Ultrafast radiationless decay mechanisms through conical intersections in cytosine: Computational insight and topological analysis of the charge density distributions

The quantum theory of atoms-in-molecules (QTAIM) in conjunction with the DFT/B3LYP/6-311++G(2d,2p) wave function are used to compute the atomic, bonded and non-bonded interactions, distributions of the charge density, ρ(r), and its Laplacian, ∇ρ(r), for the ground equilibrium structure of cytosine. The study has been further extended to include two conical intersection (CI) structures that underlie the radiationless decay of cytosine. Complete Active Space Multiconfiguration SCF level of theory with the 6-311++G** basis set are used to identify, characterize and to optimize the geometrical structures of the conical intersections between So and S1. In the case of cytosine, all ring bond critical points show ∇ρ(r) < 0 indicating covalent binding and accumulation of the electron density in the bonding regions. On the other hand, the C1-O11 bond shows, depletion of charge density, indicating the increased ionic character of this bond. This point might very well underlie the reactivity and the low keto-enol barrier in cytosine. Contour plots and relief maps have been analyzed for regions of valence shell charge concentrations and depletions in the ground state and the three CI structures of cytosine. NBO analysis reveals that the conformational and overall stability of the studied cytosine conformations is facilitated by the competitive conjugative and the lone-pair interactions. In cytosine, there is a delicate balance between these two forces, whereas, in each of the two CI configurations there is one dominant force that underlies the stability of the cytosine structure. The present work indicates that the QTAIM provide not only a graphical presentation of very important critical points on the PES but also unique quantitative descriptors of CI's that characterize it.