Gcg. Waschewsky et al., COMPETING BOND FISSION AND MOLECULAR ELIMINATION CHANNELS IN THE PHOTODISSOCIATION OF CH3NH2 AT 222 NM, Journal of physical chemistry, 99(9), 1995, pp. 2635-2645
This paper presents the first experimental investigation under collisi
onless conditions of the competing photodissociation channels of methy
lamine excited in the first ultraviolet absorption band. Measurement o
f the nascent photofragments' velocity distributions and preliminary m
easurements of some photofragments' angular distributions evidence fou
r significant dissociation channels at 222 nm: N-H, C-N, and C-H bond
fission and H-2 elimination. The data, taken on photofragments from bo
th methylamine and methylamine-d(2), elucidate the mechanism for each
competing reaction. Measurement of the emission spectrum of methylamin
e excited at 222 nm gives complementary information, evidencing a prog
ression in the amino wag (or inversion) and combination bands with one
quantum in the methyl (umbrella) deformation or with two quanta in th
e amino torsion vibration. The emission spectrum reflects the forces i
n the Franck-Condon region which move the molecule toward a ciscoid ge
ometry. The photofragment kinetic energy distributions measured for CH
3ND2 show that hydrogen elimination occurs via a four-center transitio
n state to produce HD and partitions considerable energy to relative p
roduct translation. The reaction coordinates for N-H and C-N fission a
re analyzed in comparison to that for ammonia dissociation from the (A
) over tilde state and with reference to a initio calculations of cuts
along the excited state potential energy surface of methylamine which
show these reactions traverse a small barrier in the excited state fr
om a Rydberg/valence avoided crossing and then encounter a conical int
ersection in the exit channel. The measured kinetic energy distributio
n of the C-N bond fission photofragments indicates that the NH2 (ND2)
product is formed in the (A) over tilde (2)A(1) state; the C-N fission
reactive trajectories thus remain on the upper adiabat as they traver
se the conical intersection. The mechanism for C-H bond fission is les
s clear; most of the kinetic energy distribution indicates the reactio
n evolves on a potential energy surface with no barrier to the reverse
reaction, consistent with dissociation along the excited state surfac
e or upon internal conversion to the ground state, but some of the dis
tribution reflects more substantial partitioning to relative translati
on, indicating that some molecules may dissociate via a repulsive trip
let surface. In general, the photofragment angular distributions were
anisotropic, but the measured beta approximate to -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, respe
ctively, dominate.