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

Ghigo, G.; Rehaman, A.; Gagliardi, L.; Solstad, L. M.; Cramer, C. J.
J. Org. Chem. 2007, 72, 2823.

The ground and excited electronic state properties of calicene (triapentafulvalene or 5-(cycloprop-2-en-1-ylidene)cyclopenta-1,3-diene) have been studies with a variety of density functional models (mPWPW91, PBE, TPSS, TPSSh, 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 3- and 5-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 the most important transition to occur 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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