Competing Bond Fission and Molecular Elimination Channels in the Photodissociation of CH3NH2 at 222 nm

Abstract

This paper presents the first experimental investigation under collisionless conditions of the competing photodissociation channels of methylamine excited in the first ultraviolet absorption band. Measurement of the nascent photofragments' velocity distributions and preliminary measurements of some photofragments' angular distributions evidence four significant dissociation channels at 222 nm: N-H, C-N, and C-H bond fission and H[sub 2] elimination. The data, taken on photofragments from both methylamine and methylamine-d[sub 2], elucidate the mechanism for each competing reaction. Measurement of the emission spectrum of methylamine excited at 222 nm gives complementary information, evidencing a progression in the amino wag (or inversion) and combination bands with one quantum in the methyl (umbrella) deformation or with two quanta in the amino torsion vibration. The emission spectrum reflects the forces in the Franck-Condon region which move the molecule toward a ciscoid geometry. The photofragment kinetic energy distributions measured for CH[sub 3]ND[sub 2] show that hydrogen elimination occurs via a four-center transition state to produce HD and partitions considerable energy to relative product translation. In general, the photofragment angular distributions were anisotropic, but the measured $Beta $APEQ -0.4$POM 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation. 38 refs.,more » 13 figs.« less