Singlet-triplet promotion energy

The energies of the singlet and triplet states of the simple molecules CH2 [8, 9] and SiH2 [10, 11] are well established from both theoretical and experimental studies. The singlet-triplet promotion energy, s-t> is +8.7 for CH2 and -21 kcal mol" for SiH2, a difference of-30 kcal mol". ... 

The promotion gap is determined by two basic excitations. For -electron pairs, the promotion energy is k-fold singlet—triplet excitation of the A—A dimer, while for mixed-valence cases where the number of electrons exceeds the number of centers (e.g., H3 ) and vice versa, the promotion energy is a charge-transfer excitation, (e.g., from Hr to H2 in H i ). Let us now use the promotion gap quantity to pattern the data concerning delocalized species. 

Each formulation of the state R has its own advantages (6,9), Equation 6.6 has the merit of simplicity and is easiest to apply when several bonds are broken—made in a reaction. On the other hand, Equation 6.5, which is more faithful to the sense of the VB correlation, should be preferred when subtle effects are searched for (see Exercises 6.5—6.7). What is essential for the moment is that both expressions use a gap that is either the singlet-to-triplet excitation of the bond that is broken during the reaction, or the same quantity scaled by approximately a constant 0.75. As mentioned above, a useful way of understanding this gap is as a promotion energy that is required in order to enable the A—Y bond to be broken and be replaced by another bond, X—A. 

Scheme 6.3 applies these rules by showing the HL structures for two cycloaddition reactions 11R and 11P are the structures for the reactants and products of the Woodward-Hoffmann forbidden 2 + 2 reaction, while 12R and 12P are the structures for the Woodward-Hoffmann allowed Diels—Alder reaction. In both cases, the difference in the HL structure is only the mode of spin coupling, and therefore the promotion energy G will involve only singlet-triplet excitations. In accord, we drew in Fig. 6.5 the corresponding... 

The barriers for a series of radicals have been computed (22), and were found to increase as the R H bond energy D increases the barrier is the largest for R = CF13 and the smallest for R = C(CH3)3. This trend has been interpreted by Pross et al. (23) using the VBSCD model. The promotion gap G that is the origin of the barrier involves the singlet-triplet excitation of the... 

For a silylene to have a triplet ground state, the one-electron promotion energy attendant upon moving one of the lone-pair electrons of the singlet to the p-orbital must be offset by the decrease in the repulsion upon separation of the nonbonding electrons. For the majority of carbenes. 

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