CONCENTRATION AND SOLVENT EFFECTS UPON CARBONYL STRETCHING FREQUENCY-SHIFTS OF RAMAN-SPECTRA - 4-SUBSTITUTED BENZALDEHYDES

From Raman spectra, the vC=O frequencies of 4-substituted benzaldehyde s were found to correlate with the sigma(p) values of the 4-x atom or group as well as the acceptor number (AN) values of solvents. In vario us solvents, the vC=O frequency of benzaldehyde decreased upon the inc rease of benzaldehyde concentration. This shift was mainly due to the hydrogen bonding between the carbonyl oxygen and/or aldehydic proton o f benzaldehyde and the solvent molecules. Over the 1-80 volume % chang e. the vC=O frequency of benzaldehyde down shifted from 1709.4 cm(-1) to 1700.2 cm(-1) in CCl4 solution and from 1703.0 cm(-1) to 1698.0 cm( -1) in C2H5OH solution. This is due to the fact that hydrogen bonding between the benzaldehyde and C2H5OH was much stronger than that betwee n the benzaldehyde and the other solvents.