A comparison between the construction of symmetry-correlation diagrams and the perturbation method for studying chemical reactions is carried out. The perturbation method consists of decomposing the system Hamiltonian H into a sum, H = Ho + H'. Various symmetry correlation hchemes appearing in the literature may be explained by the nonuniqueness of the decomposition scheme. All symmetry selection rules may be viewed as the varieties. By examining the symmetrycorrelation diagrams, processes under investigation may be called "forbidden" or "allowed," depending on the topological feature. Of particular importance is the topology associated with the "avoided crossing." By making the comparison, we can establish the correspondence of the two methods and conclude that the perturbation order furnishes the origin of the "forbiddenness" of ;I process.A powerful tool for a qualitative study of molecular structure and chemical reactivity problems in quantum chemistry is the method of correlation diagrams. , and many others are responsible for the development of this branch of theory. The method involves the examination of symmetry and energy levels of the electronic wave function before and after a nuclear geometric change. Certain information about the intermediate nuclear configuration may be obtained by extrapolation. It has been found very successful in codifying and rationalizing a great amount of experimental data pertaining to the electronic behavior of many systems.Another method commonly used in studying many electronic processes, including radiationless transitions, photochemical reactions, and chemical reactions in general, is the perturbation expansion approach. It is well known that a particular term may have a nonzero contribution to the full perturbation expansion only if a certain symmetry requirement is satisfied. W e thus have a reason to expect a relation between perturbation theory and the orbital correlation diagram method. We expect also that a survey of this relationship must be useful for a better understanding of the various electronic processes. Tfiis constitutes the main theme of this report.We first assume that both the reactant and the product system are stable molecular species allowing to be conveniently described by the so-called crude Born-Oppenheimer (CBO) approximated wave functions. In general, $'h (CBO) = (e0.q) x h t (Q).( 1 )Iiitcriid~ioniil .lourn31 o f Q u a n t u m Chcrnim). Vol. X V I I I . 79-81 (10x0) c I 9 X O b) .John Wile) & Sons, Inc.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.