Gcg. Waschewsky et al., WHAT WOODWARD AND HOFFMANN DIDNT TELL US - THE FAILURE OF THE BORN-OPPENHEIMER APPROXIMATION IN COMPETING REACTION PATHWAYS, Journal of the Chemical Society. Faraday transactions, 90(12), 1994, pp. 1581-1598
Citations number
44
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
The experiments presented here identity a class of organic reactions,
allowed by overall electronic symmetry but Woodward-Hoffmann forbidden
, in which the failure of the Born-Oppenheimer approximation results i
n a marked change in the expected branching between energetically allo
wed chemical bond fission channels. We first review crossed laser-mole
cular beam experiments on the competition between photodissociation pa
thways in bromoacetyl and bromopropionyl chloride at 248 nm and bromoa
cetone at 308 nm. In the competition between C-Cl and C-Br fission in
Br(CH2)(n)COCl, the barrier to C-Br fission on the lowest 1A'' potenti
al-energy surface is formed from a weakly avoided electronic configura
tion crossing, so that non-adiabatic recrossing of the barrier dramati
cally reduces the branching to C-Br fission. The experimental results
and supporting ab initio calculations investigate the strong intramole
cular distance dependence of the electronic configuration interaction
matrix elements which split the adiabats at the barrier to C-Br fissio
n. The second set of experiments reviewed investigates the competition
between C-C and C-Br bond fission in bromoacetone excited in the 1[n(
O), pi(C-O)] absorption, elucidating the role of molecular conformati
on in influencing the probability of adiabatically traversing the coni
cal intersection along the C-C fission reaction coordinate. The paper
finishes by presenting new experiments on the photodissociation of chl
oroacetone at 308 nm which test the conclusions of the earlier work. P
hotofragment velocity and angular distribution measurements show that
C-C fission competes with C-Cl fission in this molecule, while only C-
Cl fission occurs in acetyl chloride upon 1[n(O), pi(C-O)] excitation
. We investigate two contributing factors to understand the difference
in branching. Ab initio calculations show that the splitting at the a
voided crossing between the n(O)pi(C=O) and the n(pCl)sigma(C-Cl)* co
nfigurations which forms the barrier to C-Cl fission is smaller, on av
erage, in trans-chloroacetone than in acetyl chloride, so the rate con
stant for C-Cl fission is more suppressed by non-adiabatic recrossing
of the reaction barrier. In addition, C-C fission can proceed more adi
abatically from the gauche conformer of chloroacetone than from near-p
lanar geometries in acetyl chloride owing to a conformation dependence
of non-adiabatic recrossing near the conical intersection. A final me
asurement of the conformation population dependence of the branching i
nvestigates the second contributing factor.