The diatropic (sigma) ring currents of [(pi)2s+(pi)2s+(pi)2s] pericyclic transition states

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