In search of the dark state of 5-methyl-2-hydroxypyrimidine using a numerical DFT/MRCI gradient

Abstract

In a recent publication, Lobsiger et al. [Phys. Chem. Chem. Phys. 12, 5032 (2010)] presented infrared and electronic absorption spectra of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. In addition, they reported on the fast nonradiative decay of the S₁ population to a dark state. In the present paper, we have investigated the mechanism and rate constants of this nonradiative decay by means of quantum chemical multi-configuration methods. To this end, minima of the lowest excited singlet and triplet states as well as the minimum-energy crossing point of singlet and triplet potential energy hypersurfaces (PEHs) have been determined employing a numerical DFT/MRCI gradient where DFT/MRCI stands for a combination of density functional theory (DFT) and a semi-empirical multi-reference configuration interaction (MRCI) approach. Rate constants have been calculated in the Condon approximation using a time-dependent approach based on harmonic oscillator functions and electronic spin–orbit coupling matrix elements evaluated at the DFT/MRCI level. It is shown that the first excited triplet state possesses ³(n → π*) character in the gas phase. Fast intersystem crossing is mediated by the low-lying ³(π → π*) state whose PEH crosses both, the S₁ ¹(n → π*) and T₁ ³(n → π*) PEHs.

Keywords:

• multireference configuration interaction
• numerical gradient
• minimum-energy crossing point
• spin-forbidden transition
• spin–orbit coupling
• time-correlation function

Acknowledgements

One of the pioneers in the field of parallelizing quantum chemistry codes is Peter R. Taylor 53. This paper is dedicated to him on the occasion of his 60th birthday. Happy birthday, Peter, and many happy returns!