Aromatic pathways in conjugated rings connected by single bonds

Magnetically induced current densities and molecular structures for molecules consisting of two conjugated rings formally connected by a single bond have been studied at correlated ab initio and density functional theory levels. The molecular structures and magnetically induced current densities were calculated for the lowest singlet and triplet states of biphenyl, bicyclobutadiene, phenyl-cyclobutadiene, and crosslinked phenol-imidazole, which is a model system for the crosslinked histidine-tyrosine found in heme-copper oxidases. Gauge-origin independent current densities were obtained by using the gauge-including magnetically induced current method. The lowest triplet states have a significantly shorter bond between the conjugated rings and generally a more planar structure when compared with the singlet ground state. The shorter bond distance between the conjugated rings indicates a π-electron delocalization between the rings. Current-density calculations show that the singlet states sustain ring currents mainly in the individual rings. The calculations also suggest that the shorter crosslink between the rings in the triplet states is accompanied with an increased current delocalization over the connected rings.