Spectra and structure of phosphorus-boron compounds—XXVII. Low-resolution microwave, IR and Raman spectra, conformational stability, ab initio calculations, vibrational assignment and normal coordinate calculations for (chloromethyl)difluorophosphine borane

The low-resolution microwave spectrum of (chloromethyl)difluorophosphine borane, CICH2PF2 · BH3, has been investigated from 26.5 to 39 GHz. Based on the calculated dipole moments and the fact that the microwave spectrum is an a-type spectrum along with the spacing of the central transitions for the near-prolate top, it is concluded that the predominant spectrum is that for the conformer with the CCI bond trans to the PB bond. The IR (4000–40 cm−1) and Raman (3500–20 cm−1) spectra of the gas and solid of five isotopic species have been recorded. Additionally, the Raman spectra of the liquids have been recorded and qualitative depolarization values have been obtained. Both the trans and gauche conformers have been identified in the vibrational spectra of the fluid phases. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the trans and gauche conformers was determined to be 162±9 cm−1 (463±26 cal mol−1) with the gauche conformer being the more stable rotamer. Depending on experimental circumstances upon crystallization, either a solid containing only the gauche conformer or a solid containing both the gauche and trans conformers can be obtained. All of the normal modes have been assigned based on IR band contours, depolarization values and group frequencies. Additionally, ab initio calculations have been carried out utilizing both 3–21G* and 6-31G* basis sets to obtain the stabilities, barriers to internal rotation, and optimized structural parameters. A normal coordinate analysis has also been performed with a force field determined from the 3–21G* basis set for both conformers. These results are compared with the corresponding quantities of some similar compounds.