Analysis of the vibration-rotation IR spectrum of the HCl molecule

In this work, the harmonic and anharmonic vibrational frequencies of the HCl molecule havebeen determined by using the density functional theory method. Calculations have been performedat the B3LYP/6-311++G(3dp,3df) levels of theory. Transitions between energy levelshave been analyzed in terms of wavenumber. The rotational constants of the HCl molecule, thewavenumbers 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 have been calculated,and vibrational-rotational spectra have been simulated. It has been theoretically shownthat when the distance between atomic nuclei in the HCl molecule increases compared to thesteady state the R branch in the spectrum has been formed, and when this distance decreasesthe P branch has been formed.