The diatropic σ ring currents of [π2s + π2s + π2s] pericyclic transition states

Abstract

Ab initio current-density maps for the (4n + 2)-electron transition states of two thermally allowed [_π2_s + _π2_s + _π2_s] trimerisations, of ethyne to benzene (1), and of ethene to cyclohexane (2), show that both support σ diatropic ring currents, dominated by four-electron σ → σ* virtual excitation of HOMO electrons.