Materials of the International Seminar of the Institute of Spectroscopy of the Russian Academy of Sciences (ISAN), dedicated to the anniversary of Professor M.N. Popova
Spectroscopy and physics of atoms and molecules

Non-empirical analysis of isotopic shifts and resonance effects in the infrared high-resolution spectrum of freon-22 (CHF$_2$Cl), enriched with $^{13}$C

S. V. Krasnoshchekov^a, I. K. Gainullin^b, V. B. Laptev^c, S. A. Klimin<sup>c

a</sup> Chemical Faculty, Lomonosov Moscow State University, 119991 Moscow, Russia
^b Physical Faculty, Lomonosov Moscow State University, 119991 Moscow, Russia
^c Institute of Spectroscopy, Russian Academy of Sciences, 108840 Troitsk, Moscow, Russia

Abstract: The IR transmittance spectrum of an isotopic mixture of chlorodifluoromethane (CHF$_2$Cl, Freon-22) with a 33% fraction of $^{13}$C and a natural ratio of chlorine isotopes was measured in the frequency range 1400–740 cm$^{-1}$ with a resolution of 0.001 cm$^{-1}$ at a temperature of 20$^\circ$C. An ab initio calculation of the structure and sextic potential energy surface and surfaces of the components of the dipole moment has been carried out by the the electronic quantum-mechanical method of Möller–Plesset, MP2/cc-pVTZ. Then the potential was optimized by replacing the harmonic frequencies with the frequencies calculated by the electronic method of coupled clusters, CCSD(T)/aug-cc-pVQZ. The fundamental and combination frequencies were calculated using the operator perturbation theory of Van Vleck (CVPT$n$) of the second and fourth order ($n$ = 2, 4). Resonance effects were modeled using an additional variational calculation in the basis up to fourfold VCI excitation (4). The average prediction error for the fundamental frequencies of the $^{12}$C isotopologues was $\sim$ 1.5 cm$^{-1}$. The achieved accuracy made it possible to reliably predict the isotopic frequency shifts of the $^{13}$C isotopologues. It is shown that the strong Fermi resonance $\nu_4/2\nu_6$ dominates in the $^{12}$C isotopologues and is practically absent in $^{13}$C. The literature assumption [Spectrochim. Acta A, 44 : 553] about the splitting of $\nu_1$ (CH) due to the resonance $\nu_1/\nu_2+\nu_7+\nu_9$ is confirmed. The coefficients of the polyadic quantum number are determined. The analysis made it possible to carry out a preliminary identification of the centers of the vibrational-rotational bands of isotopologues $^{13}$CHF$_2$ $^{35}$Cl and $^{13}$CHF$_2$ $^{37}$Cl in the spectrum of the mixture in preparation for individual analyzes of the vibrational-rotational structures of individual vibrational transitions.

Keywords: chlorodifluoromethane, high-resolution spectrum, Fermi resonance, operator perturbation theory, hybrid potential energy surface.

Received: 06.08.2021
Revised: 01.09.2021
Accepted: 15.09.2021

DOI: 10.21883/OS.2022.01.51884.27-21

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