Aromaticity and antiaromaticity in small ring transition states, assessed by NICS values and energetics

Nucleus Independent Chemical Shift (NICS) values, pioneered by Schleyer, pr ovide detailed insights into electronic structures of transition states. Th ese show that the [2+2+2]-cycloaddition transition states, studied early by Schleyer and by us, have aromatic transition structures and that fused cyc lopropanes are aromatic in the transition structure, while fused four-membe red rings are antiaromatic. The nucleophilic ring-opening of 1- and 2-cyano bicyclo[1.1.0]butanes, studied earlier by Hoz, and ring openings of cyanocy clopropane, cyanocyclobutane, and 2-cyanobicyclo[2.2.0]hexane by hydroxide were investigated at the B3LYP/6-31+G* level. Orbital interactions through bonds explain relative facilities of ring opening.