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

Article abstract of DOI:10.1021/j100009a022

This paper presents the first experimental investigation under collisionless conditions of the competing photodisssociation 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 H2 elimination.The data, taken on photofragments from both methylamine and methylamine-d₂, 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 CH3ND2 show that hydrogen elimination occurs via a four-center transition state to produce HD and partitions considerable energy to relative product translation.The reaction coordinates for N-H and C-N fission are analyzed in comparison to that for ammonia dissociation from the <*> state and with reference to ab iniitio calculations of cuts along the excited state potential energy surface of methylamine which show these reactions traverse a small barrier in the excited state from a Rydberg/valence avoided crossing and then encounter a conical intersection in the exit channel.The measured kinetic energy distribution of the C-N bond fission photofragments indicates that the NH2 (ND2) product is formed in the <*> ²A₁ state; the C-N fission reactive trajectories thus remain on the upper adiabat as they traverse the conical intersection.The mechanism for C-H bond fission is less clear; most of the kinetic energy distribution indicates the reaction evolves on a potential energy surface with no barrier to the reverse reaction, consistent with dissociation along the excited state surface or upon internal conversion to the ground state, but some of the distribution reflects more substantial partitioning to relative translation, indicating that some molecules may dissociate via a repulsive triplet surface.In general, the photofragment angular distributions were anisotropic, but the measured β ca. -0.4 +/- 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation.We end with analyzing why in methylamine three other primary dissociation channels effectively compete with N-H fission while in CH3OH and CH3SH primarily O-H and S-H fission, respectively, dominate.