Infrared intensities of the C=C and C=N stretching modes of conjugated Schiff bases. A study based on ab initio molecular orbital calculations

Vibrational analyses of model compounds of protonated and unprotonated conjugated Schiff bases are performed by use of force fields derived from ab initio molecular orbital calculations. The calculated force fields and dipole derivatives correctly reproduce the observed infrared frequencies and intensities of the C=C and C=N stretching bands. The origin of the infrared intensities of these bands is examined in detail in terms of the magnitudes and directions of dipole derivatives. It is shown that a normal mode having a large contribution of the in-phase skeletal stretching coordinate gives rise to a strong infrared absorption. The strong infrared intensity is explained by a large change in the electronic structure that causes charge flux in the molecular skeleton.