A Computational Strategy for Organic Photochemistry

Abstract

INTRODUCTION Modeling Photochemical ReactionsThe aim of this chapter is to provide an introduction to the practical computational investigation of photochemical reaction mechanisms. During the last decade or so, the speed of computers has grown considerably, and now the computational investigation of realistic models of organic compounds is becoming a standard practice. Current applications range from the investigation of the mechanism of synthetically useful reactions to the study of shortlived organic intermediates detected in the interstellar medium. For thermal reactions, standard state-of-the-art ab initio quantum chemical methods are already capable of providing a complete description of what happens at the Reviews in Computational Chemistry, Volume 15 87 88 A Computational Strategy for Organic Photochemistry molecular level during bond-breaking and bond-forming processes. In particular, it is possible to compute the transition structure that connects a reactant to a product and the associated energy barrier with almost chemical accuracy (ca. 1 kcal/mol error). Furthermore the reaction path (i.e., the progression of the molecular structure from the reactants toward the transition state and the product) can be determined, in a completely unbiased way, by computing the minimum energy path (MEP)1 connecting the reactant to the product on the (3N -6)-dimensional potential energy surface of the system.A detailed understanding of the reaction pathway in the excited state manifold will increase our ability to design new and to control known photochemical reactions. As an example, the conversion of light into chemical energy in plants and animals involves extended conjugated moleculescarotenoids and retinals-bound in protein complexes. The use of such extended systems in optical data storage and processing technology is now being investigated. Photobiological systems exploit the ability of these chromophores to undergo cis-trans isomerization and to transduce radiative energy into thermal energy on picosecond or shorter time scales. Recent advances in timeresolved spectroscopy2 (e.g., the use of ultrafast laser pulses) have provided a powerful tool to monitor reaction dynamics on the femtosecond time scale and have made direct observation of these processes possible, increasing our understanding of the excited state structures and dynamics for model systems. UItrafast (femtosecond) radiationless decay has been observed, for example, for simple dienes,3 cyclohexadienes,4~5 and hexatrienes,6 and in both free7 and opsin-bound8 retinal protonated Schiff bases. However, a complete understanding of molecular dynamics on multiple electronic states is required to interpret these laser experiments with confidence and to understand the principles involved in the design of optical devices.Until recently, reaction path computations were mainly limited to the investigation of thermal reactions and thus to reactions occurring on a single potential energy surface. Photochemical processes, where the reactant resides on an excited ...

A survey of recent developments in ab initio valence bond theory

“…Subsequently, Michl 138 articulated this VB-based mechanism into a general photochemical model that highlighted the importance of ''funnels'' as the potential energy features that mediate the excited state species back into the ground state. Recent work by Bernardi et al 139 , Robb et al 140 showed that these ''funnels'' are conical intersections that can be predicted by simple VB arguments, and computed at a high level of sophistication. Similar applications of VB theory to deduce the structure of conical intersections in photoreactions were done by other groups.…”

Automatic generation of active coordinates for quantum dynamics calculations: Application to the dynamics of benzene photochemistry

“…This involves locating critical points on ground and excited PESs as well as any seams of conical intersections between states. 1 Mechanisms of thermal reactions are traditionally described with the concept of a static reaction coordinate along a minimum energy path. In contrast, nonadiabatic mechanisms involve ultrafast processes and depend on changes in the kinetic energy in the energized system.…”

Nonadiabatic vibrational dynamics in the HCO2−⋅H2O complex

“…[30][31][32] The very concept of a conical intersection requires an adiabatic separation of time scales of two sets of degrees of freedom, which is naturally given in the electronic case via the Born-Oppenheimer approximation, but which is less common for vibrational transitions. 18,20,22,[33][34][35][36] In the context of vibrational conical intersections, an adiabatic separation has been introduced between high-frequency OH stretch and bend vibrations on the one hand, and lower-frequency backbone vibrations 28 or torsional modes 37,38 on the other hand within one molecule.…”