Electronic states of thiophenyl and furanyl radicals and dissociation energy of thiophene via photoelectron imaging of negative ions

We report photoelectron images and spectra of deprotonated thiophene, C₄H₃S^-, obtained at 266, 355, and 390 nm. Photodetachment of the isomer of the anion is observed, and the photoelectron bands are assigned to the ground X²A^′ (π) and excited A²A^″ and B²A^″ (π) states of the thiophenyl radical. The photoelectron angular distributions are consistent with photodetachment from the respective in-plane (Δ) and out-of-plane (^π) orbitals. The adiabatic electron affinity of α_-^•C₄H₃S is determined to be 2.05 ± 0.08 eV, while the B²A^″ term energy is estimated at 1.6 ± 0.1 eV. Using the measured electron affinity and the electron affinityacidity thermodynamic cycle, the C-H_α bond dissociation energy of thiophene is calculated as DH₂₉₈(H _α-C₄H₃S) = 115 ± 3 kcalmol. Comparison of this value to other, previously reported C-H bond dissociation energies, in particular for benzene and furan, sheds light of the relative thermodynamic stabilities of the corresponding radicals. In addition, the 266 nm photoelectron image and spectrum of the furanide anion, C₄H ₃O^-, reveal a previously unobserved vibrationally resolved band, assigned to the B²A^″ excited state of the furanyl radical, ^•C₄H₃O. The observed band origin corresponds to a 2.53 ± 0.01 eV B²A^″ term energy, while the resolved vibrational progression (853 ± 42 cm ^-1) is assigned to an in-plane ring mode of α_- ^•C₄H₃O (B²A^″).