Heats of Formation of the Acetyl Radical and Ion Obtained by Threshold Photoelectron Photoion Coincidence Elizabeth A. Fogleman, Hideya Koizumi, James P. Kercher, Ba ´ lint Szta ´ ray, ² and Tomas Baer* Department of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed: February 10, 2004; In Final Form: April 12, 2004 The dissociative photoionization onsets for the production of CH 3 CO + + CH 3 • from acetone and CH 3 CO + + CH 3 CO • from butanedione have been measured by threshold photoelectron photoion coincidence (TPEPICO) in which time-of-flight (TOF) mass spectra are obtained as a function of the ion internal energy. The use of velocity focusing for threshold electrons and the subtraction of “hot” electron coincidences from the TPEPICO spectra allow the 0 K dissociation onset to be measured with a precision of 1 kJ/mol. The experimental onset for CH 3 • loss from CH 3 COCH 3 was measured to be 10.563 ( 0.010 eV and the onset for CH 3 CO • loss from CH 3 COCOCH 3 was found to be 10.090 ( 0.006 eV. A 298 K heat of formation of the CH 3 CO + of 659.4 ( 1.1 kJ/mol is obtained by combining the measured dissociation onset with the well-established heats of formation of acetone and the methyl radical. A 298 K heat of formation of the CH 3 CO • radical of -9.8 ( 1.8 kJ/mol is obtained by combining the measured dissociation onset with the well-known heat of formation of butanedione and the measured heat of formation of CH 3 CO + . The acetone and butanedione ionization energies were measured to be 9.708 ( 0.004 and 9.21 ( 0.05 eV, respectively. Introduction The heats of formation of the acetyl radical, CH 3 CO • , and its closed shell ion, CH 3 CO + , are important because they are related to a number of important thermochemical quantities, such as the C-H bond energy in acetaldehyde and the C-CH 3 bond energy in acetone. It is thus of some importance to establish these quantities to the same level of accuracy as the heats of formation of the related acetaldehyde and acetone molecules. There are several methods for determining bond energies and radical heats of formation, which have been summarized and compared by Berkowitz et al. 1 and Blanksby and Ellison. 2 Among these are negative and positive ion thermochemical cycles as well as methods based on neutral kinetics. All of these approaches for determining a radical or ion heat of formation depend on the accuracy of other measurements. The various approaches thus differ not only in their experimental techniques, but also in their dependence on ancillary thermochemical information. It is thus important to determine these thermo- chemical quantities by several methods. In this paper we present new experimental data that serve to establish the heats of formation of the acetyl radical and ion to a precision of less than 2 kJ/mol. The heat of formation of the acetyl ion can be obtained from proton affinity measurements through the reaction The gas-phase proton affinity is generally measured as a relative quantity by equilibrium methods in high-pressure mass spec- trometry, and its accuracy depends on a knowledge of the proton affinity of neighboring molecules in the scale. 3 The 298 K proton affinity of ketene as listed in the NIST webbook is 825.3 kJ/ mol, 4 a number that was verified by high-level ab initio calculations of Smith and Radom 5 (825.0 kJ/mol). On the basis of the 0 K value of 819.1 kJ/mol and the heat of formation of ketene and H + , 6 the 0 K acetyl ion heat of formation is 664.2 ( 4 kJ/mol. The error limits are difficult to determine because they are based on the reliability of the PA scale in the vicinity of ketene. We estimate it to be 4 kJ/mol. The acetyl ion heat of formation can also be determined from photoionization of a variety of precursor molecules, CH 3 COX + hV f CH 3 CO + + X. Among the factors that determine the best choice are the accuracy of the Δ f H°(CH 3 COX) and ∆ f H°- (X), the lack of a reverse activation barrier for X loss, and a rapid dissociation reaction that does not involve metastable ions. Finally it is essential that X loss be the lowest energy dissociation channel. Traeger et al. 7 investigated several precur- sors, which resulted in a broad range of derived acetyl ion heats of formation. Probably the most reliable precursor is acetone, which had a reported 298 K onset of 10.38 eV. A subsequent evaluation of this onset that takes into account the molecule’s thermal energy resulted in a reported ∆ f H 298 (CH 3 CO + ) of 654.7 ( 1.5 kJ/mol. 8 However, an earlier photoionization study by Murad and Inghram 9 had suggested an onset of 10.45 eV, while a more recent study by Trott et al. 10 using a supersonically cooled jet reported an onset of 10.52 eV. One of the problems with photoionization studies is the interpretation of the onset. Because the sample usually has a room temperature thermal energy distribution, the onset must be carefully modeled by taking this into account, as suggested by Asher et al. 11 This was not done in the previous photoionization studies. The main information about the CH 3 CO • radical heat of formation has come from neutral kinetic methods. Niiranen et al. 12 investigated the forward and backward rate constants for the reaction CH 3 CO • + HBr T CH 3 CHO + Br • as a function of temperature. Knowing the heat of formation of acetaldehyde, HBr, and the bromine atom permitted them to extract a 298 K * Corresponding author. E-mail: baer@unc.edu. ² Current address: Department of General and Inorganic Chemistry, Eo ¨tvo ¨s University Budapest, Hungary H-1117, Budapest, Pa ´zma ´ny P. se ´ta ´ny 1/a. H 2 CdCdO + H + f CH 3 CO + ∆E ) PA(H 2 CdCdO) 10.1021/jp040118s CCC: $27.50 © xxxx American Chemical Society PAGE EST: 6.7 Published on Web 00/00/0000