A parity rule for ring currents in π-clamped monocycles

Survival of ring currents in planar monocycles subject to [2 + 2] addition of one, two, three, etc., even-carbon polyene clamping groups is governed by a parity rule. If addition is across positions (i1,i1 + 1), (i2,i2 + 1),...,(ij,ij + 1) for clamps 1, 2,...,j, then, if any of i1,i2,...,ij is odd, current is quenched in the central ring; if all of i1,i 2,...,ij are even, the central ring current survives

Ring currents in large [4n + 2]-annulenes

Reported computational results for large [4n + 2]-annulenes indicate a falling off of aromaticity in D3h geometries but its retention in D6h geometries, as diagnosed on both energetic and magnetic criteria. Ipsocentric pseudo-π mapping of the current density induced by a perpendicular external magnetic field shows that ring currents follow this trend. Diatropic ring currents are quenched in D3h geometries but survive in D6h geometries of [4n + 2]-annulenes (4n + 2 = 30, 42, 54, 66). Ipsocentric orbital contributions explain this distinction in terms of the translational/diatropic, rotational/paratropic selection rules for current in monocycles, coupled with an account of differential angular-momentum mixing in D3h and D6h symmetries. The orbital model rationalises the differences between D6h and D3h geometries, accounts for the decay of aromaticity with ring size for D3h [4n + 2]-annulenes, and predicts trends for anti-aromatic [4n]-annulenes in the two symmetry groups

Angular momentum and spectral decomposition of ring currents: Aromaticity and the annulene model

By a widely accepted criterion, an aromatic (anti-aromatic) π-conjugated system is one that sustains a global diatropic (paratropic) ring current when perturbed by a perpendicular external magnetic field. Calculation of induced current densities by the specific distributed-gauge method known as the ipsocentric approach offers practical and conceptual advantages. Physically realistic distributions of current density are obtained with modest basis sets. In the ipsocentric choice, molecular orbital contributions are free of unphysical occupied - occupied mixing and so form the basis of an interpretation of current patterns in terms of frontier orbital symmetries, energies and nodal patterns. Selection rules for the sense of ring current in planar carbocycles can be formulated in terms of angular momentum: a diatropic (paratropic) current arises when a virtual excitation connects orbitals whose angular momentum quantum numbers differ by one (zero). Here we explore the extension of this reasoning to other systems based on the monocycle: to heterocycles and clamped systems, where the symmetry lowering can produce significant angular momentum mixing. It is shown that simple arguments can account for the existence and sense of current also in these systems, and hence for their aromaticity on the magnetic criterion. It is possible to understand why, for example, ab initio calculations show that clamping of a benzene (1) or cyclooctatetraene (2) cycle with cyclobutadieno-groups destroys the ring current of the free planar monocycle, but clamping with HB-BH or HN-NH units allows the current to persist, in planar geometries

Visualisation of counter-rotating ring currents in kekulene

Current-density maps for kekulene 1 computed using an ab initio all-electron distributed-origin method show global diamagnetic and paramagnetic circulations around outer and inner perimeters ('rim' and 'hub'), subsuming six local benzenoid diamagnetic π circulations, in qualitative agreement with the Hückel-London π model, but in contradiction of an [18]-in-[30] concentric-annulene description of 1

Current-density maps as probes of aromaticity: Global and Clar π ring currents in totally resonant polycyclic aromatic hydrocarbons

Calculation and visualisation of induced current density are important aids to the study of both aromaticity and observable molecular magnetic response properties. The ipsocentric method offers an accurate and economical approach to calculation of induced current density, and a physical interpretation in terms of occupied-orbital contributions. In monocyclic systems, these contributions allow rationalisation of the existence, sense, and strength of ring current using simple symmetry and node-counting arguments. Here we show maps computed with the model pseudo-π version of the ipsocentric theory for large polycyclic aromatic hydrocarbons (PAH) of the Clar type. Maps are presented for D6h-symmetric systems with up to 438 carbon centres; a full ab initio calculation for an isomer of C114H30 confirms that the success of the pseudo-π method for small PAH carries over to these larger systems. The computed maps follow a generic overall pattern that can be understood with a natural extension of the orbital arguments to a 'band theory' of totally resonant PAH. All show π current densities that combine a Clar-sextet structure of localised benzenoid diatropic ring currents (contributed by just four HOMO electrons) with a global perimeter diatropic ring current contributed by the remaining π electrons. Both currents are explained in the ipsocentric model: the localised currents arise from excitations from the HOMO; the perimeter current from excitations from the rest of the HOMO-band. © The Royal Society of Chemistry 2006

Which π-clamped conjugated monocycles exhibit ring currents?

Quenching/survival of ring currents in π-clamped conjugated monocycles is controlled by the match or mismatch in parity between the frontier orbitals of the central π-conjugated 4n+2/4n monocycle and those of the clamps. Changes in ring current are not primarily caused by bond alternation or 'Mills-Nixon' effects; current and geometry changes on clamping are both consequences of electronic structure

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

Direct evaluation of the induced π current density in [5]paracyclophane (1) shows that, despite the significant non-planarity (α = 23.2°) enforced by the pentamethylene bridge, there is only a modest (ca. 17%) reduction in the π ring current, justifying the use of shielding-cone arguments for the assignment of 1H NMR chemical shifts of 1 and the claim that the non-planar benzene ring in 1 retains its aromaticity (on the magnetic criterion). © 2011 The Owner Societies

Confirmation of syn-anti isomerism of 2,5-dimethoxydicyanoquinonediimine radical anions in solution

The conformational behaviour of 2,5-dimethoxydicyanoquinonediimine radical anions has been studied in polar solvents with ESR and proton ENDOR spectroscopy. Both syn and anti isomers of the radical anions have been identified. The two conformers differ in molecular symmetry and hyperfine coupling between the unpaired electron with nearby protons. Separate spectra of the conformers have been obtained with ENDOR-induced ESR. Finally, from electronic spin relaxation, the activation energy for the interconversion process between the syn and anti conformers was estimated at 28.5 kJ/mol (6.8 kcal/mol)

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