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Most chemical processes follow the Born-Oppenheimer
(adiabatic) approximation, in which the nuclei move on a single electronic
potential energy surface (PES). However there are important processes where
this approximation breaks down. These nonadiabatic events play an important
role in essential processes in nature such as photosynthesis, vision, charge
transfer and photochemistry. Nonadiabatic processes are facilitated by
the close proximity of two PES where radiationless transitions between
the surfaces can occur with higher probability. The efficiency for radiationless
transitions increases in the extreme case when two PES become degenerate
forming conical intersections. Thus many ultrafast nonradiative transitions
occur through conical intersections. Theoretical developments have enabled
the efficient study of conical intersections in small systems. The focus
of our group is to extend these studies to more complicated systems, particularly
of biological interest, in an effort to understand the underlying mechanisms
of photoinitiated nonadiabatic processes and their potential implications.
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| UV radiation and DNA |
The photophysical and photochemical behavior of the nucleobases is of
particular importance since they are the chromophores in DNA and RNA.
The excited states of the nucleobases are extremely short-lived,
having lifetimes of the order of femtoseconds or picoseconds.
This property has been associated with their photostability in ultraviolet (UV) radiation from the sun.
When UV radiation is absorbed by DNA photochemistry can occur which may lead to DNA damage and photocarcinogenesis. Efficient dissipation of the absorbed energy prevents extensive photodamage.
We are investigating the radiationless decay mechanisms that can explain
the ultrafast lifetimes and fluorescence quenching in these systems.
Two- and three-state conical intersections are present that can facilitate
nonadiabatic transitions between the different electronic states of these molecules.
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Although the majority of quantum mechanical studies on photoinitiated processes are carried out in vacuo, the incorporation of environmental perturbations is important for a more realistic modeling of these processes.
The effect of solvent in photophysics and photochemistry can be very important, and methods that account for these effects are essential. We have developed a simple scheme that incorporates the most important electrostatic solvation effects into the ab initio studies of excited states. In this scheme the solvent structure is obtained by molecular dynamics simulations using classical force fields. An average electrostatic potential is derived from these simulations where thousands of snapshots are included, and then is introduced into the quantum mechanical multireference configuration interaction (MRCI) calculations for the solute. This is necessary since an MRCI calculation at every step is impractical. The methodology has been used to examine solvatochromic shifts on vertical absorption maxima with very promising results.
It will be extended further to be used in studying the effect of the solvent not just at equilibrium but also along the PESs and on conical intersections.
We are working in collaboration with Prof. Thomas Weinacht on a project aimed at understanding the underlying mechanisms involved in laser control of basic chemical reactions, such as fragmentation and isomerization. Intense, shaped, ultrafast laser pulses are used to control molecular dynamics and high level ab initio calculations are used to interpret and guide the control. Close collaboration between theory and experiment is the major element in this project and this interaction has already lead to detailed understanding of fragmentation in substituted acetones. High-level, ab initio electronic structure calculations were used to interpret the fragmentation dynamics of CF3COCHBr2 following excitation with an intense ultrafast laser pulse. The calculations confirm the existence of a charge-transfer resonance during the evolution of a dissociative wave packet on the ground state potential energy surface of the molecular cation and yield a detailed picture of the dissociation dynamics observed experimentally. We currently focus on control over isomerization through conical intersections.