What Woodward and Hoffmann didn't tell us: the failure of the Born–Oppenheimer approximation in competing reaction pathways

Abstract

The experiments presented here identify a class of organic reactions, allowed by overall electronic symmetry but Woodward–Hoffmann forbidden, in which the failure of the Born–Oppenheimer approximation results in a marked change in the expected branching between energetically allowed chemical bond fission channels. We first review crossed laser-molecular beam experiments on the competition between photodissociation pathways in bromoacetyl and bromopropionyl chloride at 248 nm and bromoacetone at 308 nm. In the competition between C—Cl and C—Br fission in Br(CH₂)_nCOCl, the barrier to C—Br fission on the lowest ¹A″ potential-energy surface is formed from a weakly avoided electronic configuration crossing, so that non-adiabatic recrossing of the barrier dramatically reduces the branching to C—Br fission. The experimental results and supporting ab initio calculations investigate the strong intramolecular distance dependence of the electronic configuration interaction matrix elements which split the adiabats at the barrier to C—Br fission. 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), π*(CO)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the C—C fission reaction coordinate. The paper finishes by presenting new experiments on the photodissociation of chloroacetone at 308 nm which test the conclusions of the earlier work. Photofragment 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), π*(CO)] excitation. We investigate two contributing factors to understand the difference in branching. Ab initio calculations show that the splitting at the avoided crossing between the n_oπ^* _CO and the n_pClσ^* _C—Cl configurations which forms the barrier to C—Cl fission is smaller, on average, in trans-chloroacetone than in acetyl chloride, so the rate constant for C—Cl fission is more suppressed by non-adiabtic recrossing of the reaction barrier. In addition, C—C fission can proceed more adiabatically from the gauche conformer of chloroacetone than from near-planar geometries in acetyl chloride owing to a conformation dependence of non-adiabatic recrossing near the conical intersection. A final measurement of the conformation population dependence of the branching investigates the second contributing factor.