Spectroscopy and dynamics of methylamine. I. Rotational and vibrational structures of CH3NH2 and CH3ND2 in (A)over-tilde states

Abstract

Rovibrational structures of methylamines (CH3NH2 and CH3ND2) in predissociative A states (3s-n) are investigated using (1+1) resonant-enhanced two-photon ionization (R2PI) spectroscopy. A part of experimental results was briefly reported earlier [J. Chem. Phys. 117, 10057 (2002)], and full detailed results and analyses are given here. Spectral origins are determined to be 41 669 and 42 038 cm-1 for CH3NH2 and CH3ND2, respectively. Amino wagging and CH3 rocking modes are optically active, giving their respective fundamental frequencies of 636 (487) and 1008 (1012) cm(-1) for CH3NH2 (CH3ND2). The CH3 moiety is found to rotate nearly freely about the C-N axis with respect to the amino group with an accurately determined torsional barrier of 5.0+/-0.5 cm(-1) at the zero-point level of CH3ND2((A) over tilde). The torsional barrier increases to 19.0+/-0.5 cm(-1) at the v (ND2-wag)=1 level due to wagging-torsional mode coupling. Both internal and overall rotational fine structures are clearly resolved for the first few vibrational levels of CH3ND2((A) over tilde), providing accurate values of vibrational frequencies and associated internal and overall rotational constants. Broad spectral features of the CH3NH2 excitation spectrum are unambiguously assigned by using the internal rotor Hamiltonian established in the analysis of the CH3ND2 excitation spectrum. Linewidths of spectral bands provide lifetimes of corresponding quantum states excited at particular rovibrational levels, giving, for example, similar to8.8 and similar to1.8 ps for zero-point and v (ND2-wag)=2 levels of the CH3ND2 ((A) over tilde) state, respectively. The lifetime of CH3NH2((A) over tilde) is estimated to be much shorter, giving tausimilar to0.38 ps at the origin band. The large H/D isotope effect in lifetimes of excited states indicates that the primary dissociation channel is the N-H(D) bond dissociation and it proceeds via tunneling through a reaction barrier. Lifetimes are found to be mode specific, showing the experimental fact that energy deposition to a certain vibrational mode, which is perpendicular to the reaction coordinate, may modify the reaction barrier along the N-H(D) reaction coordinate. Ab initio results for structures and vibrational frequencies of methylamines at excited states are compared with the experiment. (C) 2003 American Institute of Physics.