Vibrational analysis of substituted benzaldehydes. Part I. Vibrational spectra, normal co-ordinate analysis and transferability of force constants of monohalogenated benzaldehydes

摘要

The Raman and Fourier transform infrared (FTIR) spectra of p-, m- and o-fluorobenzaldehydes, p-, m- and o-chlorobenzaldehydes and p-, m- and o-bromobenzaldehydes were measured. Raman polarisation measurements were made wherever possible. A normal co-ordinate analysis was carried out for both the in-plane and out-of-plane vibrations of these molecules using an 81-parameter modified valence force field. An overlay least squares technique was employed to refine the force constants using 259 frequencies of nine molecules. The reliability of these force constants was tested by making a zero-order calculation for both in-plane and out-of-plane vibration of p-, m- and o-iodobenzaldehydes; p-, m- and o-anisaldehydes; 2,4-dichlorobenzaldehyde, 2,6-dichlorobenzaldehyde and 3,4-dichlorobenzaldehyde. Unambiguous vibrational assignments of all the fundamentals were made using the potential energy distributions and eigenvectors.