Covalent-ionic state mixing

The SCD involves diabatic state curves of mixed character. Sometimes, however, the mixed character of the curves will conceal important information which can be revealed by looking at the VB configurations individually. Figures 5(a) versus 5(b) shows the HL and triple ionic structures for Sn2 reactions on carbon versus silicon. It is seen that the Si case is favored over the C case by having a much more stable triple ionic structure as well as a larger covalent-ionic VB mixing. Consequently, the Sn2(C) is typified by a TS and a barrier, while the Sn2(S1) case by a stable pentacoordinated species, (SiHsXa) . 

One way to distinguish between ionic and covalent character of the bonding in a compound is to examine the decomposed DOS. If the bonding is strongly covalent, the states from different atoms are strongly mixed and one would expect the decomposed DOS to be very similar on all sites. If the bonding is primarily ionic the decomposed DOS would be very dissimilar on sites with different atoms. To deduce the amount of hybridization in the DOS we decomposed the DOS into site contributions. 

These results can be understood using a simple model where the accessed state is a mixed state resulting from the avoided crossing of the covalent and ionic states, the covalent one bearing the oscillator strength. The simulation shown in Figure 4-3 results from the interaction of one weakly bound van der Waals potential and the ionic Hg+-Cl2 curve. 

The asymmetric reconstruction, the buckling of the ionic state Contrary to the covalent state, the ionic component should be stabilized by an asymmetric distortion if this one accommodates a positive charge on one side and a negative charge on the other one. The asymmetry allows to mix the first excited state (antisymmetric and ionic) with the upper component of the doubly excited state (symmetric and ionic) and thus to localize the electrons on a single-atom dimer. 

Upon introduction of the acceptor substituents, the above-mentioned terms lie in polar solvents in close proximity and can even cross if the wave function employed is either purely ionic or purely covalent. Such crossing can be avoided by mixing the covalent and the ionic states. 

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