Diradicaloid character

The nature of the transition state of nucleophilic reactions with LL [low lowest unoccupied molecular orbital (LUMO)] substrates is analyzed and reviewed. In cation-anion combination reactions, a partial radical character is developed on both the nucleophile and the substrate. Examination of a simple state diagram shows that this diradicaloid character is increased as the LUMO of the substrate is lowered. The model is further extended to other LL substrates such as carbonyl functions and activated olefins. Three empirical manifestations of the diradicaloid character of the transition state are discussed (1) the correlation between the ionization potentials of the nucleophiles and their nucleophilicity toward LL substrates (2) the a-effect phenomenon and (3) the variations in the positional selectivity of 9-nitromethylenefluorene in nucleophilic reactions as a function of the solvent. 

Because of the sharply avoided crossing in the region of the transition state for the [2, + 2 J concerted reaction and of the diradicaloid character of the critical points involved in the non-concerted process, the computation of this surface requires computational methods that transcend the SCF method. Recently this surface, and in particular the transition structure region, has been investigated in detail by ab initio molecular-orbital methods. The calculations have been performed at the MC-SCF level with minimal (STO-3G) and extended (4-31G) basis sets. The various critical points have been fully optimized with MC-SCF gradients and characterized by computing the corresponding Hessian matrices. 

One might think that an Si-Si double bond has formed but notice that the four substituents around the Si-Si double bond are in a highly pyramidal, cisold arrangement. We saw this same feature, not for Si but for the higher group 14 elements in Section 10.3.C. What this means is that the tt and tt levels are not split by much energy they have diradicaloid character, 23.71. We saw in Section 10.3.1... 

To account for these unusual results and the role of the free-radical character, a mechanism that implies that NH—CO—PTM- is a good leaving group in 8, 2 reactions has been proposed. It is based on the quantum-mechanical approach used to account for the kinetics of nucleophilic substitution in benzyl halides, which involves a diradicaloid configuration in the relevant transition state. The latter would be particularly stabilized by the radical character of the labelled glycine. 

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