Force field in the methylamine molecule from ab initio MO calculation

Abstract

Ab initio SCF MO calculations on the equilibrium geometry, force constants, vibrational frequencies, and dipole moment derivatives of methylamine were carried out using the STO-3G, 4–31G, 4–31G(N ∗), and 4–31G ∗ basis sets and an energy gradient method. The results were compared with the reexamined infrared absorption spectra for CH 3NH 2, CH 3ND 2, CD 3NH 2, CD 3ND 2, CH 3NHD, and CD 3NHD. A detailed discussion was made for the off-diagonal elements of the potential energy matrix ( F matrix), including the NH 2 twisting/CH 3 rocking ( A″) element. Starting from the 4–31G ∗ F matrix, a corrected F matrix was reached, by adjusting the diagonal elements to fit the observed frequencies and by keeping the off-diagonal elements at their ab initio values. The amino twisting vibration was assigned to a weak absorption band at 1335 cm −1 of CH 3NH 2 and that at 1326 cm −1 of CD 3NH 2.