COMPETING C-BR AND C-C BOND FISSION FOLLOWING (1)[N(O),PI-ASTERISK(C=O)] EXCITATION IN BROMOACETONE - CONFORMATION DEPENDENCE OF NONADIABATICITY AT A CONICAL INTERSECTION
Pw. Kash et al., COMPETING C-BR AND C-C BOND FISSION FOLLOWING (1)[N(O),PI-ASTERISK(C=O)] EXCITATION IN BROMOACETONE - CONFORMATION DEPENDENCE OF NONADIABATICITY AT A CONICAL INTERSECTION, The Journal of chemical physics, 100(5), 1994, pp. 3463-3475
These experiments investigate the competition between C-C and C-Br bon
d fission in bromoacetone excited in the (1)[n(O),pi()(C=O)] absorpti
on, elucidating the role of molecular conformation in influencing the
probability of adiabatically traversing the conical intersection along
the C-C fission reaction coordinate. In the first part of the paper,
measurement of the photofragment velocity and angular distributions wi
th a crossed laser-molecular beam time-of-flight technique identifies
the primary photofragmentation channels at 308 nm. The time-of-flight
spectra evidence two dissociation channels, C-Br fission and fission o
f one of the two C-C bonds, BrH2C-COCH3. The distribution of relative
kinetic energies imparted to the C-Br fission and C-C fission fragment
s show dissociation is not occurring via internal conversion to the gr
ound electronic state and allow us to identify these channels in the c
losely related systems of bromoacetyl- and bromopropionyl chloride. In
the second part of the work we focus on the marked conformation depen
dence to the branching between C-C fission and C-Br fission. Photofrag
ment angular distribution measurements show that C-Br fission occurs p
rimarily from the minor, anti, conformer, giving a beta of 0.8, so C-C
fission must dominate the competition in the gauche conformer. Noting
that the dynamics of these two bond fission pathways are expected to
be strongly influenced by nonadiabatic recrossing of the reaction barr
iers, we investigate the possible mechanisms for the conformation depe
ndence of the nonadiabatic recrossing with low-level ab initio electro
nic structure calculations on the C-Br reaction coordinate and qualita
tive consideration of the conical intersection along the C-C reaction
coordinate. The resulting model proposes that C-C bond fission,cannot
compete with C-Br fission in the anti conformer because the dissociati
on samples regions of the phase space near the conical intersection al
ong the CC fission reaction coordinate, where nonadiabaticity inhibits
C-C fission, while from the gauche conformer C-C fission can proceed
more adiabatically and dominate C-Br fission. A final experiment confi
rms that the branching ratio changes with the relative conformer popul
ations in accord with this model.