Intersystem crossing and characterization of dark states in the pyrimidine nucleobases uracil, thymine, and 1-methylthymine

J Phys Chem A. 2009 Oct 29;113(43):11809-16. doi: 10.1021/jp902944a.


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)n-->pi*) process cannot compete with the subpicosecond decay of the S(2) 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Electrons
  • Kinetics
  • Models, Chemical
  • Pyrimidines / chemistry*
  • Quantum Theory*
  • Thermodynamics
  • Thymine / analogs & derivatives
  • Thymine / chemistry
  • Uracil / chemistry


  • Pyrimidines
  • 1-methylthymine
  • Uracil
  • Thymine