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On the Influence of Non-Fundamental Modes on Mid-Infrared Spectra. Anharmonic DFT Study of Aliphatic Ethers.

Research paper by Krzysztof Bernard KB Beć, Justyna J Grabska, Yukihiro Y Ozaki, Jerzy Piotr JP Hawranek, Christian W CW Huck

Indexed on: 01 Feb '17Published on: 01 Feb '17Published in: Journal of Physical Chemistry A



Abstract

Fundamental and non-fundamental vibrational modes, first overtones and binary combination modes, of selected aliphatic ethers (di-n-propylether, di-iso-propylether, n-butylmethyl ether, n-butylethyl ether, di-n-butyl ether, tert-buytlmethyl ether and tert-amylmethyl ether), were modeled in a fully anharmonic generalized second-order vibrational perturbation theory (GVPT2) approach on DFT-B2PLYP/SNST level. The modeling procedure of theoretical line shapes took into account conformational isomerisms of studied molecules. The calculated spectra of the above ethers were compared with corresponding experimental spectra in the infrared (IR) region (4000-560 cm(-1)) of absorption index k(ν) derived from the neat liquid thin-film transmission data. It was found that IR spectra of aliphatic ethers are heavily influenced by the bands originating from non-fundamental modes, particularly from the combination modes in the C-H stretching region (3200-2800 cm(-1)). Due to effects of vibrational resonances, the intensities of overtones and combination bands originating from methyl and methylene deformation modes increase sufficiently to influence the experimental line shape in this region. Less significant contributions from non-fundamental vibrational modes were noticed in the lower IR region (1600-560 cm(-1)), particularly in the vicinity of C-O stretching band. The region of 2700-1600 cm(-1), which is rich of weak bands due to non-fundamental vibrations, was reproduced accurately as well. It was concluded that a fully anharmonic approach allows significantly more accurate reproduction of the complex IR line shapes, particularly in the C-H stretching region of aliphatic ethers. Based on the achieved agreement between the experimental and calculated spectra it may be concluded, that the anharmonic GVPT2 method can adequately reproduce the anharmonic effects and vibrational resonances in particular, influencing IR spectra of aliphatic ethers. The results obtained in this study show that the non-fundamental modes may play a significant role in shaping the IR spectra of aliphatic ethers and similar molecules in neat liquid phase.