A QTAIM and Electron Delocalization Computational Study of tert-Butylmethylene, Trimethylsilylmethylene, and Trimethylgermylmethylene. A New Method for Unambiguously Characterizing the Bonding between Pairs of Atoms in Reaction Intermediates

While studies on the experimental photolytic and thermolytic extrusion of nitrogen from tert-butyldiazomethane and tert-butyldiazirine and the decomposition of other precursors have shown a mixture of C−H and C−C insertion products depending on conditions, the analogous trimethylsilyldiazomethane undergoes solely Si−C insertion. Description of the singlet tert-butylmethylene intermediates potentially involved in the C−H and C−C insertion reactions and were addressed through computational means by Armstrong et al. (J. Am. Chem. Soc. 1995, 117, 3685−3689). In addition to re-examining singlet tert-butylmethylene at a higher level of theory [CCSD/6-311+G(d,p)], we have studied the silicon and germanium analogues trimethylsilylmethylene and trimethylgermylmethylene. A computational atoms-in-molecules and atomic-basin-delocalization-indices analysis established that the singlet carbenes, while exhibiting varying degrees of delocalization, are not bridged species based on the fact that none possess a pentacoordinate methyl group. In addition, from the results, we are able to make a prediction of solely a Ge−C insertion product for the extrusion of nitrogen from trimethylgermyldiazomethane. Most importantly, we demonstrated that a combination of quantum theory of atoms in molecules (QTAIM) molecular graphs, the evaluation of delocalization indices, and a visualization of the closeness of atomic basinsa QTAIM-DI-VISAB analysisshould be considered as the method of choice for unambiguously characterizing the bonding between pairs of atoms not only of carbenes but of other reaction intermediates such as carbocations, carbanions, and radicals.