Three-state conical intersections in cytosine and pyrimidinone bases

Area of Science:

• Computational chemistry
• Quantum chemistry
• Photochemistry

Background:

• Conical intersections are critical for understanding nonadiabatic dynamics in molecules.
• Cytosine and its analogs are fundamental in biological systems and undergo photochemical reactions.

Purpose of the Study:

• To locate and characterize three-state conical intersections for cytosine and 5-methyl-2-pyrimidinone.
• To investigate the influence of a third electronic state on nonadiabatic coupling terms.
• To identify general patterns in the electronic energy surfaces of 2-pyrimidinone bases.

Main Methods:

• Ab initio multireference configuration-interaction calculations.
• Characterization of potential energy surfaces.
• Calculation of nonadiabatic coupling terms.

Main Results:

• Three distinct three-state conical intersections were identified for both molecules.
• Seam paths from these intersections connect to known two-state conical intersections.
• The phase of nonadiabatic coupling terms is predictably affected by the presence of multiple seam points.

Conclusions:

• A general pattern exists for the energy surfaces of 2-pyrimidinone bases, including three-state conical intersections.
• The character and energies of these intersections are similar between cytosine and 5-methyl-2-pyrimidinone.
• Nonadiabatic coupling dynamics are influenced by the interplay of multiple electronic states.