Ab initio molecular dynamics relaxation and intersystem crossing mechanisms of 5-azacytosine.
The gas phase relaxation dynamics of photoexcited 5-azacytosine has been investigated by means of SHARC (surface-hopping including arbitrary couplings) molecular dynamics, based on accurate multireference electronic structure computations. Both singlet and triplet states were included in the simulations in order to investigate the different internal conversion and intersystem crossing pathways of this molecule. It was found that after excitation, 5-azacytosine undergoes ultrafast relaxation to the electronic ground state with a time constant of about 1 picosecond. Two important conical intersections have been identified as the funnel responsible for this deactivation mechanism. The very low intersystem crossing yield of 5-azacytosine has been explained by the size of the relevant spin-orbit coupling matrix elements, which are significantly smaller than in related molecules like cytosine or 6-azauracil. This difference is due to the fact that in 5-azacytosine the lowest singlet state is of nNπ* nature, whereas in cytosine and 6-azauracil it is of nOπ* character.
Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo, NAP-PhotoTech the USP Consortium for Photochemical Technology, Av. Prof. Lineu Prestes 748, 05508-000, São Paulo, SP, Brazil. ancborin@iq.usp.br.
Antonio
http://orcid.org/0000-0003-3047-2044
A
Carlos Borin
Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Straße 17, 1090 Vienna, Austria. sebastian.mai@univie.ac.at.
Sebastian
http://orcid.org/0000-0001-5327-8880
S
Mai
Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Straße 17, 1090 Vienna, Austria. sebastian.mai@univie.ac.at.
Philipp
http://orcid.org/0000-0002-8711-1533
P
Marquetand
Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Straße 17, 1090 Vienna, Austria. sebastian.mai@univie.ac.at.
Leticia
http://orcid.org/0000-0001-5112-794X
L
González
D003596:Cytosine / Q000031:analogs & derivatives* / Q000737:chemistry; D004581:Electronics; D056004:Molecular Dynamics Simulation
C039854:5-azacytosine; D003596:Cytosine