Abstract
Geometry, energetics and dipole moment of all possible conformers of cytosine in the ground state are calculated using density functional theory B3LYP method and the 6-311++G(3df,3pd) basis set. The most stable conformer is keto-amino conformer. The amino group hydrogen atoms are slightly out of plane by about 6.3 degrees and 9.9 degrees each. Ultrafast radiationless decay mechanism has been theoretically investigated using Complete Active Space Multiconfiguration SCF calculations. Effective pathways of ultrafast radiationless transitions from the optically allowed pi pi* state to the ground state S-0 of cytosine are explored. The n pi*, n sigma*, and the pi pi* states have been taken into account as states involved in the radiationless process. Optimized geometry and conical intersections have been searched in the full dimensional space for the vibrational degrees of freedom. Three competing direct decay mechanisms through three possible conical intersections have been found to exist. The first pathway is through the bending of molecule in a sofa-like structure leading to conical intersection with ground state at 4.23 eV. The second pathway occurs through a twisted structure that has hydrogen twisted and the cytosine ring slightly deformed leading to conical intersection at 4.08 eV. The third mechanism takes place via semi-planar conical intersection with main deformations inside the cytosine ring and C=O bond that have 3.97 eV at the intersection with ground state. The three mechanisms contribute to the stability of cytosine. The g-vector and h-vector for semi-planar conical intersection are calculated and discussed along with their geometrical parameters.