Issue 5, 2016

Analytic calculations of anharmonic infrared and Raman vibrational spectra

Abstract

Using a recently developed recursive scheme for the calculation of high-order geometric derivatives of frequency-dependent molecular properties [Ringholm et al., J. Comp. Chem., 2014, 35, 622], we present the first analytic calculations of anharmonic infrared (IR) and Raman spectra including anharmonicity both in the vibrational frequencies and in the IR and Raman intensities. In the case of anharmonic corrections to the Raman intensities, this involves the calculation of fifth-order energy derivatives—that is, the third-order geometric derivatives of the frequency-dependent polarizability. The approach is applicable to both Hartree–Fock and Kohn–Sham density functional theory. Using generalized vibrational perturbation theory to second order, we have calculated the anharmonic infrared and Raman spectra of the non- and partially deuterated isotopomers of nitromethane, where the inclusion of anharmonic effects introduces combination and overtone bands that are observed in the experimental spectra. For the major features of the spectra, the inclusion of anharmonicities in the calculation of the vibrational frequencies is more important than anharmonic effects in the calculated infrared and Raman intensities. Using methanimine as a trial system, we demonstrate that the analytic approach avoids errors in the calculated spectra that may arise if numerical differentiation schemes are used.

Graphical abstract: Analytic calculations of anharmonic infrared and Raman vibrational spectra

Article information

Article type
Paper
Submitted
02 Nov 2015
Accepted
05 Jan 2016
First published
07 Jan 2016
This article is Open Access
Creative Commons BY license

Phys. Chem. Chem. Phys., 2016,18, 4201-4215

Author version available

Analytic calculations of anharmonic infrared and Raman vibrational spectra

Y. Cornaton, M. Ringholm, O. Louant and K. Ruud, Phys. Chem. Chem. Phys., 2016, 18, 4201 DOI: 10.1039/C5CP06657C

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