Cavity ringdown spectrum of the T(1)(n,pi*) <-- S(0) transition of 4-cyclopentene-1,3-dione.

The cavity ringdown absorption spectrum of 4-cyclopentene-1,3-dione was recorded near 487 nm in a room-temperature gas cell. The very weak band system (epsilon approximately 0.05 dm3 mol-1 cm-1) in this region is due to the T1(n,pi*) <-- S0 electronic transition. The origin-band maximum was observed at 20540.0 +/- 0.5 cm-1. We have assigned about 40 vibronically resolved bands in a region extending to +1100 cm-1 relative to the origin. Assignments were aided by quantum-chemical calculations of the T1 <-- S0 0-0 excitation energy as well as ground-state vibrational frequencies. From the CRD spectral assignments, we determined fundamental frequencies for several vibrational modes in the T1 excited state, including the lowest-energy ring-bending and -twisting modes, nu19' (b1) and nu14' (a2), respectively. Their fundamentals in the T1 state are 160.5 and 246 cm-1, compared to 99 and 239 cm-1, respectively, in the S0 ground state. The increases in these ring frequencies upon electronic excitation signify that the nominal n --> pi* chromophore is delocalized to include the conjugated ring atoms. The extent of this delocalization is different in the T1(n,pi*) vs S1(n,pi*) excited states. This conclusion is based on observed differences in T1 vs S1 ring fundamental frequencies.