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

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 S ₀-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.