n,π* fluorescence from selected vibronic levels of pyrimidine vapor: Franck–Condon factors and excited state anharmonic coupling

An analysis is presented of fluorescence from various selected vibrational levels in the 1B1(S1) state of pyrimidine vapor. The intensity distribution in fluorescence from the zero point level establishes with security the general pattern of vibrational activity in the 1B1–1A1 transition, and the Franck–Condon intensities in this fluorescence set the molecular parameters required for calculation of Franck–Condon intensities in fluorescence from other levels. The fluorescence from every higher level so far reached shows marked deviations from the predicted Franck–Condon intensities. These deviations can be attributed to strong (ca., 30 cm−1) anharmonic coupling in the excited state. Every observed totally symmetric fundamental in the 1B1 state is perturbed by this mixing. Calculations which include the coupling provide a successful account of the perturbed Franck–Condon intensities. The fluorescence analyses secure the assignment of a number of absorption bands as well as several excited state fundamentals. Specifically, ν6a′, ν9a′, and ν16a′ are confirmed and the fundamentals ν1′=941 cm−1, ν6b′=335 cm−1, ν12′=1012 cm−1, and ν16b′=366 cm−1 are newly established. Fluorescence studies at various pressures show that vibrational relaxation is a poor competitor to a very efficient collision-induced electronic relaxation of the 1B1 state. Vapor phosphorescence cannot be detected under any conditions.