Infrared spectra of benzylidene aniline, p-nitrobenzylidene aniline, p-N-dimethylaminobenzylidene aniline, and their 15N-labelled azomethine analogs were obtained to assess the reliability of C=N stretching assignments and Hammett relationships based on these results. The isotopic data indicate that there is extensive mixing of internal coordinates in bands near 1600 cm−1. Bands possessing the greatest C=N stretching character are not always readily distinguishable from aromatic ring breathing modes due to variations in relative intensities and band overlap. Problems related to the use of frequency and intensity data of highly coupled normal modes in Hammett correlations are discussed.
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