Low-lying electronic states of cyclopentadienone

We report a combined experimental and theoretical study of the low-lying electronic states of cyclopentadienone (C₅H₄O). The cyclopentadienone anion (C₅H₄O^-) was generated in the gas phase via reaction of atomic oxygen radical anions (O^-) with cyclopentanone (C₅H₈O). Photoelectron imaging was used to gain access to the first three electronic states of C₅H ₄O, including the X ¹A₁ ground state and the ³B₂ and ³A₂ excited states. The first two state assignments are supported by the Franck-Condon simulations of the vibrational progressions observed in the X ¹A₁ and ³B₂ bands in the photoelectron spectra. The adiabatic electron affinity of cyclopentadienone in the ground state is determined to be EA(X ¹A₁) = 1.06 ± 0.01 eV, and the corresponding values for the first two excited states are EA(³B₂) = 2.56 ± 0.02 eV and EA(³A₂) = 3.45 ± 0.01 eV. These experimental determinations are in excellent agreement with the CCSD(T) theory predictions, lending further confidence to the above state assignments. On the basis of these results, the lowest singlet-triplet splitting (between the X ¹A₁ and ³B₂ states) in cyclopentadienone is ΔE_S-T = 1.50 ± 0.02 eV.