CHEMICAL-REACTION DYNAMICS WHEN THE BORN-OPPENHEIMER APPROXIMATION FAILS - UNDERSTANDING WHICH CHANGES IN THE ELECTRONIC WAVE-FUNCTION MIGHT BE RESTRICTED

Whether a reaction is occurring in the gas phase or condensed phase, e lectronically nonadiabatic effects can become important if the adiabat ic reaction coordinate requires a considerable change in the electroni c wavefunction. The experiments and analysis presented here seek to ma ke progress on a difficult and important problem, that of developing a back-of-the-envelope method to predict which energetically allowed pr oducts are favoured or disfavoured when significant electronic configu ration changes are required to access one or all of the possible produ ct channels. By examining the off-diagonal matrix elements responsible for coupling electronic configurations in the initially excited molec ule with those of the products, we begin to formulate a hierarchy of w hat electronic configurations 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 hiera rchy of propensity rules for electronic configuration changes from rea ctant to products. Analysis of experimental and computational results on the competition between C-Br fission and C-Cl fission in n(O) pi(C =O) excited Br(CH2)(2)COCl and on the pi pi photofragmentation channe ls of nitric acid suggest the following. If the one-electron configura tion interaction matrix elements between the reactant electronic confi guration and a product electronic configuration are zero, then the rea ction is strongly susceptible to nonadiabatic suppression of the react ion rate and/or appearance of nonadiabatic asymptotic products. One mu st then analyze the remaining two-electron configuration interaction ( Forster- and Dexter-type) matrix elements. If the two-electron change required to couple the reactant and product electronic configurations involves simultaneous configuration changes on two spatially/electroni cally isolated functional groups, then that product channel is strongl y disfavoured. We show why this is the case by examining the two-elect ron integrals for C-Br fission in Br(CH2)(2)COCl and for the forbidden NO2(1(2)B(1)) + OH(A '') channel from pi pi excited nitric acid, com paring them to those for the NO2(1(2)B(2)) + OH(A') channel where the orbitals involved are localized on the same functional moiety. This hi erarchy in electronic coupling motivates the introduction of a 'restri cted adiabatic' correlation diagram to predict which product channels are electronically accessible. In the final section of this paper we p resent new results on the photodissociation of N,N-dimethylformamide f ollowing pi(nb)pi excitation at 193 nm, where we test the ideas devel oped from analysis of the previous work. Our measurement of the photof ragment velocity and angular distributions of the dissociation product s reveals that dissociation pathway to form HCO + N(CH3)(2) results in formation of HCO((X) over tilde) + N(CH3)(2)((A) over tilde) but not HCO((A) over tilde) + N(CH3)(2)((X) over tilde). As both are energetic ally allowed product channels in the singlet A' manifold, the selectiv ity may be analyzed with respect to the required change in electronic configuration to access each asymptotic product channel. To understand the experimental results in the context of the model developed from t he prior work, we consider both one-electron and two-electron contribu tions to the configuration interaction matrix elements between the rea ctant and product electronic configurations to determine which product channels are most likely to be accessed.