Potential energy surfaces photochemistry

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]. 

Conical intersections are involved in other types of chemistry in addition to photochemistry. Photochemical reactions are nonadiabatic because they involve at least two potential energy surfaces, and decay from the excited state to the ground state takes place as shown, for example, in Figure 9.2a. However, there are also other types of nonadiabatic chemistry, which start on the ground state, followed by an ex-cnrsion npward onto the excited state (Fig. 9.2b). Electron transfer problems belong to this class of nonadiabatic chemistry, and we have documented conical intersection... 

Bernardi F, Olivucci M, Robb MA (1996) Potential energy surface crossings in organic photochemistry. Chem Soc Rev 25 321... 

Traditional Sequence of Differently Bonded Intermediates. Organic chemists have traditionally considered a reaction mechanism, in its most primitive form, to consist of a sequence of differently bonded intermediates on the path between starting materials and products. In these terms, a mechanism may be considered understood once these chemically distinct species have been correctly identified. For purposes of understanding reaction rates and stereochemistry, it is necessary to expand this set of metastable reaction intermediates to include transition structures at the saddle points between intermediates on a potential energy surface. For photochemistry one must also consider transitions between potential energy surfaces. 

F, Bemardi, M. Olivucci, andM. A. Robb, Chem. Soc. Rev., 25,321 (1996).Potential Energy Surface Crossings in Organic Photochemistry. 

M. Garavelli, F. Bernardi, M. Olivucci, T. Vreven, S. Klein, P. Celani, and M. A. Robb, Faraday Discuss., 110, 51 (1998). Potential-Energy Surfaces for Ultrafast Photochemistry Static and Dynamic Aspects. 

I trust that this book gives the flavor of the pace, excitement, and accomplishments of the last few years of cluster research. For me, the most surprising and important feature of this volume is the breadth that this new area of physical chemistry demonstrates. The various experimental chapters cover ionic chemistry, hot atom chemistry, photochemistry, neutral molecule chemistry, electron and proton transfer chemistry, chemistry of radicals and other transient species, and vibrational dynamics and cluster dissociation. Of at least equal importance is that theoretical potential energy surface studies are not accessible for cluster systems and are being pursued. All of us associated with this project have tried to convey the fresh insights and contributions that van der Waals cluster research has brought to physical chemistry. 

Thiophosgene is one of the simplest and best-studied prototype systems for which accurate potential energy surfaces can be obtained through experiment and high-level ab initio calculations. As such, thiophosgene is a molecule tailor-made for the fundamental understanding of electronic relaxation in polyatomic molecules. The results summarized in this chapter confirm the special role thiophosgene plays in the field of molecular photophysics and photochemistry. 

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