Investigating the threshold of aromaticity and antiaromaticity by variation of nuclear charge

Aromatic benzene and nonaromatic borazine can be modeled as the end-points of a continuous process evolving through intermediate systems with fractional nuclear charges. Current-density maps show a smooth, linear progression in which the global diatropic π ring current weakens as localized diatropic lone-pair currents grow with the increase in charge difference. In contrast, the progression from antiaromatic (planarized) cyclooctatetraene to nonaromatic borazocine shows an initially persistent paratropic ring current with a sharper transition to the localized diatropic system. The different behaviors of aromatic and antiaromatic systems stem from the different orbital origins of diatropic and paratropic ring current, and both can be rationalized in terms of arguments based on π distortivity and electronegativity.