Revision of the gas-phase acidity scale below 300 kcal/mol

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Revision of the gas-phase acidity scale below 300 kcal/mol Elin Raamat, Agnes Kütt, Jaan Saame, Karin Kipper, Ilmar A. Koppel, Ivar Koppel, Ivo Leito Institute of Chemistry, University of Tartu Ravila 14A, Tartu 50411, Estonia, elin.raamat@ut.ee GA (kcal mol-1) 285 295 305 275 290 300 280 Gas-phase measurements yield intrinsic acidities of the individual molecules, free of any medium effects. The gas-phase Brønsted acidity (GA) of an acid HA is the Gibbs’ free energy change on deprotonation of the acid according to the following equilibrium: GA HA A – + H+ 307.5 1.3 306.6 1.9 1.3 3.1 305.6 5.8 3.0 304.2 5.0 2.5 302.6 1.8 1.5 301.6 0.6 302.2 300.9 0.8 3.1 The gas-phase acidity (GA) scale from 307.5 kcal·mol-1 ((CF3CO)2NH) to 283.7 kcal·mol-1 ((C2F5SO2)2NH) was reexamined using the Fourier transform cyclotron resonance (FT-ICR) equilibrium measurement approach 1. 2.6 0.3 300.6 1.9 0.3 300.0 1.6 1.6 4.1 0.7 299.8 1.7 299.0 3.5 0.4 2.8 298.8 1.9 1.4 3.9 297.5 2.0 3.8 297.0 2.8 0.8 296.1 296.0 The need for verification was suggested Recently2 by high-level computations of the GA values of some strong acids. It was found that for acids stronger than ca 300 kcal·mol-1 the computed GA values seem to be systematically lower (i.e. acid stronger) than the experimental values from ref 3. 3.6 294.8 3.0 3.1 5.8 4.3 293.3 2.3 3.5 291.1 0.1 291.1 Gas-phase acidities of 18 compounds were determined for the first time but also some of the acid used previously 3 were used. 1.8 290.3 3.0 289.2 1.7 3.3 1.5 ≥2.4 287.8 287.5 An extensive set of substituted aromatic sulfonimides were used: Easily obtainable Acidity can be varied in small steps over a wide acidity range using different substituents. The additivity of substituent effects. Already used to set up the acidity scale of strong acids in acetonitrile (AN).4 1.3 1.0 286.5 1.4 2.2 285.2 2.8 2.8 2.6 1.6 1.5 284.2 0.4 283.7 The results reveal a contraction of the previously published values in the lower part of the scale (GA = 300…284 kcal·mol-1). The present results can be considered more reliable than in ref 3, because: A number of technical modifications were introduced to the earlier method. The whole range of gas-phase acidities from (CF3CO)2NH to (CF3SO2)2NH was be “covered” by three independent “pathways” of ΔGA measurements. The relative acidity of any two acids can be obtained at least two independent sets of ΔGA measurements. The consistency standard deviation is 0.3 kcal·mol-1. An excellent additivity of the substituent effects in the aromatic sulfonimides family can be observed. 1 Leito, I.; Raamat, E.; Kütt, A.; Saame, J.; Kipper, K.; Koppel, I.A.; Koppel, I.; Zhang, M.; Mishima, M.; Yagupolskii, L. M.; Garlyauskayte, R. Yu.; Filatov, A. A. J. Phys. Chem. A. 2009, 113, 8421-8424. 2 Gutowski, K. E.; Dixon, D. A. Ab Initio Prediction of the Gas- and Solution-Phase Acidities of Strong Brønsted Acids: The Calculation of pKa Values Less Than –10. J.Phys.Chem. 2006, 110,12044-12054. 3 KoppeI, I. A.; Taft, R. W.; Anvia, F.; Zbu, S.-Z.; Hu, L.-Q.; Sung, K.-S.; DesMarteau, D. D.; Yagupolskii, L. M.; Yagupolskii, Y. L.; Ignat'ev, N. V.; Kondratenko, N. V.; Volkonskii, A. Y.; Vlasov, V. M.; Notario, R.; Maria, P.-C. The Gas-Phase Acidities of Very Strong Neutral Brønsted Acids. J. Am. Chem. Soc. 1994, 116, 3047-3057. 4 Kütt, A.; Leito, I.; Kaljurand, I.; Sooväli, L.; Vlasov, V. M.; Yagupolskii, L. M.; Koppel, I. A. A Comprehensive Self-Consistent Spectrophotometric Acidity Scale of Neutral Brønsted Acids in Acetonitrile. J. Org. Chem. 2006, 71, 2829-2838.