Chemical reaction dynamics when the Born – Oppenheimer approximation fails

Whether a reaction is occurring in the gas phase or condensed phase, electronically nonadiabatic e†ects can become important if the adiabatic reaction coordinate requires a considerable change in the electronic wavefunction. The experiments and analysis presented here seek to make progress on a difficult and important problem, that of developing a back-ofthe-envelope method to predict which energetically allowed products are favoured or disfavoured when signiÐcant electronic conÐguration changes are required to access one or all of the possible product channels. By examining the o†-diagonal matrix elements responsible for coupling electronic conÐgurations in the initially excited molecule with those of the products, we begin to formulate a hierarchy of what electronic conÐgurations are strongly vs. weakly coupled. Hence, the paper focusses on understanding how an electronic wavefunction is most likely to change during a chemical reaction when it cannot adjust adiabatically during the nuclear dynamics. We begin by analyzing the results of two prior series of experiments in order to develop a hierarchy of propensity rules for electronic conÐguration changes from reactant to products. Analysis of experimental and computational results on the competition between CwBr Ðssion and CwCl Ðssion in excited and on the pp* photofragmentation chanO C/O 22 nels of nitric acid suggest the following. If the one-electron conÐguration interaction matrix elements between the reactant electronic conÐguration and a product electronic conÐguration are zero, then the reaction is strongly susceptible to nonadiabatic suppression of the reaction rate and/or appearance of nonadiabatic asymptotic products. One must then analyze the remaining two-electron conÐguration interaction (FoŽ rsterand Dexter-type) matrix elements. If the two-electron change required to couple the reactant and product electronic conÐgurations involves simultaneous conÐguration changes on two spatially/electronically isolated functional groups, then that product channel is strongly disfavoured. We show why this is the case by examining the two-electron integrals for CwBr Ðssion in 22 and for the forbidden channel from pp* excited nitric NO2(12B1)] OH(AA) acid, comparing them to those for the channel where NO2(12B2) ] OH(A@) the orbitals involved are localized on the same functional moiety. This hierarchy in electronic coupling motivates the introduction of a “restricted adiabatic Ï correlation diagram to predict which product channels are electronically accessible. In the Ðnal section of this paper we present new results on the photodissociation of N,N-dimethylformamide following excitation at 193 nb nm, where we test the ideas developed from analysis of the previous work. Our measurement of the photofragment velocity and angular distributions