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Triplet‐State Aromaticity of 4 n π‐Electron Monocycles: Analysis of Bifurcation in the π Contribution to the Electron Localization Function
Author(s) -
Villaume Sebastien,
Fogarty Heather A.,
Ottosson Henrik
Publication year - 2008
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200700540
Subject(s) - annulene , cyclobutadiene , aromaticity , chemistry , singlet state , triplet state , electron , density functional theory , ring (chemistry) , ab initio , computational chemistry , atomic physics , molecular physics , physics , molecule , excited state , quantum mechanics , organic chemistry
The π contribution to the electron localization function (ELF) is used to compare 4 n π‐ and (4 n +2)π‐electron annulenes, with particular focus on the aromaticity of 4 n π‐electron annulenes in their lowest triplet state. The analysis is performed on the electron density obtained at the level of OLYP density functional theory, as well as at the CCSD and CASSCF ab initio levels. Two criteria for aromaticity of all‐carbon annulenes are set up: the span in the bifurcation values Δ BV(ELF π ) should be small, ideally zero, and the bifurcation value for ring closure of the π basin RCBV(ELF π ) should be high (≥ 0.7). On the basis of these criteria, nearly all 4 n π‐electron annulenes are aromatic in their lowest triplet states, similar to (4 n +2)π‐electron annulenes in their singlet ground states. For singlet biradical cyclobutadiene and cyclooctatetraene constrained to D 4 h and D 8 h symmetry, respectively, the RCBV(ELF π ) at the CASSCF level is lower (0.531 and 0.745) than for benzene (0.853), even though they have equal proportions of α‐ and β‐electrons.