Photochemistry calculations

In this chapter, we look at the techniques known as direct, or on-the-fly, molecular dynamics and their application to non-adiabatic processes in photochemistry. In contrast to standard techniques that require a predefined potential energy surface (PES) over which the nuclei move, the PES is provided here by explicit evaluation of the electronic wave function for the states of interest. This makes the method very general and powerful, particularly for the study of polyatomic systems where the calculation of a multidimensional potential function is an impossible task. For a recent review of standard non-adiabatic dynamics methods using analytical PES functions see [1]. 

For this reason, there has been much work on empirical potentials suitable for use on a wide range of systems. These take a sensible functional form with parameters fitted to reproduce available data. Many different potentials, known as molecular mechanics (MM) potentials, have been developed for ground-state organic and biochemical systems [58-60], They have the advantages of simplicity, and are transferable between systems, but do suffer firom inaccuracies and rigidity—no reactions are possible. Schemes have been developed to correct for these deficiencies. The empirical valence bond (EVB) method of Warshel [61,62], and the molecular mechanics-valence bond (MMVB) of Bemardi et al. [63,64] try to extend MM to include excited-state effects and reactions. The MMVB Hamiltonian is parameterized against CASSCF calculations, and is thus particularly suited to photochemistry. 

By its nature, the application of direct dynamics requires a detailed knowledge of both molecular dynamics and quantum chemistry. This chapter is aimed more at the quantum chemist who would like to use dynamical methods to expand the tools at theh disposal for the study of photochemistry, rather than at the dynamicist who would like to learn some quantum chemishy. It hies therefore to introduce the concepts and problems of dynamics simulations, shessing that one cannot strictly think of a molecule moving along a trajectory even though this is what is being calculated. 

To use direct dynamics for the study of non-adiabatic systems it is necessary to be able to efficiently and accurately calculate electronic wave functions for excited states. In recent years, density functional theory (DFT) has been gaining ground over traditional Hartree-Fock based SCF calculations for the treatment of the ground state of large molecules. Recent advances mean that so-called time-dependent DFT methods are now also being applied to excited states. Even so, at present, the best general methods for the treatment of the photochemistry of polyatomic organic molecules are MCSCF methods, of which the CASSCF method is particularly powerful. 

The present high cost of full CASSCF direct dynamics means that it is not possible to use such calculations to run large numbers of trajectories. As a result it cannot be used to build up experience of the types of effects to be found in dynamical studies of organic photochemistry, and in their interpretation. This problem can be remedied by performing calculations using the MMVB force field [63,64]. 

Quantum chemical methods, exemplified by CASSCF and other MCSCF methods, have now evolved to an extent where it is possible to routinely treat accurately the excited electronic states of molecules containing a number of atoms. Mixed nuclear dynamics, such as swarm of trajectory based surface hopping or Ehrenfest dynamics, or the Gaussian wavepacket based multiple spawning method, use an approximate representation of the nuclear wavepacket based on classical trajectories. They are thus able to use the infoiination from quantum chemistry calculations required for the propagation of the nuclei in the form of forces. These methods seem able to reproduce, at least qualitatively, the dynamics of non-adiabatic systems. Test calculations have now been run using duect dynamics, and these show that even a small number of trajectories is able to produce useful mechanistic infomiation about the photochemistry of a system. In some cases it is even possible to extract some quantitative information. 

Being able to ntn direct dynamics calculations will add an extra, important, tool to help chemists understand photochemical systems. This chapter has outlined the present standpoint of the theory and practice of such calculations showing that, although much work remains to be done, they are already bringing new insight to mechanistic studies of photochemistry. 

Arising from studies of the photochemistry of benzenesulfonyl systems, extensive ab initio MO calculations have been made for various sulfonyl radicals and related species253. The STO-3G basis set, which includes d-type polarization functions on second-row atoms, was used. The inclusion of d orbitals on sulfur was found to be very... 

Dicarbonyls. A third area of uncertainty is the treatment of dicarbonyls formed from aromatic or terpene hydrocarbon oxidation. (The simplest is glyoxal, CHOCHO, but a large number have been identified, 47. The yields and subsequent reactions of these compounds represent a major area of uncertainty in urban air photochemistry (186) and since they may be a significant source of HOjj through photolysis, inaccuracies in their portrayal may result in errors in calculated values of HO. and HO2.. 

The assignment of the TR spectra were based on the known photochemistry of the aryl azides and comparison of the TR spectra vibrational frequencies to those predicted by density functional theory calculations for the likely photochemical intermediates. The good agreement between the experimental TR vibrational... 

In a series of studies of the spectroscopy and photochemistry of nickel(O) -a-diimine complexes, the structural differences among the complexes NiL2 and Ni(CO)2L (L Q-diimine) have been examined by means of molecular orbital calculations and electronic absorption Raman resonance studies.2471, 472 Summing up earlier work, the noninnocence of a-diimine ligands with a flat — N=C—C=N— skeleton in low-valent Ni chemistry and the course of substitution reactions of Ni° complexes with 1,4-diaza-1,3-dienes or a,a -bipyridine have been reviewed.2473... 

Molecular orbital calculations have been used to analyze the stabilities of dihydropterins <06H1705>. The photochemistry of 6-(hydromethyl)pterin in aqueous solution has been investigated <06HCA1090>. 

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