Tables of quantum chemical data. XIII. Energy characteristics of some benzoderivatives of fulvene and heptafulvene

Titz, M.; Hochmann, P.

doi:10.1135/cccc19673028

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“…Recent calculations by Ottosson and co-workers found the triplet state of 3 ( n = 1) to be above the singlet by 12 (uOLYP and CASPT2) to 18 kcal/mol (CASSCF) . Larger members of the 3 series or their derivatives have appeared only in computational surveys. ,− …”

Section: Introductionmentioning

confidence: 99%

“…The greater aromaticity gain on going from 6a to 6b should make its biradical electronic states even more favorable than those of 5 ( n = 2). These hydrocarbons have appeared previously only as part of computational surveys, ,, however, tropone analogues of 5 ( n = 1 and 2) and of 6 have been trapped or observed spectroscopically under matrix isolation conditions. − These tropones displayed chemical and spectroscopic behavior consistent with biradicaloids perturbed by a contribution from an aromatic zwitterionic resonance structure; neither experimental nor theoretical attention appears to have been given to the triplet states.…”

Section: Introductionmentioning

confidence: 99%

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Rational Design of High-Spin Biradicaloids in the Isobenzofulvene and Isobenzoheptafulvene Series

Snyder

2012

J. Phys. Chem. A

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The relative energies of singlet biradicaloid and of triplet and singlet biradical electronic states for a series of benzannelated isobenzofulvenes and isobenzoheptafulvenes were calculated at the (u-)B3LYP/6-31G(d), full π-space CASSCF-CASPT2 (≤14 π-e(-)s), and full π-space RASSCF-RASPT2 (≤24 π-e(-)s) levels of theory. Both absolute and relative CASPT2 energies were reproduced quite well by the RASPT2 approach, which can be extended to much larger active spaces. RASPT2 (and DFT) calculations find that increasing benzannelation leads to triplet ground states in both hydrocarbon series, in violation of the classical principle of maximum bonding. This confirmed the expectations that the combined effects of resonance energy and aromaticity could compensate for the extra formal π-bond of the biradicaloid singlet, and that the strong exchange coupling inherent to the embedded trimethylenemethane (TMM) would manifest itself in the biradicaloids. The relative energy of the biradicaloid singlet rises rapidly upon benzannelation, as π-bonding between the high-energy delocalized GVB orbitals decreases. The underlying π-orbital topology is revealed when this weak π-bonding is artificially eliminated by a 1:1 mixing of the nondegenerate HOMO and LUMO to produce an overcorrelated valence bond (OCVB) orbital pair. For members of both biradicaloid series, the OCVB pairs are nondisjoint, revealing a limiting triplet preference with increasing benzannelation. Within the two-electron, two-orbital approximation, the effects of π-bonding in the singlet biradicaloids and orbital localization away from the acene π-system in the triplet biradicals can be analyzed as perturbations of the singlet OCVB biradicals. The application of a VB-based spin coupling scheme is discussed, in which the unpaired electrons of these species can be considered both ferromagnetically and antiferromagnetically coupled, with the strength of the latter strongly dependent on the acene subunit.

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