DISTANCE DEPENDENCE OF NONADIABATICITY IN THE BRANCHING BETWEEN C-BR AND C-CL BOND FISSION FOLLOWING 1[N(O),PI-ASTERISK(C=O)] EXCITATION INBROMOPROPIONYL CHLORIDE
Pw. Kash et al., DISTANCE DEPENDENCE OF NONADIABATICITY IN THE BRANCHING BETWEEN C-BR AND C-CL BOND FISSION FOLLOWING 1[N(O),PI-ASTERISK(C=O)] EXCITATION INBROMOPROPIONYL CHLORIDE, The Journal of chemical physics, 99(6), 1993, pp. 4479-4494
These experiments on bromopropionyl chloride investigate a system in w
hich the barrier to C-Br fission on the lowest 1A'' potential energy s
urface is formed from a weakly avoided electronic configuration crossi
ng, so that nonadiabatic recrossing of the barrier to C-Br fission dra
matically reduces the branching to C-Br fission. The results, when com
pared with earlier branching ratio measurements on bromoacetyl chlorid
e, show that the additional intervening CH2 spacer in bromopropionyl c
hloride reduces the splitting between the adiabatic potential energy s
urfaces at the barrier to C-Br fission, further suppressing C-Br fissi
on by over an order of magnitude. The experiment measures the photofra
gment velocity and angular distributions from the 248 nm photodissocia
tion of Br (CH2)2COCl, determining the branching ratio between the com
peting primary C-Br and C-Cl fission pathways and detecting a minor C-
C bond fission pathway. While the primary C-Cl:C-Br fission branching
ratio is 1:2, the distribution of relative kinetic energies impar-ted
to the C-Br fission fragments show that essentially no C-Br fission re
sults from promoting the molecule to the lowest 1A'' potential energy
surface via the 1[n(O),pi(C-O)] transition; C-Br fission only results
from an overlapping electronic transition. The results differ markedl
y from the predictions of statistical transition state theories which
rely on the Born-Oppenheimer approximation. While such models predict
that, given comparable preexponential factors, the reaction pathway wi
th the lowest energetic barrier on the 1A'' surface, C-Br fission, sho
uld dominate, the experimental measurements show C-Cl bond fission dom
inates by a ratio of C-Cl:C-Br=1.0: <0.05 upon excitation of the 1[n(O
),pi(C=O)] transition. We compare this result to earlier work on brom
oacetyl chloride, which evidences a less dramatic reduction in the C-B
r fission pathway (C-Cl:C-Br = 1.0:0.4) upon excitation of the same tr
ansition. We discuss a model in which increasing the distance between
the C-Br and C=O chromophores decreases the electronic configuration i
nteraction matrix elements which mix and split the 1n(O)pi(C=O) and n
p(Br)sigma(C-Br) configurations at the barrier to C-Br bond fission i
n bromopropionyl chloride. The smaller splitting between the adiabats
at the barrier to C-Br fission increases the probability of nonadiabat
ic recrossing of the barrier, nearly completely suppressing C-Br bond
fission in bromopropionyl chloride. Preliminary ah initio calculations
of the adiabatic barrier heights and the electronic configuration int
eraction matrix elements which split the adiabats at the barrier to C-
Br and C-Cl fission in both bromopropionyl chloride and bromoacetyl ch
loride support the interpretation of the experimental results. We end
by identifying a class of reactions, those allowed by overall electron
ic symmetry but Woodward-Hoffmann forbidden, in which nonadiabatic rec
rossing of the reaction barrier should markedly reduce the rate consta
nt, both for ground state and excited state surfaces.