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

Relativistic ring currents in metallabenzenes:an analysis in terms of contributions of localised orbitals

Ring currents calculated in the ipsocentric CTOCD-DZ formalism are presented for four representative metallabenzenes, compounds in which a benzene CH group is formally replaced by a transition metal atom with ligands. Aromaticity is probed using ring currents computed using non-relativistic and relativistic orbitals (derived with relativistic effective core potentials or ZORA). Maps computed at different levels of relativistic theory turn out to be similar, showing that orbital nodal character is the main determinant of ring current. Diatropic/paratropic global ring currents in these compounds, and also circulations localised on the metal centre, are interpreted in terms of contributions of localised pi-type orbitals and metal d-orbitals, respectively. All four considered metallabenzenes should be regarded as 6 pi electron species, despite the fact that three support diatropic ('aromatic') ring currents and one a paratropic ('anti-aromatic') current. The current-density maps determine the correct way to count electrons in these species: differential occupation of d-orbitals of formal pi-symmetry contributes to circulation on the metal centre, but not around the benzenoid ring. The overall trend from strongly diatropic to weakly paratropic ring currents along the series 1 to 4 is explained by the increasing strength of interaction between formally non-bonding orbitals on the metal centre and C5H5 moiety, which together make up the six-membered ring