The homotropenylium cation : a system with a pinched π ring current

The homotropenylium cation (1, C8H9+) is a key species in the discussion of homoaromaticity. Constrained optimisations around the minimum structure have been performed, varying the size of the gap spanned by the CH2-bridge and optimising all other geometrical parameters. At each bridging distance, ab initio current-density maps have been calculated and plotted using the ipsocentric approach. Analysis of the maps, including decomposition into localised orbital contributions, gives a clear indication of a global diatropic ring current passing through the gap. The change in p(pi)-p(pi) interaction, from conventional p overlap around the conjugated seven-carbon perimeter to s overlap (p(sigma)-p(sigma)) in the gap, results in a distinctive pinched topology, with two streams of current pinched down into one for part of the circuit. This ring current is diatropic and therefore the species 1 is aromatic on the magnetic criterion

Aromaticity of strongly bent benzene rings:persistence of a diatropic ring current and its shielding cone in [5]paracyclophane

Direct evaluation of the induced pi current density in [5]paracyclophane (1) shows that, despite the significant non-planarity (alpha = 23.2 degrees) enforced by the pentamethylene bridge, there is only a modest (ca. 17%) reduction in the pi ring current, justifying the use of shielding-cone arguments for the assignment of H-1 NMR chemical shifts of 1 and the claim that the non-planar benzene ring in 1 retains its aromaticity (on the magnetic criterion)

The effect of syn-anti isomerism on the lowest valence transitions of 1,1′-bicyclohexylidene. An ab initio MRDCI investigation

Conformational effects on the ground-state and excited-state properties of 1,1′-bicyclohexylidene were studied. The energy difference between the two conformers anti- (anti-1a) and syn-1,1′-bicyclohexylidene (syn-1b) was determined at the RHF/6-31G, MP2/6-31G//RHF/6-31G, RHF/6-311G * * //RHF/6-31G and MP2/6-311G * * //RHF/6-31G levels of theory. Syn-1b is lower in energy by 0.051 kcal/mol at the MP2/6-311G * * //RHF/6-31G level. The valence transitions of syn-1b were calculated using the MRDCI method using its 6-31G geometry and molecular orbitals. In contrast to the predicted UV data of anti-1a for which two absorptions are found, only one absorption for syn-1b, a π→π * transition at ~6.0 eV, is discernible. The next transition with appropriate oscillator strength is at ~7.8 eV (π→σ *). Inclusion of polarization functions on the carbon atoms hardly affects the calculated transition energies, oscillator strengths and CI vectors