ON the accuracy of ab initio CALCULATIONs of ABSOLUTE band INTENSITies in the IR SPECTRa of molecules
A. I. Pavlyuchko, E. V. Vasiliev, L. A. Gribov
Keywords: organic compounds, IR spectra, absolute intensities, quantum chemical calculations
Pages: 278-284
Abstract
Absolute intensities are calculated in the harmonic approximation for the IR spectrum of 18 hydrocarbons, oxygen- and nitrogen-containing organic compounds. The quantum chemical calculation is carried out in the 6-311G(3df,3pd) basis set. The calculated data are compared to the experimental values of absolute absorption intensities. It is found that calculations in the HF approximation substantially overestimate (on average by 87%) the integral absolute intensity of fundamental vibrations in the 575-4000 cm-1 range. Most part of this overestimation falls on the stretching vibrations with large amplitudes (C-H and C=O bonds). When the MP2 method is used without electron correlation treatment, this overestimation decreases to 22% and becomes more uniform in the whole spectral region.
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