title: Cavity ringdown spectrum of the T(1)(n,pi*) <-- S(0) transition of 4-cyclopentene-1,3-dione.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54701-4004, USA.
authors.forename: Mitchell G
authors.initials: MG
authors.lastname: Springer
authors.forename: Nikolaus C
authors.initials: NC
authors.lastname: Hlavacek
authors.forename: Sydney P
authors.initials: SP
authors.lastname: Jagusch
authors.forename: Andrew R
authors.initials: AR
authors.lastname: Johnson
authors.forename: Stephen
authors.initials: S
authors.lastname: Drucker
mesh_terms: D003198:Computer Simulation; D003517:Cyclopentanes / Q000737:chemistry*; D011789:Quantum Theory; D013057:Spectrum Analysis / Q000379:methods*
chemical_list: D003517:Cyclopentanes; C017848:4-cyclopentene-1,3-dione