Sigma-antiaromaticity in cyclobutane, cubane, and other molecules with saturated four-membered rings.

Dissected nucleus-independent chemical shift (NICS) analyses of cycloalkanes and cage hydrocarbons reveal contrasting ring current effects, diatropic in three- and five-membered and paratropic in four-membered ring systems. The large shielding effects of the C-C bonds of the archetypal sigma-aromatic, cyclopropane, are magnified in tetrahedrane and related structures. The remarkable deshielding effect of the cyclobutane C-C(sigma) bonds is general: cubane and cages with four-membered rings are strongly deshielding (i.e., sigma-antiaromatic).[structure--see text]