CHEMICAL-REACTION DYNAMICS WHEN THE BORN-OPPENHEIMER APPROXIMATION FAILS - UNDERSTANDING WHICH CHANGES IN THE ELECTRONIC WAVE-FUNCTION MIGHT BE RESTRICTED
Nr. Forde et al., CHEMICAL-REACTION DYNAMICS WHEN THE BORN-OPPENHEIMER APPROXIMATION FAILS - UNDERSTANDING WHICH CHANGES IN THE ELECTRONIC WAVE-FUNCTION MIGHT BE RESTRICTED, Faraday discussions, (108), 1997, pp. 221-242
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.