Ab Initio Study of the Large Amplitude Motions of Various Monosubstituted Isotopologues of Methylamine (CH3-NH2)

11 pags., 4 figs., 6 tabs.CCSD(T)-F12 theory is applied to determine electronic ground state spectroscopic parameters of various isotopologues of methylamine (CH-NH) containing cosmological abundant elements, such as D, C and N. Special attention is given to the far infrared region. The studied isotopologues can be classified in the G, G and G molecular symmetry groups. The rotational and centrifugal distortion constants and the anharmonic fundamentals are determined using second order perturbation theory. Fermi displacements of the vibrational bands are predicted. The low vibrational energy levels corresponding to the large amplitude motions are determine variationally using a flexible three-dimensional model depending on the NH bending and wagging and the CH torsional coordinates. The model has been defined assuming that, in the amine group, the bending and the wagging modes interact strongly. The vibrational levels split into six components corresponding to the six minima of the potential energy surface. The accuracy of the kinetic energy parameters has an important effect on the energies. Strong interactions among the large amplitude motions are observed. Isotopic effects are relevant for the deuterated species.The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding the research through the Research Group Project No. RG-333. This research was supported by the Ministerio de Ciencia, Innovación y Universidades of Spain through the grants EIN 2019-103072 and FIS 2016-76418-P. The author acknowledges the “Red Española de Computación for the grants AECT-2020-2-0008 and RES-AECT-2020-3-001