1 ISSN: 1469-0667 © IM Publications LLP 2009 doi: 10.1255/ejms.1055 All rights reserved EUROPEAN JOURNAL OF MASS SPECTROMETRY Organic peroxy radicals (ROO) play an integral role in the chemistry of the troposphere. 1–3 These radicals are formed as a result of OH-initiated oxidation of volatile organic compounds (RH) emitted from biogenic and anthropogenic sources; this process efficiently removes hydrocarbons from the atmos- phere and serves as an important sink for the OH radical. Once formed, alkyl radicals (R) rapidly combine with O 2 via a three- body reaction to produce organic peroxy radicals. RH + OH → R + H 2 O (1) R + O 2 + M → ROO + M (2) Peroxy radicals react with the NO x and HO x radical fami- lies. In urban areas, reactions with NO x species dominate. By oxidizing NO to NO 2 , peroxy radicals directly influence tropo- spheric ozone formation, since the photolysis of NO 2 forms O( 3 P), which rapidly reacts with O 2 to form O 3 in the sequence shown below. 4,5 ROO + NO → RO + NO 2 (3) NO 2 + hν → NO + O( 3 P) (4) O( 3 P) + O 2 + M → O 3 + M (5) In areas of low NO x , organic peroxy radicals primarily react with other peroxy radicals (mainly HOO), propagating chain reactions that eventually lead to the removal of radicals from the atmosphere. Photoelectron spectroscopy and thermochemistry of the peroxyacetate anion Stephanie M. Villano, a Nicole Eyet, a,b Scott W. Wren, a G. Barney Ellison, a Veronica M. Bierbaum a and W. Carl Lineberger a a JILA, University of Colorado and the National Institute of Standards and Technology and Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0440, USA. E-mails: veronica.bierbaum@colorado.edu; wcl@jila.colorado.edu b Current Address: Department of Chemistry, Saint Anselm College, 100 Saint Anselm Dr. #1760 Manchester, NH 03102, USA The 351.1 nm photoelectron spectrum of the peroxyacetate anion, (CH 3 C(O)OO – ) was measured. Analysis of the spectrum shows that the observed spectral features arise almost exclusively from transitions between the trans-conformer of the anion and the X ~ 2 A″ and A ~ 2 A′ states of the corresponding radical. The electron affinity of trans-CH 3 C(O)OO is 2.381 ± 0.007 eV and the term energy splitting of the A ~ 2 A′ state is 0.691 ± 0.009 eV, in excellent agreement with two prior values [Zalyubovsky et al. J. Phys. Chem. A 107 , 7704 (2003); Hu et al. J. Phys. Chem. 124, 114305/1 (2006); Hu et al. J. Phys. Chem. 110, 2629 (2006)]. The gas-phase acidity of trans-peroxyacetic acid was brack- eted between the acidity of acetic acid and tert-butylthiol at Δ a G 298 (trans-CH 3 C(O)OOH) = 1439 ± 14 kJ mol –1 and Δ a H 298 (trans-CH 3 C(O) OOH) = 1467 ± 14 kJ mol –1 . The acidity of cis-CH 3 C(O)OOH was found by adding a calculated energy correction to the acidity of the trans- conformer; Δ a G 298 [cis-CH 3 C(O)OOH] = 1461 ± 14 kJ mol –1 and Δ a H 298 [cis-CH 3 C(O)OOH] = 1490 ± 14 kJ mol –1 . The O–H bond dissociation energies for both conformers were determined using a negative ion thermodynamic cycle to be D 0 [trans-CH 3 C(O)OOH] = 381 ± 14 kJ mol –1 and D 0 [cis-CH 3 C(O)OOH] = 403 ± 14 kJ mol –1 . The atmospheric implications of these results and relations to the thermochemistry of peroxyacetyl nitrate are discussed briefly. Keywords: acylperoxy radical, acyl peroxide, peroxyacetyl radical, peroxyacetic acid, photoelectron spectroscopy, FA-SIFT, negative ion cycle, bond dissociation energy, atmospheric chemistry, peroxyacetyl nitrate Introduction S.M. Villano et al., Eur. J. Mass Spectrom. 15, xxx–xxx (2009) Received: 31 August 2009 ■ Accepted: 19 October 2009 ■ Publication: 18 December 2009