Triplet State Z/E-Photoisomerizations of Polyenes:  A Comparison of ab Initio and Density Functional Methods

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 102; no. 32; pp. 6513 - 6524
• Main Authors: Brink, Maria, Jonson, Helene, Ottosson, Carl-Henrik
• Format: Journal Article
• Language: English
• Published: American Chemical Society 06.08.1998
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp973384x

In the search for less CPU-costly methods for study of triplet state Z/E-photoisomerization of olefins, Kohn−Sham density functional theory (DFT) has been tested on 1,3-butadiene (BD) and 1,3,5-hexatriene (HT). Computed T1 energies were compared to those from CASSCF, CASPT2, and spin-projected UMP4(SDTQ) calculations as well as experiments. For both molecules it is necessary that nonlocal gradient corrections are made to the exchange functional since usage of the local spindensity approximation for exchange in most cases leads to vertical and relaxed T1 energies that are too high. Gradient-corrected DFT as well as hybrid functional methods lead to T1 energies that are bracketed by the corresponding UMP4(SDTQ) and CASPT2 energies and lie at most 4 kcal/mol below measured values. The relaxed T1 energies for planar geometries are in slightly better agreement with experiment when calculated by pure nonlocal gradient-corrected DFT than by hybrid functional methods. However, T1-state potential energy surfaces obtained by either type of method explain the experimental observations on triplet-state Z/E-photoisomerizations of BD and HT, and geometries of T1 isomers of BD and HT compare well with those from UMP4(SDQ), UMP2, and CASSCF calculations. Finally, it should be noted that for both molecules UHF deviates from the higher computational levels in T1 energies by 20−30 kcal/mol and should be avoided in all computations of T1 states of olefins.