Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignments for methylaminothiophosphoryl difluoride

Infrared spectra (4000-50 cm(-1)) of the vapor, amorphous and crystalline s olids and Raman spectra (3600-10 cm(-1)) of the liquid with qualitative dep olarization data as well as the amorphous and crystalline solids of methyla minothiophosphoryl difluoride, CH3N(H)P(=S)F-2, and three deuterated specie s, CD3N(H)P(=S)F-2, CH3N(D)P(=S)F-2, and CD3N(D)P(=S)F-2, have been recorde d. The spectra indicate that in the vapor, liquid and amorphous solid a sma ll amount of a second conformer is present, whereas only one conformer rema ins in the low temperature crystalline phase. The near-infrared spectra of the vapor confirms the existence of two conformers in the gas phase. Asymme tric top contour simulation of the vapor shows that the trans conformer is the predominant vapor phase conformer. From a temperature study of the Rama n spectrum of the liquid the enthalpy difference between the trans and near -cis conformers was determined to be 368 +/- 15 cm(-1) (4.41 +/- 0.2 kJ/mol ), with the trans conformer being thermodynamically preferred. Ab Initio ca lculations with structure optimization using the 6-31G(d) and 6-311+G(d,p) basis sets at the restricted Hartree-Fock (RHF) and/or with full electron c orrelation by the perturbation method to second order (MP2) support the occ urrence of near-trans (similar to5 degrees from trans) and near-cis (simila r to 20 degrees from cis) conformers. From the RHF/6-31G(d) calculation the near-tra,ls conformer is predicted to be the more stable form by 451 cm(-1 ) (5.35 kJ/mol) and from the MP2/6-311+G(d,p) calculation by 387 cm(-1) (4. 63 kJ/mol). All of the normal modes of the near-tl-ans rotamer have been as signed based on infrared band contours, depolarization values and group fre quencies and the assignment is supported by the normal coordinate calculati on utilizing harmonic force constants from the MP2/6-31G(d) ab initio calcu lations. (C) 2001 Elsevier Science B.V. All rights reserved.