How much anharmonicity is in vibrational spectra of CH3I and CD3I?

Swapnil Singh; Mirosław Antoni Czarnecki

doi:10.1016/j.saa.2020.119176

How much anharmonicity is in vibrational spectra of CH₃I and CD₃I?

Spectrochim Acta A Mol Biomol Spectrosc. 2021 Mar 5:248:119176. doi: 10.1016/j.saa.2020.119176. Epub 2020 Nov 16.

Authors

Swapnil Singh¹, Mirosław Antoni Czarnecki²

Affiliations

¹ Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland.
² Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland. Electronic address: miroslaw.czarnecki@chem.uni.wroc.pl.

PMID: 33257247
DOI: 10.1016/j.saa.2020.119176

Abstract

This work presents new experimental and theoretical insights on vibrational spectra of CH₃I and CD₃I in the liquid phase. For the first time, we provided the contributions from different vibrational modes to mid-infrared (MIR) and near-infrared (NIR) spectra and estimated the extent of anharmonicity in the MIR region. Direct comparison of the intensities from ATR-IR and NIR transmission spectra was possible due to normalization of ATR-IR spectra. As a reference for normalization, we applied the area of the ν_s(CH₃)/ν_s(CD₃) band recorded in transmission mode. Our results show that the corresponding vibrational modes of CH₃I and CD₃I have similar contributions to the total intensity (MIR + NIR), however, these contributions are distributed in a different way between MIR and NIR regions. As expected, most of intensity in MIR spectra originates from the fundamental transitions (>90%). The fundamental bands together with the first overtones and the binary combinations contribute to more than 99% of MIR intensity for both compounds. Therefore, reliable reconstruction of MIR spectra can be achieved by considering only these vibrational modes. On the other hand, accurate simulation of NIR spectra requires including the higher-order transitions. In the case of CD₃I, the fourth-order transitions contribute to 12.7% of NIR intensity. The contributions from NIR region are significantly smaller than those from MIR range and were estimated to be 6.7% for CH₃I and 2.3% for CD₃I. The theoretical calculations provide a reasonable estimation of the total contribution from the fundamental bands. Yet, the calculated contributions from the anharmonic transitions are different from those obtained from the experimental data. MIR spectra of CH₃I and CD₃I reveal an unexpected increase in the intensity of some overtones and combination bands indicating the presence of Fermi resonances. These resonances are responsible for differences in contributions from the first overtones and binary combinations between CH₃I and CD₃I.

Keywords: Anharmonic calculations; Anharmonicity; Band assignments; CD(3)I; CH(3)I; Mid-infrared (MIR); Near-infrared (NIR); Vibrational intensities; Vibrational spectroscopy.