Computational
Photochemistry
(Cramer, Dupuis, Garrett, Truhlar, Valiev)
Electronically nonadiabatic processes (also called non-Born-Oppenheimer processes) are those in which the dominant dynamics involves an exchange of electronic energy with energies of nuclear vibration, rotation, and translation. Since electronic energies are much greater than nuclear-motion energies, electronically activated chemical systems usually have dynamical mechanisms quite different from those of thermally activated systems. Chemical processes that involve radiationless transitions between electronic states include reactions that involve nonadiabatic recrossings due to a change in the character of the electronic wave function near the transition state, the decay of photoexcited species, charge-transfer reactions, and collisions of electronically excited species.
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There has been tremendous progress in recent years in our ability to predict the outcomes of chemical reactions, but most of this progress has been limited to non-catalytic reactions occurring in the ground electronic state. Excited-state reactions are a new frontier that is only now being opened up to detailed study. Just as for ground-state reactions, the problem breaks into two steps – prediction of the potential energy surface and calculation of the dynamics of nuclear motions. In this case, however, there is more than one potential energy surface, and one must know the terms that couple the surfaces as well as the surfaces themselves. Furthermore, the dynamics are more complicated too – one has motion on coupled potential energy surfaces rather than just motion on a single surface. There has been recent progress on both of these problems for small-molecule reactions in the gas phase.
The time is now ripe to extend
this progress to larger systems and to reactions in the condensed phase. By
taking advantage of the expertise built up at Minnesota and PNNL
in the areas of electronic structure, dynamics, and solvation, we can build a
general set of convenient tools that will be available to the community for
such calculations. At the same time, we can systematize the treatment of
excited electronic states in solution for the calculation of spectra. Fundamental
understanding of electronically excited states provided by these new methods
and computational tools will be instrumental in the design of photocatalytic
processes for advanced materials with tailored properties, the design of novel
fuels and new energetic processes, and technologies based on optical and
ultraviolet wavelengths.
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Updated: Feb. 18, 2008 |