Ab initio study of the cyclodimerization of uracil through butane-like and oxetane-like conical intersections

• Performance of the study of the excited-state reaction paths of the photocyclodimerization of uracil.
• Finding of conical intersections S0/S1 of the cyclodimerizations.
• Explanation of the experimental observations connected with the cyclodimerization processes.
• Clarification of the vertical excitation energies of the stable minima – cyclodimers and stacked dimers.

In the present work we explored the mechanisms of the formations of the uracil cyclobutane and oxetane cyclodimers using ab initio methods through excited states and the S0–S1 conical intersections. The excited-state reaction paths showed that the 1ππ* excited states of the stacked dimers can relax, in a barrierless manner, to the ground states via S0–S1 conical intersections. We found that the destruction of the cyclodimers occurs through 1ππ* excited states and the same S0–S1 conical intersections. In other words, the formation and the destruction of the uracil cyclodimers are competitive reactions initiated by UV light. Excited-state reaction paths show that the production of the cyclobutane dimer is much more favorable than the oxetane adduct.