title: Quantum chemical characterization of low-energy states of calicene in the gas phase and in solution.
abstract: 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.
authors: Dipartimento di Chimica Generale ed Organica Applicata, Università di Torino, C.so M. d'Azeglio 48, I-10125 Torino, Italy. giovanni.ghigo@unito.it
authors: Giovanni
authors: G
authors: Ghigo
authors: Abdul Rehaman Moughal
authors: AR
authors: Shahi
authors: Laura
authors: L
authors: Gagliardi
authors: Lee M
authors: LM
authors: Solstad
authors: Christopher J
authors: CJ
authors: Cramer