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.