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

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Abstract
The microwave spectrum of chloromethylphosphonic difluoride, ClCH2P(O)F2, has been investigated in the region from 26.5 to 39 GHz. The a‐type R branch transitions have been assigned for both the 35Cl and 37Cl isotopic species for the trans conformer on the basis of the rigid rotor model. For the ground vibrational state the rotational constants for the 35Cl isotope were found to be A = 4392.4±2.3, B=1543.36±0.01, and C=1512.30±0.01 MHz and for the 37Cl isotope: A=4395.3±2.7, B=1502.04±0.01, and C=1472.54±0.01 MHz. With reasonably assumed structural parameters for the C–H and P=0 distances as well as the HCH angle, a diagnostic least‐squares adjustment was utilized to obtain the other six structural parameters. The dipole moment components were determined from the Stark effect to be ‖μa‖ =2.28±0.05, ‖μb‖ =0.75±0.02, and ‖μt‖ =2.40±0.02 D. The infrared (3500–40 cm−1) and Raman (3500–20 cm−1) spectra of the gas and solid have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative polarization values have been obtained. Both the trans and gauche conformers have been identified in the vibrational spectrum of the fluid phases. From a temperature study of the Raman spectrum of the liquid phase the enthalpy differencebetween the trans and gauche conformers was determined to be 370±50 cm−1 (1.06 kcal/mol) with the trans conformer being thermodynamically preferred. Band contour simulation of the infrared gas phase bands also shows that the trans conformer is more stable in this phase. Upon crystallization only the trans conformer remains in the solid state. The asymmetric torsion for the trans conformer was observed as a series of closely spaced Q branches beginning at 82.5 cm−1 and falling to lower frequency and the corresponding transitions for the gauche conformer begin at 72.9 cm−1. These transitions have been used to obtain the potential constants for the asymmetric rotation. All of the normal modes have been assigned based on band contours, depolarization values, and group frequencies. A normal coordinate calculation has been carried out by utilizing a modified valence force field to calculate the frequencies and the potential energy distribution. All of these results are compared to similar quantities in some corresponding molecules.

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