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Locating Cytosine Conical Intersections by Laser Experiments and ab Initio Calculations.

The decay mechanism of S0 → S1 excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (ns resonant-two photon ionization (R2PI) and ps lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at N1, C5 and C6, rapid internal conversion sets in at 250 - 1200 cm-1 above the 00 bands. The break-off in the spectra correlates with the cal- culated barriers towards the "C5-C6 twist" CI, which unambiguously establishes the decay mechanism at low S1 state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5 and C6 following (1ππ∗) excitation. The R2PI spectra of the clamped derivatives 5,6-trimethyleneCyt (TMCyt) and 1-methyl-TMCyt (1M-TMCyt), which decay along an N3 out-of-plane coordinate, extend up to +3500 and +4500 cm-1.