Remco W. A. Havenith

Unconventional ring currents in an 'all-metal aromatic', Al42-

Abstract The square planar anion Al 4 2− supports an unconventional diamagnetic ring current which originates in the σ system, has negligible contribution from the two-electron π system, and survives in pyramidal bimetallic clusters MAl 4 − (M=Li, Na, Cu).

Ring current and electron delocalisation in an all-metal cluster, Al42−

Abstract Localised-orbital analysis of the current density induced by a perpendicular magnetic field in square-planar Al 4 2− demonstrates the intrinsic non-localisability of the σ electrons of this metallic cluster and confirms their dominant role in its diamagnetic ring current. Though delocalised, the π electrons do not give rise to a significant ring current.

Aromaticity and antiaromaticity of LixAl4 clusters: Ring current patterns versus electron counting

Maps of magnetic-field induced current density are computed for a series of lithium–aluminium clusters based on the planar Al4 cycle: formal 2π systems LiAl4− (C4v), Li2Al4 (D4h, Cs), and formal 4π systems Li3Al4− (Cs), and Li4Al4 (C2h). All four species sustain a diatropic σ ring current in the Al4 cycle. In the 2π systems, although the 4n + 2 π electron count suggests aromaticity and hence diatropicity, the π orbital is magnetically inactive, as in the Al42− dianion. However, in the 4π formally antiaromatic systems, the π-like HOMO supports an additional paratropic current. Considerations of orbital symmetry and energy, but not electron counting alone, rationalise both computed currents. All calculations were carried out at the coupled Hartree–Fock level in a 6-31G** basis using the CTOCD-DZ (continuous transformation of origin of current density – diamagnetic zero), or ipsocentric, formulation of magnetic response, where current density at any point is obtained with that point itself chosen as the origin of vector potential.

Calibration of the n-electron valence state perturbation theory approach.

Extensive tests have been performed to benchmark and to compare with second-order perturbation theory based on a complete active space self-consistent field reference function (CASPT2), the recently developed n-electron valence state perturbation theory at second order (NEVPT2). Test calculations included the group fifteen diatomic molecules X(2) (X=N, P, As, and Sb) and the (4)S/(2)D and (4)S/(2)P splittings for the corresponding atoms, the (1)A(1)-(3)B(1) splittings for CH(2) and SiH(2), and the absorption spectra of pyrrole and of Cu(Imidazole)(2)(SH)(SH(2))(+), which is a model for plastocyanin. Comparisons with full configuration-interaction calculations and experimental data show that the accuracy of NEVPT2 is in most cases even better than CASPT2. Where intruder states hamper the CASPT2 calculations, NEVPT2 performs significantly better. Care is needed in the choice of active orbitals, for example in the calculation of the (4)S/(2)D and (4)S/(2)P splittings for the group fifteen atoms. This is due to the different treatment of orbitals belonging to the inactive or active spaces, making the NEVPT2 not invariant for the choice of active space, even in cases where the multiconfiguration self-consistent field energy is invariant.

Ring Currents in a Proposed System Containing Planar Hexacoordinate Carbon, CB62−

Current-density maps at the coupled Hartree-Fock level calculated in the CTOCD (continuous transformation of origin of current density) approach demonstrate the magnetic response of the hypothetical planar hexacoordinate carbon species, CB(2-)(6). In contrast with the empty CB(2-)(6) framework, which supports paramagnetic currents, the carbon-containing species has a typical diamagnetic pi-ring current that circulates undisturbed by the central atom. In spite of the unconventional nature of the species, the properties of 6pi CB(2-)(6) and 4pi CB(2-)(6) can be rationalised with the same orbital model that accounts for the diamagnetic pi current of benzene and the paramagnetic pi current of planar cyclooctatetraene.

Local and Global Paratropic and Diatropic Ring Currents in Pyrene and Its Cyclopenta‐Fused Congeners

An ab initio distributed-origin coupled Hartree−Fock method has been used to compute the σ-, π- and total (σ+π)-current density maps of a family of polycyclic aromatic hydrocarbons comprising pyrene (1) and its externally cyclopenta-fused congeners 2−8. The number and the distribution of pentagons strongly affect the local patterns of induced currents, and global aromaticity is reduced as more pentagons are added to the pyrene perimeter. The pentagons in 2 and 3 have only minor effects on the pyrene core and contain cyclopenteno double bonds. In the case of the other species 4−8, in marked contrast, intense paratropic currents in the pentagons result in stepwise destruction of the 14π pyrene perimeter motif and the breaking up of the currents into single-ring contributions. The visualisation adds detail to the interpretation of Nucleus-Independent Chemical Shift (NICS) values, and the pattern of paratropic contributions can in fact, with the exception of 6 and 7, be explained qualitatively by simple Huckel−London π-electron theory.

Localisation and reversal of paratropic ring currents in molecules with formal anti-aromatic electron counts

The aromatic character of the six smallest members of the α,ω-bicyclopentadiene-polyacene series (1(n)) has been assessed using magnetic and energetic criteria. Current density maps (CTOCD-DZ/6-31G**//B3LYP/6-31G**) show that, along the series, the ring current changes from paratropic in pentalene (1(0)) and s-indacene (1(1)), through quenched in 1(2) and 1(3), to diatropic in 1(4) and 1(5). These changes are rationalised in terms of orbital contributions to current. Valence bond calculations (VB/6-31G//B3LYP/6-31G**) in which the π system is composed of strictly atomic p-orbitals, show that the electronic structures in these homologous series can be described in terms of the two closed-shell Kekule resonance structures for the smallest molecules, but that in the larger molecules allyl-polyacene-allyl biradical structures prevail, owing to the larger resonance energies. This trend in electronic structure parallels the switch from paratropic to diatropic character.

Ring currents that survive bond alternation in constrained 8π and 6π monocycles

Abstract Current density maps are computed at the ipsocentric CTOCD-DZ/6-31G ** //RHF/6-31G ** level for angle-constrained planar 1,3,5,7-cyclooctatetraenes (COT) and benzene. Constraint of α (CCH) angles to 90° in D 4 h COT ( 3b ) leads to endo - 4 and exo - 4 valence isomers. The exo structure, with CH bonds perpendicular to long sides of the octagon, is lower in energy by 202 kJ/mol and has stronger bond alternation (Δ R 0.265 A). However, endo - 4 , exo - 4 and COT 3b at its best planar geometry all sustain intense paratropic ring currents, attributed to the rotational character of the HOMO–LUMO transition, and consistent, mutatis mutandis , with the diatropic current in D 3 h benzene 5 .

Survival and extinction of delocalised ring currents in clamped benzenes

Direct visualisation of induced current density in clamped benzenes 1-4 distinguishes between saturated clamping groups, for which the central benzene ring retains a conventional diamagnetic ring current, and strongly interacting, unsaturated clamps, for which the central ring supports only the localised circulations expected of a 1,3,5-cyclohexatriene with fully fixed double bonds.

Paratropic ring currents in cubane

Abstract Visualisation, at the ab initio CTOCD-DZ/RHF/6-31G ** level, of current density induced in the cubane molecule, C 8 H 8 , by a magnetic field along a fourfold axis shows a pair of strong face-localised paratropic ring currents in this saturated system that account for the strong deshielding effects at both face and cube centres predicted in previous calculations.