ISSN:
0020-7608
Keywords:
MRMP
;
multireference-based perturbation theory
;
excited states
;
polyenes
;
excited-state energy gaps
;
Chemistry
;
Theoretical, Physical and Computational Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the valence π→π* excited states of 1,3-butadiene, 1,3,5-hexatriene, 1,3,5,7-octatetraene, and 1,3,5,7,9-decapentaene. Our focus was put on determining the nature of the two lowest-lying singlet excited states, 11Bu+ and 21Ag-, and their ordering. The 11Bu+ state is a singly excited state with an ionic nature originating from the HOMO→LUMO one-electron transition while the covalent 21Ag- state is the doubly excited state which comes mainly from the (HOMO)2→(LUMO)2 transition. The active-space and basis-set effects are taken into account to estimate the excitation energies of larger polyenes. For butadiene, the 11Bu+ state is calculated to be slightly lower by 0.1 eV than the doubly excited 21Ag- state at the ground-state equilibrium geometry. For hexatriene, our calculations predict the two states to be virtually degenerate. Octatetraene is the first polyene for which we predict that the 21Ag- state is the lowest excited singlet state at the ground-state geometry. The present theory also indicates that the 21Ag- state lies clearly below the 11Bu+ state in decapentaene with the energy gap of 0.4 eV. The 0-0 transition and the emission energies are also calculated using the planar C2h relaxed excited-state geometries. The covalent 21Ag- state is much more sensitive to the geometry variation than is the ionic 11Bu+ state, which places the 21Ag- state significantly below the 11Bu+ state at the relaxed geometry. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 157-175, 1998
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
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