Journal
JOURNAL OF PHYSICAL CHEMISTRY A
Volume 113, Issue 43, Pages 11809-11816Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp902944a
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Funding
- Deutsche Forschungsgemeinschaft (DFG) [SFB 663]
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The ground and low-lying excited states of the pyrimidine nucleo bases uracil, thymine, and 1-methylthymine have been characterized using ab initio coupled-cluster with approximate doubles (CC2) and a combination of density functional theory (DFT) and semiempirical multireference configuration interaction (MRCI) methods. Intersystem crossing rate constants have been determined perturbationally by employing a nonempirical one-center mean-field approximation to the Breit-Pauli spin-orbit operator for the computation of electronic coupling matrix elements. Our results clearly indicate that the S-2((1)pi-pi*) -> T-2((3)eta-pi*) process cannot compete with the subpicosecond decay of the S2 population due to spin-allowed nonradiative transitions, whereas the T-1((3)pi ->pi*) state is populated from the intermediate S-1((1)n-pi*) state on a subnanosecond time scale. Hence, it is very unlikely that the S-1((1)n ->pi*) state corresponds to the long-lived dark state observed in the gas phase.
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