Heats of Formation of the Propionyl Ion and Radical and 2,3-Pentanedione by Threshold Photoelectron Photoion Coincidence Spectroscopy James P. Kercher, † Elizabeth A. Fogleman, † Hideya Koizumi, † Ba ´ lint Szta ´ ray, ‡ and Tomas Baer* ,† Department of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290, and Department of General and Inorganic Chemistry, Eo ¨tVo ¨s Lora ´ nd UniVersity, Budapest, Hungary ReceiVed: September 14, 2004; In Final Form: NoVember 19, 2004 The dissociative photoionization onsets for the formation of the propionyl ion (C 2 H 5 CO + ) and the acetyl ion (CH 3 CO + ) were measured from energy selected butanone and 2,3-pentanedione ions using the technique of threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Ion time-of-flight (TOF) mass spectra recorded as a function of the ion internal energy permitted the construction of breakdown diagrams, which are the fractional abundances of ions as a function of the photon energy. The fitting of these diagrams with the statistical theory of unimolecular decay permitted the extraction of the 0 K dissociation limits of the first and second dissociation channels. This procedure was tested using the known energetics of the higher energy dissociation channel in butanone that produced the acetyl ion and the ethyl radical. By combining the measured dissociative photoionization onsets with the well-established heats of formation of CH 3 • , CH 3 CO + , CH 3 CO • , and butanone, the 298 K heats of formation, Δ f H° 298K , of the propionyl ion and radical were determined to be 618.6 ( 1.4 and -31.7 ( 3.4 kJ/mol, respectively, and ∆ f H° 298K [2,3-pentanedione] was determined to be -343.7 ( 2.5 kJ/mol. This is the first experimentally determined value for the heat of formation for 2,3- pentanedione. Ab initio calculations at the Weizmann-1 (W1) level of theory predict ∆ f H° 298K values for the propionyl ion and radical of 617.9 and -33.3 kJ/mol, respectively, in excellent agreement with the measured values. Introduction Establishing the heats of formation of radicals, ions, and neutrals by measuring dissociative photoionization onsets is based on the following reaction: in which the heats of formation of the three species are related to the threshold energy, E 0 , by the thermochemical cycle: The ideal reaction should meet several criteria, among which are the following: (a) there should be no activation energy for the reverse reaction, (b) the heats of formation of two of the three species must be well established, and (c) the reaction of interest should in general be the lowest energy dissociation channel. The last requirement is a result of the so-called competitive shift, 1-3 which shifts the observed onset for a higher energy channel to higher energies. This is because, at the dissociation limit for the second channel, the rate of the lowest energy reaction may be orders of magnitude higher than the rate of the second reaction, thereby preventing the observation of products at the dissociation limit. In this paper, we utilize the statistical theory of unimolecular decay 4 to model the experimental data for higher energy dissociation channels in order to remove this last limitation associated with the photo- ionization method. The benefit of this analysis is the ability to investigate new species not otherwise accessible. We have recently studied the heats of formation of the acetyl radical (CH 3 CO • ) and ion (CH 3 CO + ) through the photoioniza- tion of acetone and butanedione. 5 In the present study, we use two starting molecules and three reactions to establish the heats of formation of the propionyl radical (C 2 H 5 CO • ), the propionyl ion (C 2 H 5 CO + ), and 2,3-pentanedione (C 2 H 5 COCOCH 3 ). The reactions involved are the following: The heat of formation of butanone is known to within 1 kJ/ mol, as are the heats of formation of CH 3 • ,C 2 H 5 • , CH 3 CO + , and CH 3 CO • . 5 With the aid of velocity focusing optics for electrons and a method for the subtraction of the “hot” electron contamination in the threshold signal, 6 we are now able to determine the first dissociation onsets to within 1 kJ/mol and the second dissociation onsets to within 2 kJ/mol. The propionyl ion production channels (3a and 4a) are the lowest energy dissociation channels, whereas the acetyl ion production chan- nels (3b and 4b) are the second lowest energy dissociation channels. We can test our ability to extract the second onset energies by using the known thermochemistry of reaction 3b. The onset of the C 2 H 5 CO + ion from butanone (reaction 3a) was investigated some years ago by Murad and Inghram 7 as † University of North Carolina. ‡ Eo ¨tvo ¨s Lora ´nd University. AB + hν f A + + B (1) E 0 ) ∆ f H° [A + ] + ∆ f H° [B] - ∆ f H° [AB] (2) C 2 H 5 COCH 3 + hν f C 2 H 5 CO + + CH 3 • (3a) f CH 3 CO + + C 2 H 5 • (3b) C 2 H 5 COCOCH 3 + hν f C 2 H 5 CO + + CH 3 CO • (4a) f CH 3 CO + + C 2 H 5 CO • (4b) 939 J. Phys. Chem. A 2005, 109, 939-946 10.1021/jp0458497 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/19/2005