Solvent Effects on the Formation of Ground-State Twisted Intermediate during Electronic Relaxation of Pyrimidine Nucleobases.

This study investigates solvent effects on the formation of ground-state twisted intermediates recently found in the electronic relaxation of uracil and thymine [Obara et al., J. Am. Chem. Soc. 2025, 147, 15077]. We newly identify a similar reaction intermediate characterized by a twisted C5═C6 bond for aqueous cytidine, establishing a structural motif common to all pyrimidine nucleobases and their derivatives. Unlike uracil and thymine, the cytidine intermediate lacks a blue-shifted band in its IR spectrum associated with C═O stretching, precluding its identification using the same spectral criteria applied to uracil and thymine. Nevertheless, quantum chemical calculations assuming a twisted geometry reproduce the experimental IR spectrum, notably identifying the 1580 cm-1 band previously misassigned to the 1nπ* state of cytidine as the marker band for the twisted intermediate. The marker band appears within 1 ps after UV excitation in polar protic solvents such as water and methanol, indicating that the twisted intermediate is generated directly from the 1ππ* state via ultrafast internal conversion through the ethylenic conical intersection. The polar aprotic solvent acetonitrile suppresses the formation of the twisted intermediate and increases the yield of the 3ππ* state for uracil and thymine. The primary difference between protic and aprotic solvents arises from the destabilization of the 1nπ* state in the former. The complexity of solvent effects warrants detailed theoretical modeling to elucidate the interplay between the solute and solvent in nonadiabatic dynamics in solution.