Scientific article
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Quantum Chemical Characterization of Low-energy States of Calicene in the Gas Phase and Solution

Published inJournal of organic chemistry, vol. 72, no. 8, p. 2823-2831
Publication date2007

The ground and excited electronic state properties of calicene (triapentafulvalene or 5-(cycloprop-2-en-1-ylidene)cyclopenta-1,3-diene) have been studied with a variety of density functional models (mPWPW91, PBE, TPSS, TPSh, B3LYP) and post-Hartree−Fock models based on single (MP2 and CCSD(T)) and multideterminantal (CASPT2) reference wave functions. All methods agree well on the properties of ground-state calicene, which is described as a conjugated double bond system with substantial zwitterionic character deriving from a charge-separated mesomer in which the three- and five-membered rings are both aromatic. Although the two rings are joined by a formal double bond, contributions from the aromatic mesomer reduce its bond order substantially. A rotational barrier of 40−41 kcal mol-1 is predicted in the gas phase and solvation effects reduce the barrier to 37 and 33 kcal mol-1 in benzene and water, respectively, because of increased zwitterionic character in the twisted transition-state structure. Multi-state CASPT2 (MS-CASPT2) is used to characterize the first few excited singlet and triplet states and indicates that the most important transition occurs at 4.93 eV (251 nm). A cis−trans photoisomerization about the inter-ring double bond is found to be inefficient.

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Citation (ISO format)
GHIGO, Giovanni et al. Quantum Chemical Characterization of Low-energy States of Calicene in the Gas Phase and Solution. In: Journal of organic chemistry, 2007, vol. 72, n° 8, p. 2823–2831. doi: 10.1021/jo062420y
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ISSN of the journal0022-3263

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