Cations with pentacoordinate carbon atoms (carbonium ions) prove to be an interesting topic for debate, and although their existence is now established, they remain an interesting check on our conceptions of what constitutes a chemical bond. In this study an equilibrium between carbenium and carbonium ions (see below) was discovered and studied with B3LYP/6-31G(d) and MP2/6-31G(d) calculations.
It was discovered that, although in most cases the carbonium ion is higher in energy than the classical carbenium ion, cation 1 is actually favored by 5.7 (B3LYP/6-31G(d); 14.8, MP2) kcal/mol relative to the ring opened carbenium ion. Further, the barrier for ring closure (that is, going from the classical to the nonclassical structure) is only 1.4 (B3LYP; 0.8, MP2) kcal/mol.
Siebert, M.R. and Tantillo, D.J. Journal of Organic Chemistry, 2006, 71, 645-654. “Tetracoordinate Carbon as a Nucleophile? Interconversion of Carbenium Ions Possessing Nearly Square Pyramidal Pentacoordinate Carbons.” 10.1021/jo052053a.