Effect on Ring Current of the Kekulé Vibration in Aromatic and Antiaromatic Rings

Abstract

Derivative current-density maps are used to follow the changes in ring-current (and hence, on the magnetic criterion, the changes in aromaticity) with the Kekulé vibrations of the prototypical aromatic, antiaromatic, and nonaromatic systems of benzene, cyclooctatetraene (COT), and borazine. Maps are computed at the ipsocentric CHF/6-31G**//RHF/6-31G** level. The first-derivative map for benzene shows a growing-in of localized bond currents, and the second-derivative map shows a pure, paratropic "antiring-current", leading to the conclusion that vibrational motion along the Kekulé mode will reduce the net aromaticity of benzene, on average. For planar-constrained D(4h) COT, the Kekulé mode (positive for reduction of bond-length alternation) increases paratropicity at both first and second order, indicating an average increase in antiaromaticity with zero-point motion along this mode. On the ring-current criterion, breathing expansions of benzene and D(4h) COT reduce aromaticity and increase antiaromaticity, respectively.

Cyclic tris-[5]helicenes with single and triple twisted Möbius topologies and Möbius aromaticity

“…The C–C bonds connecting the [5]helicene units in 2 and 3 exhibit a bond length of 1.48–1.49 Å which is considerably longer than the bond length in the prototypic aromatic molecule benzene (∼1.40 Å). Upon distortion of D 6h benzene towards a D 3h 1,3,5-hexatriene structure with alternating C–C bond lengths of 1.553 Å and 1.337 Å, benzene retains 72% of its diatropic ring current 33. We conclude that a corresponding bond length elongation by 0.01 Å in 2 and 3 does not significantly reduce the ring currents.…”

1,3,2,5-Diazadiborinine featuring nucleophilic and electrophilic boron centres

“…Almost a century after the first identification of benzene by Faraday 2 , borazine (B 3 N 3 H 6 ), also referred as inorganic benzene, was prepared 3 . Despite the isoelectronic and isosterism relationships between the C=C and B–N units, however, B 3 N 3 H 6 displays a different electronic property from that of benzene 4 5 , which is due to polarization of the B–N units arising from the variation of electronegativity between boron and nitrogen atoms. Thus, introduction of B–N units into the aromatic skeleton of isoelectronic organic counterparts leads to unique electronic structures, which indicate the potential to expand the diversity of aromatic molecules 6 7 8 .…”

Inorganic Triphenylphosphine

“…both are colourless liquids under ambient conditions) but the aromaticity of borazine, as determined computationally and experimentally, is significantly lower than the aromaticity of benzene (e.g. NICS values for benzene and borazine are −11.5 and −2.1 respectively), and the dominant mechanisms by which they undergo substitution are fundamentally different: electrophilic aromatic substitution for benzene, addition‐elimination for borazine …”