Spectra and structure of organophosphorus compounds. XXIII. Microwave spectra, electric dipole moment, and molecular structure of two conformers of ethyldifluorophosphine

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Abstract
The microwave spectrum of ethyldifluorophosphine, CH3CH2PF2, has been investigated in the region from 18.0 to 39.5 GHz. The spectra of two conformers have been identified. The ground state and seven vibrationally excited states have been assigned for the more stable gauche conformer (phosphorus lone pair with respect to CH3 group), whereas the ground state and one excited state have been assigned for the trans form. Rotational and most quartic centrifugal distortion constants have been determined with A=6784.501(3), B=2930.156(2), C=2352.576(2) MHz for gauche and A=5444.910(3), B=3290.133(3), C=2871.915(3) MHz for trans, both in the ground state. Electric dipole moments were obtained from ground state Stark effect measurements: ‖ μa‖=1.990(4), ‖ μb‖=0.855(4), ‖ μc‖=0.091(7), ‖ μt‖=2.168(2) D for gauche and ‖ μa‖=1.780(10), ‖ μb‖=0, ‖ μc‖=1.077(10), ‖ μt‖=2.081(10) D for trans. Using a new computer program, a diagnostic least-squares fit of the structural parameters suggests significant structural differences between the conformers, notably the C–C–P angle which changes from 110.7° in the gauche form to 114.9° in the trans form. From a temperature study of the microwave spectrum, it was found that the gauche conformer is at least 80 cm−1 (229 cal/mol) more stable than the trans conformer. These results are compared to similar quantities in some corresponding molecules.

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