The kinetic method is used to corroborate the description of ground state ionized α-diketones as loosely electron-bonded acylium ion dimers: R 1-C=O+ - e- - +O=C-R2. The abundance ratio of both the acylium ion fragments R1CO + and R2CO+ (summed to those of their respective secondary fragments) formed upon low energy (5 eV) collision-induced dissociation of several ionized α-diketones is found to correlate linearly with the ionization energies (IEs) of the corresponding R 1CO• and R2CO• free radicals as predicted by density functional theory calculations at the B3LYP/6-311++G(d,p) level. However, when these abundances are taken from 70 eV electron ionization mass spectra, lower and sometimes inverted ratios (2,3-pentanedione and 2,3-hexanedione) are observed. Inverted ratios are also observed via charge-exchange mass spectrometry/mass spectrometry (MS/MS) experiments for ionized 2,3-pentanodione formed with relatively high internal energies. Ionized α-diketones are found to display an effective temperature of 1705 K, which indicates an intermediate loosely-bonded nature. B3LYP/6-311++G(d,p) optimized geometries and charge and spin densities also corroborate the description of ground state ionized α-diketones as loosely electron-bonded diacylium ion dimers.94295304Cooks, R.G., Kruger, T.L., Intrinsic basicity determination using metastable ions (1977) J. Am. Chem. Soc., 99, p. 1279Cooks, R.G., Patrick, J.S., Kotiaho, T., McLuckey, S.A., Thermochemical determinations by the kinetic method (1994) Mass Spectrom. Rev., 13, p. 287Cooks, R.G., Wong, P.S.H., Kinetic method of making thermochemical determinations: Advances and applications (1998) Acc. Chem. Res., 31, p. 379Yang, S.S., Chen, G.D., Ma, S.G., Cooks, R.G., Gozzo, F.C., Eberlin, M.N., Relative carbonyl isocyanate cation [OCNCO](+) affinities of pyridines determined by the kinetic method using multiple-stage (MS(3)) mass-spectrometry (1995) J. Mass Spectrom., 30, p. 807Wong, P.S.H., Ma, S.G., Yang, S.S., Cooks, R.G., Gozzo, F.C., Eberlin, M.N., Sulfur trifluoride cation (SF3 +) affinities of pyridines determined by the kinetic method: Stereoelectronic effects in the gas phase (1997) J. Am. Soc. Mass Spectrom., 8, p. 68Wong, P.S.H., Ma, S.G., Wang, F., Cooks, R.G., Stereoelectronic effects and gas phase Co+, Ni+, CpFe+, CpCo+ and CpNi+ affinities of pyridines studied by the kinetic method (1997) J. Organomet. Chem., 539, p. 131Schroeter, K., Wesendrup, R., Schwarz, H., Substituent effects on the bond-dissociation energies of cationic arene-transition-metal complexes (1998) Eur. J. Org. Chem., p. 565Stöckigt, D., Hrusák, J., Schwarz, H., Isotope effects in radiative cooling - The Al(C6H 6)(+) systems (1995) Int. J. Mass Spectrom. Ion Proc., 150, p. 1Wright, L.G., McLuckey, S.A., Cooks, R.G., Wood, K.V., Relative gas-phase acidities from triple quadrupole mass spectrometers (1982) Int. J. Mass Spectrom. Ion Proc., 42, p. 115McLuckey, S.A., Cameron, D., Cooks, R.G., Proton affinities from dissociations of proton-bound dimers (1981) J. Am. Chem Soc., 103, p. 1313Boand, G., Houriet, R., Gaumann, T., Gas-phase acidity of aliphatic-alcohols (1983) J. Am. Chem. Soc., 105, p. 2203Ma, S.G., Wang, F., Cooks, R.G., Gas-phase acidity of urea (1998) J. Mass Spectrom., 33, p. 943Burinsky, D.J., Fukuda, E.K., Campana, J.E., Electron-affinities from dissociations of mixed negative-ion dimers (1984) J. Am. Chem. Soc., 106, p. 2770Chen, G., Cooks, R.G., Corpuz, E., Scott, L.T., Estimation of the electron affinities of C-60, corannulene, and coronene by using the kinetic method (1996) J. Am. Soc. Mass Spectrom., 7, p. 619Denault, J.W., Chen, G.D., Cooks, R.G., Electron affinity of 1,3,5,7-cyclooctatetraene determined by the kinetic method (1998) J. Am. Soc. Mass Spectrom., 9, p. 1141Chen, G.D., Ma, S.G., Cooks, R.G., Bronstein, H.E., Best, M.D., Scott, L.T., Electron affinities and C-60 anion clusters of certain bowl-shaped polycyclic aromatic hydrocarbons (1997) J. Mass Spectrom., 32, p. 1305Chen, G.D., Cooks, R.G., Estimation of ionization energies of polycyclic aromatic hydrocarbons using the kinetic method (1997) J. Mass Spectrom., 32, p. 333Eberlin, M.N., Kotiaho, T., Shay, B.J., Yang, S.S., Cooks, R.G., Gas-phase Cl+ affinities of pyridines determined by the kinetic method using multiple-stage [MS(3)] mass-spectrometry (1994) J. Am. Chem. Soc., 116, p. 2457Ma, S.G., Wong, P., Cooks, R.G., Gozzo, F.C., Eberlin, M.N., Stereoelectronic effects in phosphorus dichloride cation pyridine complexes (1997) Int. J. Mass Spectrom. Ion Process, 163, p. 89Yang, S.S., Wong, P., Ma, S.G., Cooks, R.G., SiCl3 + and SiCl+ affinities for pyridines determined by using the kinetic method with multiple stage mass spectrometry: Agostic effects in the gas phase (1996) J. Am. Soc. Mass Spectrom., 7, p. 198Wang, F., Ma, S., Wong, P., Cooks, R.G., Gozzo, F.C., Eberlin, M.N., Gas phase agostic bonding in pyridine SiFn + (n = 1, 3) cluster ions investigated by the kinetic method (1998) Int. J. Mass Spectrom. Ion Proc., 180, p. 195Jarrold, M.F., Bower, J.E., Collision-induced dissociation of metal cluster ions - Bare aluminum clusters, Al+ (n = 3-26) (1987) J. Chem. Phys., 86, p. 3876Grese, R.P., Cerny, R.L., Gross, M.L., Senge, M., Determination of structure and properties of modified chlorophylls by using fast atom bombardment combined with tandem mass-spectrometry (1990) J. Am. Soc. Mass Spectrom., 1, p. 172Hanley, L.H., Whitten, J.L., Anderson, S.L., Collision-induced dissociation and Ab initio studies of boron cluster ions - Determination of structures and stabilities (1992) J. Phys. Chem., 92, p. 5803Cheng, X.H., Wu, Z.C., Fenselau, C., Collision energy-dependence of proton-bound dimer dissociation - Entropy effects, proton affinities, and intramolecular hydrogen-bonding in protonated peptides (1993) J. Am. Chem. Soc., 115, p. 4844Cerda, B.A., Wesdemiotis, C., Li+, Na+, and K+ binding to the DNA and RNA nucleobases. Bond energies and attachment sites from the dissociation of metal ion-bound heterodimers (1996) J. Am. Chem. Soc., 118, p. 11884Cerda, B.A., Hoyau, S., Ohanesian, G., Wesdemiotis, C., Na+ binding to cyclic and linear dipeptides. Bond energies, entropies of Na+ complexation, and attachment sites from the dissociation of Na+-bound heterodimers and ab initio calculations (1998) J. Am. Chem. Soc., 120, p. 2437Gozzo, F.C., Eberlin, M.N., Primary and secondary kinetic isotope effects in proton (H +/D+) and chloronium ion (35Cl(+)/ 37Cl(+)) affinities (2001) J. Mass Spectrom., 36, p. 1140Tao, W.A., Zhang, D.X., Wang, F., Thomas, P.D., Cooks, R.G., Kinetic resolution of D,L-amino acids based on gas-phase dissociation of copper(II) complexes (1999) Anal. Chem., 71, p. 4427Shen, W.Y., Wong, P.S.H., Cooks, R.G., Stereoisomeric distinction by the kinetic method: 2,3-Butanediol (1997) Rapid. Commun. Mass Spectrom., 11, p. 71Guo, J.H., Wu, J.Y., Siuzdak, G., Finn, M.G., Measurement of enantiomeric excess by kinetic resolution and mass spectrometry (1999) Angew. Chem. Int. Ed., 38, p. 1755Vékey, K., Czira, G., Distinction of amino acid enantiomers based on the basicity of their dimers (1997) Anal. Chem., 69, p. 1700Tao, W.A., Gozzo, F.C., Cooks, R.G., Mass spectrometric quantitation of chiral drugs by the kinetic method (2001) Anal. Chem., 73, p. 1692Tao, W.A., Cooks, R.G., Chiral analysis by MS (2003) Anal. Chem., 75, pp. 25AAugusti, D.V., Carazza, F., Augusti, R., Tao, W.A., Cooks, R.G., Quantitative chiral analysis of sugars by electrospray ionization tandem mass spectrometry using modified amino acids as chiral reference compounds (2002) Anal. Chem., 74, p. 3458Drahos, L., Vékey, K., How closely related are the effective and the real temperature (1999) J. Mass Spectrom., 34, p. 79Denault, J.W., Wang, F., Cooks, R.G., Gozzo, F.C., Eberlin, M.N., Structural characterization of clusters formed from alkyl nitriles and the methyl cation (2000) J. Phys. Chem. A, 104, p. 11290Burinsky, D.J., Fukuda, E.K., Campana, J.E., Electron-affinities from dissociations of mixed negative-ion dimers (1984) J. Am. Chem. Soc., 106, p. 2770Wong, P.S.H., Ma, S.G., Cooks, R.G., Ionization energy determination by the kinetic method (1996) Anal. Chem., 68, p. 4254Chen, G.D., Wong, P., Cooks, R.G., Estimation of free radical ionization energies by the kinetic method and the relationship between the kinetic method and the Hammett equation (1997) Anal. Chem., 69, p. 3641Eberlin, M.N., Triple-stage pentaquadrupole (QqQqQ) mass spectrometry and ion/molecule reactions (1997) Mass Spectrom. Rev., 16, p. 113Juliano, V.F., Gozzo, F.C., Eberlin, M.N., Kascheres, C., Dolago, C.L., Fast multidimensional (3D and 4D) MS(2) and MS(3) scans in a high-transmission pentaquadrupole mass spectrometer (1996) Anal. Chem., 68, p. 1328Tiernan, T.O., Futrell, J.H., Ionic reactions in unsaturated compounds. 2. Ethylene (1968) J. Phys. Chem., 72, p. 3080Becke, A.D., Density-functional thermochemistry. 3. The role of exact exchange (1993) J. Chem. Phys., 98, p. 5648Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Zakrzewski, V.G., Pople, J.A., (1998) Gaussian 98, Revision A.6., , Gaussian, Inc., Pittsburgh, PA, USADrahos, L., Vekey, K., MassKinetics: A theoretical model of mass spectra incorporating physical processes, reaction kinetics and mathematical descriptions (2001) J. Mass Spectrom., 36, p. 237Lias, S.G., Bartmess, J.E., Liebman, J.F., Holmes, J.L., Levin, R.D., Mallard, W.G., Gas-phase ion and neutral thermochemistry (1988) J. Phys. Chem. Reference Data, 17, p. 1Stevenson, D.P., (1951) Discussions of the Faraday Society, 10, p. 35Depuy, C.H., Bierbaum, V.M., Damrauer, R., Relative gas-phase acidities of the alkanes (1984) J. Am. Chem. Soc., 106, p. 4051Damrauer, R., Depuy, C.H., Barlow, S.E., Gronert, S., The gas-phase chemistry of the silaacetylide anion, HCSi- (1988) J. Am. Chem. Soc., 110, p. 2005Murad, E., Inghram, M.G., Thermodynamic properties of acetyl radical + bond dissociation energies in aliphatic carbonyl compounds (1964) J. Chem. Phys., 41, p. 404Griffin, L.L., Traeger, J.C., Hudson, C.E., McAdoo, D.J., Why are smaller fragments preferentially lost from radical cations at low energies and larger ones at high energies? An experimental and theoretical study (2002) Int. J. Mass Spectrom., 217, p. 23Robinson, P.J., Holbrook, K.A., (1972) Unimolecular Reactions, , Wiley-Interscience, New YorkBaer, T., Mayer, P.M., Statistical Rice-Ramsperger-Kassel-Marcus quasiequilibrium theory calculations in mass spectrometry (1997) J. Am. Soc. Mass Spectrom., 8, p. 103Derrick, P.J., Lloyd, P.M., Christie, J.R., (1995) Adv. Mass Spectrom., 13, p. 23McLuckey, S.A., Collision energy effects in tandem mass-spectrometry as revealed by a proton-bound dimer ion (1984) Org. Mass Spectrom., 19, p. 545Lund, K.H., Bojesen, G., Measurements of the kinetic energy released in decompositions of proton-bound dimers of primary amines (1996) Int. J. Mass Spectrom. Ion Processes, 156, p. 203Lias, S.G., Bartmess, J.E., Liebman, J.F., Holmes, J.L., Levin, R.D., Mallard, W.G., (2001) Ion Energetics Data, , Ed by P.J. Linstrom and W.G. Mallard. National Institute of Standards and Technology, Gaithersburg, MDMcLuckey, S.A., Cameron, D., Cooks, R.G., Proton affinities from dissociations of proton-bound dimers (1981) J. Am. Chem. Soc., 103, p. 1313McLuckey, S.A., Cooks, R.G., Fulford, J.E., Gas-phase thermochemical information from triple quadrupole mass spectrometers - Relative proton affinities of amines (1983) Int. J. Mass Spectrom. Ion Phys., 52, p. 165Trikoupis, M.A., Terlouw, J.K., Burgers, P.C., Peres, M., Lifshitz, C., How do dimethyl oxalate ions CH3O-C(=O)-C(=O)-OCH 3 •+ break in half? Loss of CH3 • + CO2 versus CH3O-C=O (1999) J. Am. Soc. Mass Spectrom., 10, p. 869. , and references cited thereinDewar, M.J.S., Zoebisch, E.G., Healy, E.F., Stewart, J.J.P., The development and use of quantum-mechanical molecular-models. 76. Am1 - A new general-purpose quantum-mechanical molecular-model (1985) J. Am. Chem. Soc., 107 (13), p. 3902Kishore, K., Anklam, E., Aced, A., Asmus, K.D., Formation of intramolecular three-electron-bonded 2 sigma/1 sigma* radical cations upon reduction of dialkylsulfinyl sulfides by H-atoms (2000) J. Phys. Chem. A, 104, p. 9646De Visser, S.P., Bickelhaupt, F.M., De Koning, L.J., Nibbering, N.M.M., Sulfur-sulfur three-electron bond dissociation enthalpies of dialkyl sulfide dimer radical cations (1998) Int. J. Mass Spectrom., 180, p. 43Harcourt, R.D., Valence bond and molecular orbital descriptions of the three-electron bond (1997) J. Phys. Chem. A, 101, p. 5962