Computational photochemistry calculations

Parallel to the developments achieved in methodology and hardware, the conventional methods and some of the new approaches have been employed to study several types of photoinduced processes which are relevant mainly in biology and nanotechnology. In particular, important contributions have been made related to the topics of photodissociations, photostability, photodimerizations, photoisomerizations, proton/hydrogen transfer, photodecarboxylations, charge transport, bioexcimers, chemiluminescence and bioluminescence. In contrast to earlier studies in the field of computational photochemistry, recent works include in many cases analyses in solution or in the natural environment (protein or DNA) of the mechanisms found in the isolated chromophores. In addition, semi-classical non-adiabatic molecular dynamics simulations have been performed in some studies to obtain dynamical attributes of the photoreactions. These latter calculations are however still not able to provide quantitative accuracy, since either the level of theory is too low or too few trajectories are generated. Within this context, theory and hardware developments aimed to decrease the time for accurate calculations of the PESs will certainly guide future achievements in the field of photodynamics. 

On-the-fly molecular dynamics have been employed in order to simulate the photochemistry of carbonyl-containing compounds. The on-the-fly mechanism implemented in the MNDO program is the velocity-Verlet algorithm. Here an additional aspect of the usage of a computational cheap semiempirical method is visible. In order to provide realistic relative yields of different photochemical reactions, a large enough sample of trajectories is needed. For these systems, a substantial amount of trajectories (around 100) has been calculated for a relatively long timescale (up to 100 ps). 

Figure 2.4 Computationally generated plots of log kEj versus AG°, where the continuous and dashed lines are calculated by using data obtained from semi-classical and quantum theories, respectively. Reprinted from K.S. Schanze and K.A. Walters, in Molecular and Supramolecular Photochemistry, Vol. 3, V. Ramamurthy and K.S. Schanze (Eds), Marcel Dekker, New York, 1998, Ch. 3, p. 80, Figure 2, by courtesy of Marcel Dekker Inc.

In many instances spectroscopic constants can now be computed to an accuracy comparable with that which they can be measured. The choice between calcula-lation and measurement then rests solely on grounds of convenience and cost. It is in the realm where the constants are needed but cannot be measured that the potential of this work lies. For full exploitation there needs to be some change of emphasis in the work of theoreticians it is ceasing to be a matter of wonder if the calculations can reproduce the experiments. It is now time to seek out the problems wherever the application of computation is going to be really beneficial, providing answers to questions for which the answer is really sought, perhaps in photochemistry or astrophysics. 

The UV resonance Raman spectrum of thymine was revisited in 2007, with a slightly different approach, by Yarasi, et al. [119]. Here, the absolute UV resonance Raman cross-sections of thymine were measured and the time-dependent theory was used to experimentally determine the excited-state structural dynamics of thymine. The results indicated that the initial excited-state structural dynamics of thymine occurred along vibrational modes that are coincident with those expected from the observed photochemistry. The similarity in a DFT calculation of the photodimer transition state structure [29] with that predicted from the UV resonance Raman cross-sections demonstrates that combining experimental and computational techniques can be a powerful approach in elucidating the total excited-state dynamics, electronic and vibrational, of complex systems. 

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