Estimation of absorption band contribution for overtones and combined frequencies in the fingerprint region for alkanes, nitriles, amines, and nitroalkanes

Abstract

In the anharmonic approximation the intensities of fundamental, overtone, and combined absorption bands are calculated in the range from 100 cm-1 to 4000 cm-1 for a series of alkanes, nitriles, amines, and nitroalkanes. The first and second derivatives of the molecular dipole moment are calculated by quantum chemical methods in the ab initio approximation using the MP2/6-31G(1d) basis set. Overtones and combined frequencies are found to make a significant contribution (5–20%) to the total absorption. The spectral distribution of this contribution depends on the molecular structure. For nitriles and amines significant absorption of the overtones and combined frequencies is observed in the same regions where fundamental absorption bands characteristic of these compounds are located.