Ben Said Ridha , Theoretical Chemistry Laboratory, Email: baha.tangour@fsm.rnu.tn
Abstract
Keywords The study of the halophosphines PX3, PX2H and PXH2 (X=F, Cl, Br, I) is topical because of their involvement in chemistry and in biochemistry [1]. Although some of them have been known for more than a century, in particular the trihalophosphines PX3, the majority still resist synthesis or identification e.g. PFH2 [2] and PF2H [3] were only formally identified after they had been trapped in a argon matrix at low temperature. At ambient temperatures, halophosphine compounds have a high tendency to polymerise. Iodine presents more difficulties than do the other halogens. While information about
the PI3 molecule is available for the gas state, the PIH2 and PI2H
molecules have only been detected in mixtures with PI3, PH3, I2
and P2I4 in CS2 by Schmidt and coworkers [4]. Their
presence was confirmed by the analysis of the NMR 31P and H1
spectra. In particular, the NMR 31P chemical shift and 1JPH coupling
constant of PIH2 and PI2H are (-156, 168Hz) and (-9,145Hz)
respectively. The PIH2 molecule was also detected by Beckers and coworkers[5]
in a mixture of P2H4 and HI. Experimental vibrational frequencies
values are also available although they do not concern the isolated or synthesized
molecule and were determined in situ. This paper is aimed at a better understanding and description of the iodophosphines from physico-chemical vibrational properties. Computational details Ab-intio calculations were supported by the PSHONDO algorithm[6], a modified version of the HONDO program [7], including the pseudo-potential suggested by Durand and Barthelat. Effective core potentials were used for each atom other than Hydrogen leaving five or seven electrons in the atomic valence space of the Phosphorus and Iodine atoms respectively. The valence electrons were described through a basic set of quadruple gaussian functions for both Phosphorus and Hydrogen and quintuple gaussian functions for Iodine. These functions were optimized in the ground state of each atom. We contracted the denoted DZ gaussian functions of double zeta by means of a 3+1 process for both Hydrogen and Phosphorus, and a 4+1 process for Iodine. The triples zeta denoted TZ, contraction was executed by means of a 2+1+1 process for the Hydrogen and Phosphorus, and a 3+1+1 process for the Iodine. We added to these basic sets, successively, the polarisation function 2p, 3d and 5d on Hydrogen, Phosphorus and Iodine atoms respectively. The exponents for these polarisation functions were optimized for each molecule. These basic sets will be described as DZP or TZP. We have shown in Table 1 the optimized values of the polarisation functions used. Geometry optimization was performed using PSG[8] or MONSTERGAUSS[9] programs with a self-consistent-field (SCF) gradient technique. The convergence threshold on the gradient components was fixed at 10-4. The vibrational frequencies and corresponding normal modes of each state were determined under the same conditions. Results and Discussion Geometry Optimization In spite of the use of relatively large dimension basis sets and of polarisation function optimization, we obtain for the molecule PI3 2.489 Å for the P-I bond length which is larger by 2.4% than the experimental value of 2.43Å. This could have as its origin the influence of relativistic effects which are completely ignored in the pseudopotentials determinations. We note a substantial decrease of the bond lengths when the basic set dimension increases. The P-I bond length decreases, when we pass from the DZ basis sets to the TZP by 5.9%, 4.8% and 4.3%, for PIH2, PI2H and PI3 respectively. The P-H bond length decreases only in this series by 1.3% and 1.5% for PIH2 and PI2H respectively. We also note that the introduction of the polarisation function has a more important effect than the basis set relaxation on the decrease of the bond length. For example, for the PIH2 molecule, the former effect decreases the P-I bond length by 5.9% and the latter only by 1.3%. As usually observed, the basis set dimension is less significant on changes in the bond angles than it is on the bond lengths. So the HPH angle increases only by 1.1% in passing from the TZP basis set to the DZ. Similar results are observed on HPI and IPI bond angles. Vibrational Frequencies And Normal Modes The experimental values (Table 2c) are 325cm-1, 303cm-1, 111cm-1 and 79cm-1 attributed to the modes n 3(e), n 1(a1), n 2(a1) and n 4(e). We begin by noting that our calculated values are basis set independent and they are in
excellent agreement with the experimental values in spite of the fact that the basic sets
used do not contain polarisation functions. The largest error of 14% is for the modes For PHI2, whose symmetry classification is Cs , Schmidt and coworkers [4]
suggest the following values: 2260-2270cm-1 for n
(PH), 781-782cm-1 for d (PHI), For PIH2 molecule, whose point group is Cs, Schmidt et al suggest the following values: 2360cm-1 for n (PH),750 cm-1 for d (PH2). The valence stretching n (PI) mode was not assigned with precision. The authors indicate simply that it is in the domain 337-320cm-1 "common to all PI vibrations". However, Beckers and coworkers suggest for this molecule P the following values: 2306cm-1 for n (PH), 1030cm-1, one band of C type, for d (PH2), 752cm-1, one band with the A type*, for d (PHI) and 338cm-1, one band of the A type, for n (PI). For the PI2H molecule, we think that the assignment by Schmidt and coworkers of the two valence modes n as(PI) and n s(PI) as two very close frequencies is hazardous considering the large number of compounds in the mixture#. The value of 337cm-1, in spite of the fact that it also exists in PI3, is more probable. Conclusion In this work, we have obtained results in good agreement with the available experimental data. We have proposed theoretical values for the several vibrational modes of PI3, PI2H and PIH2 molecules. We hope that our investigations give chemists useful information leading to a better characterisation and identification for the as yet unsynthetised molecules PI2H and PIH2. [Editor's Notes: * Vapour phase band contours in infrared absorptioon. The subject will be covered in future editions. # But this is normal in a molecule Abn where the mass of B>A. An example is the sulphur and selenium molecule X2Y2. Where X = and Y = the sym and assym XY stretching…………by only cm-1].
Table 1a. PIH2 geometry parameters in the different basis sets.
Table 1b. PI2H geometry parameters in the different basis sets.
Table 1c. PI3 geometry parameters in the different basis sets
Table 2a. Normal modes PIH2 vibrationnal frequencies , expressed in cm-1.
Table 2b. Normal modes PI2H
vibrationnal frequencies, expressed in cm-1.
Table 2c. Normal modes PI3 vibrationnal frequencies in cm-1. References
Received in original format 27th May 1999, accepted
7th June 1999 |
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HPH
HPI