The C-H bond dissociation energy of furan: photoelectron spectroscopy of the furanide anion.

Using photoelectron spectroscopy, we interrogate the cyclic furanide anion (C(4)H(3)O(-)) to determine the electron affinity and vibrational structure of the neutral furanyl radical and the term energy of its first excited electronic state. We present the 364-nm photoelectron spectrum of the furanide anion and measure the electron affinity of the X̃(2)A(') ground state of the α-furanyl radical to be 1.853(4) eV. A Franck-Condon analysis of the well-resolved spectrum allows determination of the harmonic frequencies of three of the most active vibrational modes upon X̃(2)A(') ← X̃(1)A(') photodetachment: 855(25), 1064(25), and 1307(40) cm(-1). These modes are ring deformation vibrations, consistent with the intuitive picture of furanide anion photodetachment, where the excess electron is strongly localized on the α-carbon atom. In addition, the Ã(2)A('') excited state of the α-furanyl radical is observed 0.68(7) eV higher in energy than the X̃(2)A(') ground state. Through a thermochemical cycle involving the known gas-phase acidity of furan, the electron affinity of the furanyl radical yields the first experimental determination of the C-H(α) bond dissociation energy of furan (DH(298)(C(4)H(3)O-H(α))): 119.8(2) kcal mol(-1).