulphide

This is a very important technological area. Sulphide catalysts (MoS2 based) are used in almost every refinery in the world, essentially to remove sulphur and/or nitrogen from hydrocarbons and to hydrogenate petroleum, e.g. aromatics to cycloakanes.

RuS2 is 15 times more active that MoS2 in hydro desulphuration of dibenzothiophene and 6 times more active in hydrogenation of biphenyl. It has been postulated that hydrogen may be adsorbed either homolytically or heterolytically. On sulphides, accordingly, hydrogen chemisorption should form not only SH groups but also metal-hydrogen bonds. Hydroxyl and sulphhydryl groups have previously been measured but hydride-type hydrogen has never been clearly identified. In particular MoH species have not been observed on MoS2 in spite of several INS experiments.


Figure 2: INS spectra obtained at 25 K with ruthenium sulphide:
(a) unreduced catalyst.
(b) 2. hydrogen adsorbed at low pressure (less than 1 mbar) on partially desulphurised RuS2.
(c) hydrogen adsorbed at higher pressure (0.5 bar at 295 K) on partially desulphurised RuS2.


30 The interaction of hydrogen with RuS2 has been recently studied by INS [5] because this sulphide is 10 times more active than MoS2 in hydrogenation and hydrodesulphurization reactions. This could be due to the larger adsorption capacity of ruthenium sulphide or to the presence of different hydrogen species. Unsupported RuS2 consisting of homodispersed spheres with a diameter 45-50 Ĺ was used. By changing the degree of reduction and the experimental conditions, several hydrogen species have been observed:

(i) When the catalyst is sulphided at 673 K under H2S flow, chemical analysis gives a stoichiometry S/Ru = 2.25. The INS spectrum shown in Fig. 2(a) indicates only the presence of SH groups on the surface. The peak measured at 737 cm-1 is assigned to SH bending modes, in agreement with previous INS studies performed on other sulphides: these modes were measured at 694 cm-1 on WS2 [6] and at 650 cm-1 on MoS2 [7].

(ii) If the catalyst is partially desulphurized under H2 flow at 513 K, the solid composition becomes RuS1.88. After hydrogen adsorption at low pressure (less than 1 mbar), a new peak and a shoulder appear respectively at 823 and 550 cm-1, Fig. 2(b). When the hydrogen pressure is increased (0.5 bar at 295 K), the peak at 542 cm-1 gains further in intensity, Fig. 2(c). This peak and the one measured at 826 cm-1 are assigned to the bending modes of two different RuH species. It appears that the hydridic group which gives rise to the peak at 542 cm-1 is more weakly adsorbed than the species which yields the peak at 826 cm-1. Since it has been mentioned in the previous section that on Raney nickel, reactive hydrogen is the more weakly adsorbed species, it can be proposed that the hydrogen species active on ruthenium sulphide for hydrogenation reactions is the one giving rise to the peak at 542 cm-1.

Since the integrated intensities of the bands are a direct measure of the populations of the various species, it can be concluded that the reduction of the catalyst creates new SH groups: their intensity is 3 times larger in Fig. 2(b) than in Fig. 2(a). The shift to lower frequency observed for the SH modes after reduction, from 737 to 683 cm-1, indicates that the Brönsted acidity of the catalyst increases.



31 2. Water in interaction with acidic sites in zeolites

A question which is much debated at the moment is whether the Brönsted acidity of several solids, e.g. zeolites or sulphated zirconia, is high enough to protonate water. Two possible structures have been envisagedÝ: a hydrogen-bonded water molecule and a protonated molecule, H3O+, the hydroxonium ion. Most of the experimental results have been obtained by NMR or infrared spectroscopies, but the interpretation of the results is complicated. In NMR, rapid exchange can take place at room temperature between molecules in different adsorption states. In infrared, there are resonant interactions in the stretching OH region with overtones of OH deformations.

Recent ab initio calculations performed on small clusters indicate that only the hydrogen-bonded structure is a minimum whereas the hydroxonium species is a transition structure for proton transfer [9]. However the energy difference between the two structures is small, a few kJ/mol.

Here, the comparison between INS and theoretical calculations is fruitful. INS spectra can be simulated for the two possible water structures using theoretical frequencies and atomic displacements as inputs [8]. The two calculated spectra are compared with the experimental data in Fig. 3.


Figure 3. INS spectra of water adsorbed at low loading in H-ZSM-5
(a) experimental spectrum
(b) spectrum simulated for a water molecule hydrogen-bonded to a bridging hydroxyl group.
(c) spectrum simulated for a hydroxonium ion.


32 The experimental INS spectrum, Fig. 3(a), corresponds to a ratio H2O/H+ of 0.61 in the H-ZSM-5 zeolite. The contribution from the dehydrated zeolite has been subtracted which explains the negative peak at 1080 cm-1. This band corresponding to (OH) deformations of the acidic groups. The negative contribution in the difference spectrum indicates interaction of water with the acid sites. In the case of water interaction with acid sites in zeolites, ab initio calculations give the frequencies and the atomic displacements so that INS spectra can be calculated without adjustable parameters. For larger systems: benzene in NaY, ab initio calculations are not possible so that the frequencies and the atomic displacements have to be derived from empirical force fields calculations. To compare experimental and simulated spectra, they can be superposed one on top of the other, as in Figure 3; the comparison is made by eye. The second method is to fit the force field directly to the observed INS profile.

The two calculated spectra are shown in Figs. 3(b) and (c ). It is worth noting that there are no adjustable parameters, only the resolution function has been introduced. It is clear from a comparison of the spectra in Fig. 3 that the hydrogen-bonded water model, Fig. 3(b) reproduces better the experimental profile than the hydroxonium model, Fig. 3 (c).

The only deficiency in the simulation is that the calculated frequencies are situated 80 cm-1 too low in energy. Ab initio quantum calculations yield harmonic vibrational frequencies systematically shifted with respect to the observed frequencies. Therefore the calculated frequencies have to be scaled by a value ranging from 0.8 to 1.1. The assignment of the main INS bands is however simple. The band measured at 1650 cm-1 is assigned to the bending of the water. The peak at 1380 cm-1 corresponds to the perturbed deformation mode of the acid site, (OzHz). The band having a maximum at 900 cm-1 corresponds to the out-of plane deformation of the proton hydrogen-bonded to the zeolite: (OwHb). The largest peak at 455 cm-1 is the sum of several contributions: the out-of-plane deformation of the free water proton, (OwHf) and intermolecular modes of water (twisting and rocking). Translational modes of water give a band at 62 cm-1 in the calculated spectrum, compared with 90 cm-1 in the experiment.



33 3. Benzene adsorbed in NaY zeolite

Ab initio calculations are not very often available to simulate INS spectra without adjustable parameters. Generally, a quantitative interpretation is based on an empirical force field. The first approach was to compare visually observed and calculated spectra to test the vibrational assignment. At a later stage [10], a method was proposed by which the vibrational analysis is performed by the refinement of the force constants to give a least-squares fit of the calculated spectrum to the observed profile.


Figure 4: Comparison of experimental and calculated INS spectra of benzene adsorbed in NaY zeolite.


An example concerning benzene adsorbed in NaY zeolite will be given here [11]. Part of the experimental spectrum, obtained for a loading of one molecule per supercage, is shown in Fig. 4 as a dotted line. The frequencies of the fundamentals are indicated as sticks. Side bands due to combinations with the external modes can be measured in between fundamentals.

34 The force constants of adsorbed benzene were refined directly to the observed profile, starting from a reasonable force field. The refinement includes the intensities from fundamentals, overtones and combinations, the contributions from all atoms being added up. The calculated profile is shown in Fig. 4 as a continuous line. The agreement with the experimental data is reasonable. Not all the frequencies are resolved, but the method of treating overlapping modes is similar to the Rietveld method# in powder diffraction. The entire INS profile is refined, instead of just adjusting the frequencies of the normal modes. The frequencies of all fundamentals are thus obtained, whereas they have to be derived from overtones or combinations bands in infrared.

It appears that much progress has been made in the data treatment of the INS spectra, since the first quantitative interpretation of benzene [12]. In former times, the instrumental resolution was low and only the intensities from the fundamentals were calculated. The experimental spectra can be now simulated with a good accuracy because the resolution varies with the energy transfer. At the present time, it is of ~10 cm-1 (FWHM) at 200 cm-1, rising to 30 cm-1 at 1500 cm-1

#The Rietveld method is used for X-ray or neutron diffraction with powders (as opposed to single crystals). With a powder, there is usually overlap between adjacent peaks, so that individual reflections cannot be measured. This produces a complex diffraction profile that is calculated from an initial structural model as a sum of overlapping reflections. It is then compared to the observed pattern, assuming a peak-shape function. By varying the unit-cell parameters and the atomic parameters (positions, occupancies, Debye-Waller factors...), the best least-squares fit between observed and calculated profiles can be obtained, and hence the `best' crystal structure is refined.



35 Conclusion

The information that is extracted from the INS results cannot usually be obtained from other vibrational methods. INS is well suited to characterise different adsorbed hydrogen species or the Bronsted acidity of a catalyst. The nature of adsorbed molecules can also be identified and the adsorption geometry and strength of bonding can be determined. In the future, quantitative interpretations will be more common, to use the information contained in the intensities. However the small number of high resolution spectrometers (only four in the world) will always limit the number of applications to highly selected systems.



References

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6  Wright C. J., Fraser D., Moyes R. B. and Wells P. B., Appl. Catal. 1 (1981) 49-58.

7  Sundberg P., Moyes R. B. and Tomkinson J., Bull. Soc. Chim. Belg. 100 (1991) 967-976.

8  Jobic H., Tuel A., Krossner M. and Sauer J. J. Phys. Chem. 100 (1996) 19545.

9  Krossner, M. and Sauer J., J. Phys. Chem. 100 (1996) 6199-6211.

10  Kearley, G. J., J. Chem. Soc., Faraday Trans. 2 82 (1986) 41-48.

11  Jobic H. and Fitch A. N., Progress in Zeolite and Microporous Materials, Studies in Surface Science and Catalysis, Vol.105A, H. Chon, S. K Ihm and Y. S. Uh, Eds. (Elsevier, 1997) pp. 559-566.

12  Jobic H., Tomkinson J., Candy J.P., Fouilloux P. and Renouprez A. Surface Science 95 (1980) 496-510.




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