Photoelectron Spectroscopy and Thermochemistry of the Peroxyformate Anion Stephanie M. Villano, Nicole Eyet, † Scott W. Wren, G. Barney Ellison, Veronica M. Bierbaum,* and W. Carl Lineberger* JILA, UniVersity of Colorado and the National Institute of Standards and Technology and Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0440 ReceiVed: August 5, 2009; ReVised Manuscript ReceiVed: September 28, 2009 The 351.1 nm photoelectron spectrum of the peroxyformate anion has been measured. The photoelectron spectrum displays vibronic features in both the X ˜ 2 A′′ ground and A ˜ 2 A′ first excited states of the corresponding radical. Franck-Condon simulations of the spectrum show that the ion is formed exclusively in the trans- conformation. The electron affinity (EA) of the peroxyformyl radical was determined to be 2.493 ( 0.006 eV, while the term energy splitting was found to be 0.783 -0.020 +0.060 eV. Extended progressions in the C-OO (973 ( 20 cm -1 ) and O-O (1098 ( 20 cm -1 ) stretching modes are observed in the ground state of the radical. The fundamental frequency of the in-plane OCO bend was found to be 574 ( 35 cm -1 . The gas-phase acidity of peroxyformic acid has been determined using an ion-molecule bracketing technique. On the basis of the size of the trans- to cis- isomerization barrier, the measured acidity was assigned to the higher energy trans- conformer of the acid. The gas-phase acidity of the lower energy cis-conformer of peroxyformic acid was found from the measured acidity for the trans-form and a calculated energy correction: Δ a G 298 (cis-peroxyformic acid) ) 346.8 ( 3.3 kcal mol -1 and Δ a H 298 (cis-peroxyformic acid) ) 354.4 ( 3.3 kcal mol -1 . Using a negative ion EA/acidity thermochemical cycle, the O-H bond dissociation energy (D 0 ) values of the trans- and cis- conformers of peroxyformic acid to form the trans-radical were determined to be 94.0 ( 3.3 and 97.1 ( 3.3 kcal mol -1 , respectively. The heat of formation (Δ f H 298 ) of the trans-peroxyformyl radical was found to be -22.8 ( 3.5 kcal mol -1 . Introduction The reaction of formyl radical with molecular oxygen has received considerable attention, due to its prominent role in tropospheric and hydrocarbon combustion processes. A variety of techniques have been employed to determine the room temperature reaction rate, 1–13 and the formation of HO 2 has been observed using laser magnetic resonance spectrosco- py. 14 While there has been some controversy on the exact temperature dependence 6,9,10,13 as well as the effect of isotopic substitution, 7,13 the consensus is that this reaction proceeds via an activated peroxyformyl collision complex, [HC(O)OO]*. 7,15–20 Hsu et al. 15 have investigated the various product channels using ab initio molecular orbital and statistical calculations. These results indicate that the vibrationally excited peroxyformyl complex rapidly dissociates via a four-membered cyclic transi- tion state structure to form HO 2 + CO. Direct formation of these products via hydrogen abstraction was found to be negligible at temperatures below 2000 K. At pressures below 2000 Torr, deactivation of the peroxyformyl radical by a third body collision was also found to be insignificant. The results of this study are consistent with previous experimental observations, which found that the rate constant is pressure-independent over the range of 5-1000 Torr. 2,7,8 To date, the only experimental observation of the peroxy- formyl radical has been via matrix isolation IR-spectroscopy. In this technique, transient reaction intermediates, which readily dissociate in the gas phase, are stabilized through multiple collisions when trapped in the matrix environment. In an early study, this radical was produced by the photolysis of formal- dehyde in a solid oxygen matrix. 17,18 The assignment of the spectral features, however, was complicated by the presence of the HO 2 radical, which was likely present in the photolysis cage. More recently, the peroxyformyl radical has been studied by discharge production and deposition of the formyl radical onto a solid argon matrix followed by an association reaction with oxygen. 19 Two vibrational transitions at 1821.5 and 957.3 cm -1 have been assigned to the CdO and C-OO stretching modes, respectively. The spectral assignments were confirmed by 18 O isotopic labeling and by electronic structure calculations. In this work, we report the first observation of the isolated peroxyformyl radical. This has been achieved through electron photodetachment from the peroxyformyl anion. Analysis of the photoelectron energy spectrum allows determinations of the electron affinity, term energy splitting, and vibrational frequen- cies of the peroxyformyl radical and provides insight into the molecular structure of both the anion and radical species. Additionally, the gas-phase acidity of peroxyformic acid was determined from ion-molecule bracketing experiments. Com- bining this gas-phase acidity with the peroxyformyl radical electron affinity in a negative ion thermochemical cycle 21–24 allows the determination of the O-H bond dissociation energy in peroxyformic acid. Electronic structure calculations provide additional insight into the structural and energetic aspects of the peroxy ion, the radical, and the parent acid. This paper * Corresponding authors. E-mail: veronica.bierbaum@colorado.edu (V.M.B.); wcl@jila.colorado.edu (W.C.L.). † Current address: Department of Chemistry, Saint Anselm College, 100 Saint Anselm Dr. #1760, Manchester, New Hampshire 03102. HCO + O 2 f HO 2 + CO (1) J. Phys. Chem. A 2010, 114, 191–200 191 10.1021/jp907569w  2010 American Chemical Society Published on Web 10/14/2009