The large amplitude motions of methylamine from the perspective of the highly correlated ab initio methods

Abstract

CCSD(T)-F12 theory in connection with extended basis sets is employed to determine the electronic ground state spectroscopic parameters of methylamine at low temperatures. The geometry, the rotational constants, all the fundamental frequencies, the dipole moment and its components, and the centrifugal distortion constants, are provided. The ground vibrational state rotational constants were found to be A0=103067.15MHz, B0=22588.29MHz, and C0=21710.50MHz and the dipole moment to be 1.4071D. 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 three variables: the HNH bending, the NH2 wagging and the CH3 torsional coordinates. The computed levels are compared with previous experimental and calculated energies. Methylamine parameters are very sensitive to the level of ab initio calculations.