How far do electrons move? A semiempirical investigation of thermal electron-transfer distances in cationic bis(hydrarine) and bis(hydrazyl) mixed-valence compounds
Rc. Johnson et Jt. Hupp, How far do electrons move? A semiempirical investigation of thermal electron-transfer distances in cationic bis(hydrarine) and bis(hydrazyl) mixed-valence compounds, J AM CHEM S, 123(9), 2001, pp. 2053-2057
A computational approach for estimating thermal electron-transfer reaction
distances in symmetrical mixed-valence compounds is described and applied t
o a series of bis(hydrazine) and bis(hydrazyl) radical cations and derivati
ves, some of which have been investigated experimentally by Nelsen and co-w
orkers. Ground-state semiempirical charge distributions are obtained by usi
ng optimized reactant geometries. Advantage is then taken of the approximat
e C-2 symmetry, or the approximate mirror symmetry, of each of the targeted
compounds, and the inherent degeneracy of the corresponding electron-trans
fer reactions, such that the change in dipole moment (Delta mu) upon charge
transfer can be estimated from an appropriately distance-weighted sum of c
harge differences between approximately symmetry-equivalent atoms found on
the donor and acceptor sides of the molecule. Delta mu can then be related
directly to the effective one-electron-transfer distance. We find that calc
ulated adiabatic electron-transfer distances can differ appreciably from th
e geometric donor-site/acceptor-site separation distances. Furthermore, for
a fixed geometric separation distance, the effective electron-transfer dis
tance can vary considerably, depending on chemical substituent composition
and/or isomeric configuration. Further advantage is taken of the approximat
e donor-site/acceptor-site symmetry, in the context of a Newton-Cave type a
nalysis, to establish the relative importance of electronic delocalization
effects versus self-polarization and inductive effects in diminishing or en
hancing effective one-electron-transfer distances.