Locating Cytosine Conical Intersections by Laser Experiments and Ab Initio Calculations

The decay mechanism of S-0 -> S-1 excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at Ni, C5, and C6, rapid internal conversion sets in at 250-1200 cm(-1) above the 0(0)(0) bands. The break-off in the spectra correlates with the calculated barriers toward the C5-C6 twist conical intersection, which unambiguously establishes the decay mechanism at low S-1 state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5, and C6 following ((1)pi pi*) 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).