Infrared Study of 4-Substituted Benzaldehydes in Dilute Solution in Various Solvents: The Carbonyl Stretching Mode

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Abstract
The carbonyl stretching mode in some of the 4- x-benzaldehydes is in Fermi resonance with an overtone of a fundamental which occurs at lower frequency. In general, the unperturbed vC=O frequencies for 4- x-benzaldehydes do appear to correlate with the σp values of the 4- x atom or group. The AN values of the solvents show a pseudo-correlation with the carbonyl and vasym. NO2 stretch vibrations of 4- x-benzaldehyde. However, neither σp nor AN values appear to take into account solute/solvent interactions such as intermolecular hydrogen bonding with the C=O group and with other sites in the molecule such as the phenyl group π system and with the electronic system of other functional groups, since the points in each plot do not correlate in the exact manner in each case.