WHAT WOODWARD AND HOFFMANN DIDNT TELL US - THE FAILURE OF THE BORN-OPPENHEIMER APPROXIMATION IN COMPETING REACTION PATHWAYS

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.