Structural studies of nonclassical cyclobutylmethyl cations by the ab initio method

Ab initio calculations at the MP2/cc-pVTZ level show that the cyclobutylmethyl cation is a nonclassical sigma-delocalized species, which is distinct from the global minimum C2-symmetric cyclopentyl cation (Schleyer, P. v. R.; Carneiro, J. W. de M.; Koch, W.; Raghavachari, K. J. Am. Chem. Soc. 1989, 111, 5475). Relatively lower level DFT calculations, on the other hand, show that the primary cyclobutylmethyl cation spontaneously collapses into the cyclopentyl cation (Prakash, G. K. S.; Reddy, V. P.; Rasul, G.; Casanova, J.; Olah, G. A. J. Am. Chem. Soc. 1998, 120, 13362). The secondary 1-cyclobutylethyl cation is also a nonclassical carbocation, as shown by calculations at the MP2/cc-pVTZ level. Two structures having energy minima are identified for the latter cation on the potential energy surface. The conformer in which the methyl group is in the exo orientation is a global minimum and is favored over the corresponding endo conformer by 1.2 kcal/mol at the MP2/cc-pVTZ//MP2/cc-pVTZ +ZPE level of calculations. The tertiary 1-cyclobutyl-1-methylethyl cation, at this level of calculations, also involves substantial nonclassical sigma-delocalization, showing that the nonclassical stabilization is more important for cyclobutylmethyl cations relative to the cyclopropylmethyl cations. The 13C NMR chemical shifts obtained from GIAO-CCSD(T)/tzp/dz calculations further substantiate the nonclassical structures for these carbocations.