Spectra and structure of organophosphorus compounds. XXXIII. Microwave, infrared, and Raman spectra, structure, conformational stability, and vibrational assignment for ethylphosphonothioic difluoride

Abstract

The microwave spectra of ethylphosphonothioic difluoride, CH3CH2P(S)F2, and eight isotopic species have been investigated in the region from 26.5 to 39.5 GHz. Only a-type transitions were observed and R-branch assignments have been made for all the isotopic species in the ground vibrational state for both the gauche and trans (methyl group trans to the P=S bond) conformers from which the rotational constants were determined. From these data the complete r0 structural parameters were determined for the gauche conformer with the values for the heavy atom parameters being: r(C–C)=1.532±0.006 Å, r(C–P)=1.800±0.007 Å, r(P=S)=1.880±0.003 Å, r(P–F)=1.555±0.005 Å, ∢CCP=112.6±0.3°, ∢CPS=119.4±0.4°, ∢CPF=102.0±0.2°, dih ∢FPCS=129.3±0.2°, and dih ∢CCPS=56.9±0.2°. The parameters of the trans conformer which differed significantly from the values for the corresponding ones in the gauche conformer were: r(C–P)=1.814±0.011 Å, r(P=S)=1.861±0.007 Å, and ∢CCP=114.8±0.2°. The infrared (3500 to 40 cm−1) and Raman (3500 to 20 cm−1) spectra of the gaseous and solid CH3CH2P(S)F2 and CD3CD2P(S)F2 as well as the Raman spectrum of the liquid have been recorded. Both trans and gauche conformers have been identified in the vibrational spectra of the fluid phases, but only the trans corformer remains in the solid state and a complete vibrational assignment is proposed for the trans conformer. The barrier to methyl rotation for the trans conformer was determined to be 808 cm−1 (2.31 kcal/mol). The asymmetric torsion for the trans conformer was observed as a series of closely spaced Q branches beginning at 73.25 cm−1 and falling to lower frequency and the gauche transitions begin at 70.82 cm−1. These transitions along with the dihedral angle for the gauche conformer have been used to obtain the potential function for the asymmetric rotation which indicates that the trans conformer is more stable than the gauche conformer in the gas phase by 63±37 cm−1 (180±106 cal/mol). All of these results are compared with corresponding quantities for several similar organophosphorus compounds.