Torsional motions of the free and H-bonded hydroxyl groups of the catechol molecule

Torsional vibrations of free and hydrogen-bonded hydroxyl groups of the 1,2-dihydroxybenzene molecule are analyzed within the harmonic and anharmonic approximations, as well as by constructing a 2D potential energy surface formed by varying the torsional coordinates of the two O-H groups. Calculation of the vibrational spectrum of the molecule in the harmonic and anharmonic approximations was performed at the MP2/acc-pVTZ level of theory. Calculations of the 2D potential energy surface were performed at the MP2/acc-pVTZ, MP2/CBS (T,Q), and CCSD(T)/acc-pVDZ levels of theory. Calculations of the 2D surfaces of dipole moment components and kinetic coefficients were performed at the MP2/cc-pVTZ and MP2/acc-pVTZ levels of theory. In the latter case, the formalism of Wilson's (s)over-right-arrow vectors was used. The energies of the first 100 stationary torsional levels are calculated and classified by irreducible representations of the molecular symmetry group C-2V (M). Furthermore, the analysis of wave functions allowed us to assign most of the torsional states to the fundamentals, combinations, and overtones of the torsional vibrations of the hydroxyl groups of two conformers that have the point symmetry groups C-S (I) and C-2V (II). The tunneling frequencies between equivalent configurations of conformer I in the ground and excited torsional states are estimated. The calculated values of the frequencies of fundamental torsional vibrations of two O-H groups and the tunneling splitting value of the first excited torsional state of the free hydroxyl group of conformer I are compared with the experimental data presented in a recently published work [J. Bruckhuisen et al., Molecules 26 (2021) 3645]. Torsional IR spectra of the molecule are modeled at different temperatures.