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.jpg): Failed to open stream: No such file or directory in C:\Inetpub\vhosts\kidney.de\httpdocs\pget.php on line 117 J+Chem+Phys
2015 ; 143
(19
): 194101
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Nonadiabatic rate constants for proton transfer and proton-coupled electron
transfer reactions in solution: Effects of quadratic term in the vibronic
coupling expansion
#MMPMID26590521
Soudackov AV
; Hammes-Schiffer S
J Chem Phys
2015[Nov]; 143
(19
): 194101
PMID26590521
show ga
Rate constant expressions for vibronically nonadiabatic proton transfer and
proton-coupled electron transfer reactions are presented and analyzed. The
regimes covered include electronically adiabatic and nonadiabatic reactions, as
well as high-frequency and low-frequency proton donor-acceptor vibrational modes.
These rate constants differ from previous rate constants derived with the
cumulant expansion approach in that the logarithmic expansion of the vibronic
coupling in terms of the proton donor-acceptor distance includes a quadratic as
well as a linear term. The analysis illustrates that inclusion of this quadratic
term in the framework of the cumulant expansion framework may significantly
impact the rate constants at high temperatures for proton transfer interfaces
with soft proton donor-acceptor modes that are associated with small force
constants and weak hydrogen bonds. The effects of the quadratic term may also
become significant in these regimes when using the vibronic coupling expansion in
conjunction with a thermal averaging procedure for calculating the rate constant.
In this case, however, the expansion of the coupling can be avoided entirely by
calculating the couplings explicitly for the range of proton donor-acceptor
distances sampled. The effects of the quadratic term for weak hydrogen-bonding
systems are less significant for more physically realistic models that prevent
the sampling of unphysical short proton donor-acceptor distances. Additionally,
the rigorous relation between the cumulant expansion and thermal averaging
approaches is clarified. In particular, the cumulant expansion rate constant
includes effects from dynamical interference between the proton donor-acceptor
and solvent motions and becomes equivalent to the thermally averaged rate
constant when these dynamical effects are neglected. This analysis identifies the
regimes in which each rate constant expression is valid and thus will be
important for future applications to proton transfer and proton-coupled electron
transfer in chemical and biological processes.