Intermediate state energies, molecular construction

As an example of why linear interpolation is not always a useful way to initialize an NEB calculation, consider the molecule HCN in the gas phase. This molecule can rearrange to form CNH. Optimize the structures of HCN and CNH, then use these states to examine the bond lengths in the structures that are defined by linear interpolation between these two structures. Why are the intermediate structures defined by this procedure not chemically reasonable Construct a series of initial images that are chemically reasonable and use them in an NEB calculation to estimate the activation energy for this molecular isomerization reaction. 

A more complete analysis of interacting molecules would examine all the involved molecular orbitals in a similar way. A correlation diagram would be constructed to determine which reactant orbital is transformed into which product orbital. Reactions which permit smooth transformation of the reactant orbitals to product orbitals without the intervention of high-energy transition states or intermediate can be identified in this way. If no such transformation is possible, a much higher activation energy is likely since the absence of a smooth transformation implies that bonds must be broken before they can be reformed. This treatment is more complete than the frontier orbital treatment since it focuses attention not only on the reactant but also on the product. We will describe such analysis in detail in Chapter 10. The PMO approach which has been described here is a useful and simple way to apply MO theory to reactivity problems in a qualitative way and we will employ it in subsequent chapters to problems in reactivity which are best described in MO terms. 

Three modem developments have been produced in the last years that are the key for the comprehension of the photophysics and photochemistry of many chemical and biochemical phenomena (1) rapid advances in quantum-chemical methods allow to study the excited states with high accuracy (2) improved molecular beams techniques permit studies of isolated molecules, despite their sometimes low vapor pressme and propensity for thermal decomposition, and (3) the revolutionary impact that femtosecond laser and multiphoton techniques have had on the study of the electronic energy relaxation processes. Indeed, now it is possible to get information about reaction intermediates at very short times from femtochemical techniques, and, more than ever, the participation of quantum chemistry to interpret such findings has become crucial. A constructive interplay between theory and experiment can provide an insight into the chemistry of the electronic state that cannot be easily derived from the observed spectra alone. 

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