Vibrational study of molecules with a geminal diphenyl group: A reappraisal of the Raman and i.r. spectra of 1,1′-diphenylethene and the low-frequency spectra of benzophenone, diphenylketimine, diphenylmethane and diphenylether

The Raman and i.r. spectra of 1,1′-diphenylethene and the low-frequency spectra of benzophenone, diphenylketimine, diphenylmethane and diphenylether have been reinvestigated in order to attain a consistent attribution of the characteristic vibrations of geminal diphenyl substitution. Most of the internal vibrations of the benzene rings were seen in pairs of bands with very close frequencies, corresponding to symmetric (in-phase) and asymmetric (out-of-phase) modes. Of those monosubstituted-benzene vibrations usually considered to be sensitive to substitution, ν 13 , ν 6a and ν 16b were found to span a narrow frequency interval, with a separation of 10–20 cm −1 between the respective symmetric and asymmetric modes. On the contrary, quite far apart frequencies were attributed to the symmetric and asymmetric modes of the vibrations ν 1 , ν 9b and ν 10b ; for these, frequency separations ranging from about 50 to 140 cm −1 were found, with a pronounced dependence of the covered spectral interval on the nature of the central group. The presence of a very strong Raman band at 90—80 cm −1 was the most striking spectral characteristic for all of the molecules under consideration: it was attributed to the PhXPh angle bending vibration, although considerable mixing of it with other low-frequency vibrations was considered likely. Based on internal potential energy functions computed by a semi-empirical method (C-INDO), an approximate calculation of the frequencies for the conrotatory torsion of Ph 2 CCH 2 , PH 2 CO and Ph 2 O and both torsions of Ph 2 CH 2 was performed, so as to predict their relative magnitude and to assist the interpretation of the spectra in the lowest region. I.r. bands observed at 55 and 70 cm −1 for Ph 2 O and Ph 2 CO, respectively, might be considered for assignment to torsional vibrations.