Close-lying electronic states

The higher-energy D and E states of Bz+ represent an example of simultaneous JT and PJT interactions between close-lying electronic states. From the energetic proximity (difference of vertical IPs of only — 0.4 eV) the PJT interaction can be expected to affect both states significantly. According to the symmetry selection... 

Compounds can have close-lying electronic states of different spin for several reasons. For convenience, we have divided these into three partially... 

The g-factors also point to the problem of excited state mixing, the values of gL and gs in particular being too far from the free electron values for comfort. The single state effective Hamiltonian, derived by perturbation theory, may be inadequate in molecules where there are several close-lying electronic states which are appreciably mixed. 

Diyls are short-lived intermediates and in most cases are not directly observed during cycloaddition reactions. The course of the reaction is complicated by the fact that diyls possess several close-lying electronic states which might be populated thermally singlet diradical, triplet diradical, and zwitterionic states1. These various electronic states differ in reactivity as well as in selectivity. Triplet reactions are generally slower by several orders of magnitude and are less selective. 

For an overall view of transition metal systems one has to confront a number of problems besides correlation, or sometimes as a part of it many electrons in open shells producing large number of close-lying electronic states, different spin multiplicities, magnetism, metal-metal bonds, core or " semicore electrons that occupy the same part of the space as the valence shell, fast electrons requiring relativistic corrections etc. In addition, many of transition metal systems of practical interest are highly complex and often incompletely characterized, so that careful modeling is to be added to the list of difficulties. 

Multiconhgurational methods have been particularly successful in studies of transition metal complexes, both for ground state and excited states. This is an area where the alternative methods are few. Many open shells with varying values of the spin together with many close lying electronic states (not only singly excited) makes it difficult to use simple methods like Density Functional Theory, or other methods which assume a Hartree-Fock like ground states. 

However, molecular distortion to lower symmetry can be real and, in fact, occurs quite frequently in radical ions and diradicals, as discussed in the section on problems associated with close-lying electronic states. Even in the case of allyl, where higher level calculations and experiments both show that molecular distortion of the C2. equilibrium geometry to Q is an artifact of ROHE calculations, antisymmetric distortion of the C—C bonds will invariably occur for sufficiently long C—C bond lengths. It is obviously more favorable to dissociate allyl radical into a vinyl radical plus methylene than into two methylenes and a CH radical. 

PROBLEMS ASSOCIATED WITH CLOSE-LYING ELECTRONIC STATES... 

Problems Associated with Close-Lying Electronic States 45... 

Problems Associated tvith Close-Lying Electronic States 47... 

Figure 14 Spin-orbit splitting in C2H for the lowest vibronic levels (indicated by points) between 0 and 5000 cm" which can be looked upon as mixing between unperturbed vibrational levels of three close-lying electronic states with different characteristics, (x) Measured data...

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