title: Second hyperpolarizability (gamma) of singlet diradical system: dependence of gamma on the diradical character.
abstract: The dependence of the second hyperpolarizability (gamma) on the diradical character (y) for singlet diradical systems is investigated using a model compound, the p-quinodimethane (PQM) molecule with different both-end carbon-carbon (C-C) bond lengths, by several ab initio molecular orbital and density functional theory methods. The diradical character based on UHF calculations indicates that at equilibrium geometry PQM is in a singlet ground state and primarily exhibits a quinoid structure, whereas the diradical character increases when increasing both-end C-C bond lengths. At the highest level of approximation, that is, using the UCCSD(T) method with the 6-31G+diffuse p (zeta = 0.0523) basis set, the longitudinal static gamma of PQM presents a maximum value for intermediate diradical character (y approximately 0.5) while the gamma values are larger for intermediate and large diradical character (y approximately 0.5-0.7) than for small diradical character (y < 0.2). This feature suggests that the gamma values of singlet diradical systems in the intermediate and somewhat strong correlation regimes are significantly enhanced as compared to those in the weak correlation regime. These results are substantiated by a complementary study of the variation in gamma upon twisted ethylene.
authors.affiliation: Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan.
authors.forename: Masayoshi
authors.initials: M
authors.lastname: Nakano
authors.forename: Ryohei
authors.initials: R
authors.lastname: Kishi
authors.forename: Tomoshige
authors.initials: T
authors.lastname: Nitta
authors.forename: Takashi
authors.initials: T
authors.lastname: Kubo
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Nakasuji
authors.forename: Kenji
authors.initials: K
authors.lastname: Kamada
authors.forename: Koji
authors.initials: K
authors.lastname: Ohta
authors.forename: Benoît
authors.initials: B
authors.lastname: Champagne
authors.forename: Edith
authors.initials: E
authors.lastname: Botek
authors.forename: Kizashi
authors.initials: K
authors.lastname: Yamaguchi