JOURNAL OF PHYSICAL ORGANIC CHEMISTRY J. Phys. Org. Chem. 2004; 17: 656–664 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.801 298 K enthalpies of formation of monofluorinated alkanes: theoretical predictions for methyl, ethyl, isopropyl and tert-butyl fluoride Bethany L. Kormos, 1 Joel F. Liebman 2 and Christopher J. Cramer 1 * 1 Department of Chemistry and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, USA 2 Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA Received 18 December 2003; revised 9 February 2004; accepted 11 February 2004 epoc ABSTRACT: Experimentally measured 298 K enthalpies of formation are not well established for monofluoroalk- anes. To supplement available experimental data, the multi-coefficient G3 (MCG3) quantum mechanical model has been applied to estimate this thermochemical quantity for methyl fluoride, ethyl fluoride, 2-fluoropropane (isopropyl fluoride) and 2-fluoro-2-methylpropane (tert-butyl fluoride). The following 298 K standard enthalpies of formation are suggested for these monofluoroalkanes: ÁH  f ;298 (MeF) ¼57.1  0.2 kcal mol 1 , ÁH  f ;298 (EtF) ¼66.5  0.4 kcal mol 1 , ÁH  f ;298 (i-PrF) ¼75.4  0.5 kcal mol 1 , and ÁH  f ;298 (t-BuF) ¼86.0  2.0 kcal mol 1 (1 kcal ¼ 4.184 kJ). Copyright # 2004 John Wiley & Sons, Ltd. Additional material for this paper is available in Wiley Interscience KEYWORDS: enthalpy of formation; fluoroalkanes; methyl fluoride; ethyl fluoride; isopropyl fluoride; tert-butyl fluoride INTRODUCTION Owing to their unique properties, interest in fluorinated compounds continues unabated. Their thermochemical properties, and in particular their standard enthalpies of formation, have been of key importance. However, as perusal of relevant review articles and book chapters shows, 1–4 the majority of research has been focused on species characterized by a very high degree of fluorination. By contrast, very few contemporaneous experimental thermochemical studies address monofluorinated spe- cies. 5 Insofar as the effects of fluorine substitution are manifestly non-additive, we are thus missing key data for furthering our understanding of the energetics of organic compounds. For example, it is well accepted that the following reaction is significantly exothermic: 1–4 4CH 3 F ! 3CH 4 þ CF 4 However, we are thwarted from knowing the exothermi- city quantitatively because of the absence of enthalpy of formation data for methyl fluoride from the experimental calorimetric literature. Even in those instances where the experimental litera- ture has addressed monofluorinated alkanes, the results have been open to question with respect to their accu- racy. 6–8 For example, there is only one measured enthalpy of formation for a sec-alkyl fluoride, that of isopropyl fluoride. 9 The value reported differs from that for the iso- meric n-propyl fluoride by 1.8  0.3 kcal mol 1 (1 kcal ¼ 4.184 kJ). 10 This difference is substantially smaller than that observed for the corresponding isomeric amines (3.3  0.1 kcal mol 1 ), alcohols (4.2  0.1 kcal mol 1 ) and the other halides (Cl, 3.1  0.3; Br, 3.0  0.7; I, 2.4  0.9 kcal mol 1 ). 10 Not only are enthalpies of formation rather rare in the literature for alkyl fluorides, but so too are enthalpies of reaction. Again, for those cases where data are available, there are troubling issues. For example, from an analysis of measured gas-phase dissociative protonation attach- ment reactions, Abboud et al. 11 asserted the halogen exchange reaction 1-AdF þ HCl ! HF þ 1-AdCl (Ad ¼ adamantyl) to have a Gibbs free energy change of 6.4  2.8 kcal mol 1 . If we assume that the entropy change is negligible, the enthalpy of reaction is the same value (this entropy assumption seems reasonable, insofar as the entropy change for MeF þ HCl ! HF þ MeCl Copyright # 2004 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2004; 17: 656–664 *Correspondence to: C. J. Cramer, Department of Chemistry and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, USA. E-mail: cramer@chem.umn.edu Contract/grant sponsor: National Science Foundation; Contract/grant number: CHE-0203346.