Conformational stability of CH3CH2P(Z)F2(Z=O,S) from temperature dependent FT-IR spectra of rare gas solutions and ro structural parameters

Abstract

Variable temperature (from −55 to −150°C) studies of the infrared spectra (3500–400cm−1) of ethylphosphonic difluoride, CH3CH2P(O)F2 and ethylphosphonothioic difluoride, CH3CH2P(S)F2 dissolved in liquid xenon or krypton have been recorded. From these data, the enthalpy differences have been determined to be 76±9cm−1(0.91±0.11kJ/mol), for CH3CH2P(O)F2 with the trans conformer the more stable rotamer and 53±7cm−1(0.63±0.08kJ/mol) for CH3CH2P(S)F2 but with the gauche conformer the more stable form. Complete vibrational assignments are presented for both molecules, which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G(d) calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been obtained from RHF/6-31G(d) and/or MP2/6-31G(d) ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate as well as with some corresponding results for some similar molecules. The ro adjusted structural parameters have been obtained for both molecules from a combination of the microwave rotational constants and ab initio predicted parameters. The corresponding ro structural parameters have been obtained for some similar molecules for comparison.