Excited-State Structure and Dynamics of Keto–AminoCytosine: The 1ππ* State Is Nonplanar and ItsRadiationless Decay Is Not Ultrafast.

Wehave measured the mass- and tautomer-specific S0→S1vibronic spectra and S1state lifetimes ofthe keto–amino tautomer of cytosine cooled in supersonic jets,using two-color resonant two-photon ionization (R2PI) spectroscopyat 0.05 cm–1resolution. The rotational contoursof the 000bandand nine vibronic bands up to +437 cm–1are polarizedin the pyrimidinone plane, proving that the electronic excitationis to a 1ππ* state. All vibronic excitationsup to +437 cm–1are overtone and combination bandsof the low-frequency out-of-plane ν1′(butterfly), ν2′(boat),and ν3′(H–N–C6–H twist) vibrations. UVvibronic spectrum simulations based on approximate second-order coupled-cluster(CC2) calculations of the ground and 1ππ* statesare in good agreement with the experimental R2PI spectrum, but onlyif the calculated ν1′and ν2′frequencies are reduced by afactor of 4 and anharmonicity is included. Together with the highintensity of the ν1′and ν2′overtone vibronic excitations, thisimplies that the 1ππ* potential energy surfaceis much softer and much more anharmonic in the out-of-plane directionsthan predicted by the CC2 method. The 1ππ*state lifetime is determined from the Lorentzian line broadening necessaryto reproduce the rotational band contours: at the 000band it is τ = 44 ps, remainsat τ = 35–45 ps up to +205 cm–1, anddecreases to 25–30 ps up to +437 cm–1. Theselifetimes are 20–40 times longer than the 0.5–1.5 pslifetimes previously measured with femtosecond pump–probe techniquesat higher vibrational energies (1500–3800 cm–1). Thus, the nonradiative relaxation rate of keto–amino cytosineclose to the 1ππ* state minimum is knr∼ 2.5 × 1010s–1, much smaller than at higher energies. An additionalnonradiative decay channel opens at +500 cm–1excessenergy. Since high overtone bands of ν1′and ν2′are observed in the R2PIspectrum but only a single weak 2ν3′band, we propose that ν3′is a promotingmode for nonradiative decay, consistent with the observation thatthe ν3′normal-mode eigenvector points toward the “C6-puckered”conical intersection geometry. [ABSTRACT FROM AUTHOR]