| Popis: |
The infrared and Raman spectra of 1,1,3,3,5,5-hexafluoro-1,3,5-trisilacyclohexane (c-C 3 H 6 Si 3 F 6 ) as a solid powder have been recorded. The vibrational spectra reveal that the crystalline compound exists as a chair conformer with C 3v symmetry at ambient temperature. Additional conformers such as twist or boat were not detected although quantum chemical calculations indicated a negligible energy difference between chair, twist and boat forms. The wavenumbers of the IR and Raman bands were measured and the assignments were initially supported by quantum chemical B3LYP/cc-pVTZ calculations in the harmonic approximation. These vibrational modes were scaled with a common factor of 0.98, and the 11 totally symmetric A 1 and 16 doubly degenerate E fundamentals were assigned on the basis of the infrared and Raman intensities. An average relative deviation of ca. 2.9% between the observed and the scaled harmonic calculations was found. Additional calculations were made in the anharmonic approximation, in which the symmetry was reduced to C s symmetry by a slight distortion of the hexagonal ring. Employing these calculations without scaling, a better agreement between the observed and calculated wavenumbers was obtained (1.5%). Further calculations, involving a twist conformer was not consistent with the spectral results, neither in terms of the number of modes nor their wavenumbers. |
Full Text from ScienceDirect