1,721,029 research outputs found
Ipsocentric and allocentric methods of mapping induced current density
No full textDistributed-origin strategies for calculating magnetic response in terms of locally paramagnetic (DZ) or diamagnetic (PZ) induced currents are compared in pure and in modified forms that push the origin towards nearby nuclei (DZ2, PZ2). For visualisation of ring-currents, ipsocentric DZ has practical and conceptual advantages, with interpretation in terms of well-defined orbital contributions, whereas for nuclear shieldings allocentric PZ2 has greater accuracy. Maps are presented for diatropic, paratropic and localised systems: benzene, planarised cyclooctatetraene, borazine, clamped benzenes. All variants give indistinguishable π-maps, but differ in the σ plane, near heavy nuclei. Mixed ipsocentric-mapping/allocentric-property-calculation combines efficiency and insight into magnetic aromaticity. © 2004 Elsevier B.V. All rights reserved
Counter-rotating spin-polarised ring currents in odd-electron carbocycles
No full textWe propose a molecular-orbital model to explain how majority and minority spins in odd-π-electron carbocycles sustain counter-rotating magnetic-field-induced ring currents. The model is based on the ipsocentric approach to magnetic response, in which ring currents are dominated by frontier-orbital contributions obeying angular-momentum selection rules. Coupled unrestricted Hartree-Fock ab initio calculations of the ring-current responses for singly charged benzene and planarised cyclo-octatetraene ions confirm the predictions of the qualitative model, and are consistent with correlated MP2 spin-polarised current calculations. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Frontier-orbital ring currents and the annulene analogy
No full textPerimeter and ring currents induced in planar monocycles and poly cycles by perpendicular magnetic fields are commonly taken as diagnostic of aromaticity and antiaromaticity in these systems. Diatropic π currents are associated with aromaticity, paratropic with its opposite. The ipsocentric method is an accurate and economical way of calculating ab initio current-density maps. In π systems it provides a natural interpretation of ring currents in terms of nonredundant orbital contributions, governed by simple symmetry rules for π-π* virtual excitations and dominated by the frontier electrons. Thus, in planar [4n + 2] monocycles, the product of π HOMO and π* LUMO symmetries includes that of the in-plane translations and leads to a 4-electron diatropic current. In planar [4n] monocycles, this product includes the symmetry of the in-plane rotation and leads to a 2-electron paratropic current. Perturbation arguments based on the monocycle explain the opposite senses of the π ring currents in naphthalene (diatropic) and pentalene (paratropic) as consequences of their different frontier-orbital symmetry products. In a generalization to heterocycles, ring current maps for benzotriazole (BtH), its conjugate base (Bt-) and the cation (Bt +) are calculated at an ab initio level. The diatropic current of the two 10π systems and the paratropic current of the 8π system are rationalized in terms of the perturbed-annulene orbital model, giving an explanation of the applicability of simple electron counting in these cases: where the frontier-orbital structure remains close to that of the [4n+2]/[4n] annulene, so does the current-density map
Ring-current aromaticity in open-shell systems
No full textThe ab initio ipsocentric approach to calculation and mapping of induced orbital current density is extended to open-shell π systems. ROHF/6-31G** calculations document the (magnetic) aromaticity of planar 4nπ triplet states of 4π/8π annulenes and isobenzofulvene, and quintet excited-state azulene. An orbital model for currents rationalises the generalised form of Baird's rules, by which 4n + 2 annulenes are aromatic (antiaromatic) in states of even (odd) total spin (vice versa for 4nπ annulenes). In contrast to geometric criteria, ring-currents predict antiaromaticity for the doublet naphthalene radical anion. © 2007 Elsevier B.V. All rights reserved
Non-linear ring currents: Effect of strong magnetic fields on π-electron circulation
No full textFinite-field calculations of non-linear induced current density in representative [4n + 2] and [4n] π systems, benzene and flat cyclooctatetraene (COT), show that a strong uniform perpendicular magnetic field enhances benzene π-diatropicity and decreases COT π-paratropicity. The non-linear current is stronger by two orders of magnitude in COT, but still small: at 1a 0 height and in a field of 25 T, third-order effects contribute -5 ppm to the first-order π ring current. Classical arguments based on radial contraction of charge density rationalise the non-linear response of benzene, but that of COT depends on a specific orbital-rotation effect, characteristic of paratropic π systems. © 2004 Elsevier B.V. All rights reserved
Aromaticity, polarisability and ring current
No full textElectric dipole polarisability has been proposed as an alternative to magnetic criteria of aromaticity. The ipsocentric formulation of the molecular response to a magnetic field gives selection rules for diatropic ring current that coincide with those for in-plane polarisability. Thus, induced π current strength might correlate, if at all, with HOMO contributions to the in-plane components of polarisability of diatropic systems. No equivalent correlation is expected a priori for anti-aromatic paratropic systems, or clamped systems in which diatropic current is quenched by interaction with π systems. Ab initio calculations of π current density maps and dipole polarisability contributions confirm the expected correlation for monocycles, but show the limitations of polarisability as a measure of aromaticity. © 2003 Elsevier B.V. All rights reserved
Visualising aromaticity of bowl-shaped molecules
No full textSuperposition of slices (planar maps of induced current density calculated within the ipsocentric pseudo-π model of electronic response to a magnetic field) gives a simple route to visual diagnosis of ring-current aromaticity in bowl-shaped molecules. Results are presented for currents in the recently synthesized indenocorannulene precursors of [60]fullerene. © the Owner Societies 2011
Magnetic properties of C-60 calculated by continuous transformation of the origin of the current density
No full textThe strongly diamagnetic character of an isolated molecule of C-60, predicted by several theoretical treatments, is confirmed by a gauge-origin-independent calculation, performed within the CTOCD method at the Hartree-Fock level of approximation and using a large properties-oriented basis set
Ring currents in the porphyrins: π shielding, delocalisation pathways and the central cation
No full textIt is shown that the ipsocentric orbital-based model explains how the charge of the central cation drives the delocalisation pathway in metalloporphyrins. A positive charge +Ze at the centre of the porphin ring gives rise to a two-way radial transfer of charge within the π structure of the porphin macrocycle. This manifests itself in a change of pathway of the global π current, as Z increases from Z = 0, from an inner- through a bifurcated- to an outer-pathway. Changes of pathway can be interpreted in terms of a specific π shielding effect whereby electrons in high-lying π orbitals are screened from the central charge by the electrons in lower-lying orbitals of the same symmetry. These changes in π structure are essentially independent of accompanying changes in the a structure. © The Royal Society of Chemistry 2005
Electric-field perturbations of ring currents in π systems
No full textIpsocentric distributed-gauge calculations are used to map differential ring currents induced in [4n+2] and [4n] π systems by simultaneous application of an axial magnetic and electric field (field gradient). In benzene, electric fields and gradients produce stratified first-order π ring currents, opposing/reinforcing the intrinsic diatropic current, according to the height profile of charge depletion/enhancement induced by the electric perturbation. In (planarised) cyclooctatetraene, action of a uniform electric field is similarly rationalised, but response to the (totally symmetric) field gradient is dominated by a specific orbital effect, closing/widening of the small HOMO-LUMO gap leading to reinforcement/reduction of the intrinsic paratropic current. © 2005 Elsevier B.V. All rights reserved
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