The Mechanism of Formation of the Vibrational-Rotational Spectrum of the HCL Molecule

In this work, the harmonic and anharmonic vibrational frequencies of the HCl molecule have been determined by using the density functional theory method. Calculations have been performed at the B3LYP/6-311++G (3dp, 3df) levels of theory. Transitions between energy levels are analyzed in terms of wavenumber. The rotational constants of the HCl molecule, the wavenumbers corresponding to the rotational structure of the P, R, and Q branches in the spectrum, and their relative intensities at temperatures of 100, 200, and 300 K are calculated, and vibrational-rotational spectra are simulated. The vibrational-rotational spectrum of the HCl molecule is obtained in the range of 2600-3100 cm-1. It has been theoretically shown that when the distance between atomic nuclei in the HCl molecule increases compared to the steady state, the R branch in the spectrum, and when it decreases, the P branch is formed. Rotational constants and corresponding frequencies are calculated for each fundamental transition in rotational energy levels. All calculations use empirical and non-empirical methods. For H35Cl and H37Cl molecules, graphs of the number of fundamental transitions (m) in rotational energy levels and the wave number corresponding to these transitions are drawn. It is in good agreement with all scale experiment results. It is verified that the rotational constants (B) corresponding to the general degree of degeneration (2J+1)2 are similar to the values observed in the literature.