Ra. Marcus, THEORY OF RATES OF S(N)2 REACTIONS AND RELATION TO THOSE OF OUTER-SPHERE BOND RUPTURE ELECTRON TRANSFERS, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(22), 1997, pp. 4072-4087
A model is considered for S(N)2 reactions, based on two interacting st
ates. Relevant bond energies, standard electrode potentials, solvent c
ontributions (nonequilibrium polarization), and steric effects are inc
luded. A unified approach is introduced in which there can be a flux d
ensity for crossing the transition state, which is either bimodal, one
part leading to S(N)2 and the other to ET products, of unimodal with
a less marked energy-dependent separation of the rates of formation of
these products. in a unified description an expression is given for t
he reorganization energy, which reduces in the appropriate limits to t
he pure S(N)2 and ET/bond rupture cases. Expressions are obtained for
the S(N)2 rate constant and for its relation to that of the concerted
electron transfer/bond rupture reaction. Applications of the theory ar
e made to the cross-relation between rate constants of cross and ident
ity reactions, experimental entropies and energies of activation, the
relative rates of S(N)2 and ET reactions, and the possible expediting
of an outer sphere ET reaction by an incipient S(N)2-type interaction.
Results on the photoelectron emission threshold energies of ions in s
olution provide some information on a solvation term, and another quan
tity can be estimated using data from gas phase S(N)2 reactions or fro
m quantum chemistry calculations. Also introduced for comparison is an
adiabatic model that is an extension of a bond energy-bond order form
ulation for gas phase reactions.