Infrared and Raman Spectra of and Isotopomers: A DFT-PT2 Anharmonic Study

Andrea Alparone

doi:10.1155/2013/741472

Abstract

IR and Raman spectra of selenophene and of its perdeuterated isotopomer have been obtained in gas phase through density-functional theory (DFT) computations. Vibrational wavenumbers have been calculated using harmonic and anharmonic second-order perturbation theory (PT2) procedures with the B3LYP method and the 6-311 basis set. Anharmonic overtones have been determined by means of the PT2 method. The introduction of anharmonic terms decreases the harmonic wavenumbers, giving a significantly better agreement with the experimental data. The most significant anharmonic effects occur for the C–H and C–D stretching modes, the observed H/D isotopic wavenumber redshifts being satisfactorily reproduced by the PT2 computations within 6–20 cm−1(1–3%). In the spectral region between 500 cm−1and 1500 cm−1, the IR spectra are dominated by the out-of-plane C–H (C–D) bending transition, whereas the Raman spectra are mainly characterized by a strong peak mainly attributed to the C=C + C–C bonds stretching vibration with the contribution of the in-plane C–H (C–D) bending deformation. The current results confirm that the PT2 approach combined with the B3LYP/6-311 level of calculation is a satisfactory choice for predicting vibrational spectra of cyclic molecules.

Highlights

Calculated harmonic vibrational wavenumbers of organic compounds typically deviate from experimental fundamental data, especially overestimating observed wavenumbers of high-energy X–H (X = C, O, N) stretches [1]
Two principal procedures can be employed in practice to correct the shortcomings of the harmonic approximation: (1) scaling methods [2, 3] and (2) anharmonic computations [4,5,6,7]
Scaling factors are usually derived for a certain level of theory and basis set by fitting computed harmonic frequencies to experimental data for restricted subsets of molecules

Summary

Introduction

Anharmonic perturbative approaches are generally proven to be less accurate than variational schemes [10], they are reliable for predicting fundamental wavenumbers of semirigid cyclic compounds [10,11,12,13,14,15,16,17]. In this contribution we report some interesting results on the performances of the anharmonic second-order perturbation theory (PT2) [6] as implemented in the GAUSSIAN 09 program [18] to predict vibrational wavenumbers. The B3LYP/6-311G∗∗ geometry agree reasonably well with experiment, with a root mean square (rms) deviation exp

Computational Details

Result and Discussion

Conclusions