Section: 10 | Proton Affinities |
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John R. Rumble, ed., CRC Handbook of Chemistry and Physics, 102nd Edition (Internet Version 2021), CRC Press/Taylor & Francis, Boca Raton, FL.
If a specific table is cited, use the format: "Physical Constants of Organic Compounds," in CRC Handbook of Chemistry and Physics, 102nd Edition (Internet Version 2021), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.


Proton affinity is a useful parameter for describing gas-phase ion-molecule reactions in fields such as atmospheric chemistry, plasma chemistry, mass spectrometry, and astrophysics. The proton affinity Epa (often designated in the literature as PA) of a molecular species M is defined as the negative of the enthalpy change for the gas-phase reaction

M + H+ → MH+

A closely related quantity is the gas-phase basicity ΔbaseG° (often designated as GB), which is the negative of the Gibbs energy change for the same reaction. Thus, the two are related by

ΔbaseG° = Epa + TΔS

where T is the temperature and ΔS is the entropy change in the reaction (which can be calculated if the molecular structure of M and M+ is known).

Direct measurement of the proton affinity is possible for only a few molecules, mainly olefins and carbonyl compounds. However, these measurements have been used to establish a scale of Epa values that permits proton affinities to be determined for many other molecules, including unstable species and reaction intermediates. The basis for this scale is described by Hunter and Lias in Ref. 1.

The Epa and ΔbaseG° values at a temperature of 298 K are tabulated below for selected molecules. Many values are given to one decimal place, but the majority are not accurate to better than one or two kilojoules per mole. The methods of measurement are described in Ref. 1, which contains a much more extensive and detailed tabulation.

Column definitions for the table are as follows.

Column heading Definition
Name Name of compound; listed alphabetically by name
Mol. form. Molecular formula of compound, in Hill order
Epa Proton affinity, in units kJ mol-1
ΔbaseG° Gas-phase basicity, in units kJ mol-1 (negative of the Gibbs energy change for the reaction)
Note Reference or additional information of interest


  1. Hunter, E. P. L., and Lias, S. G., J. Phys. Chem. Ref. Data 27, 413, 1998. []
  2. Hunter, E. P., and Lias, S. G., “Proton Affinity Evaluation”, in NIST Chemistry WebBook, NIST Standard Reference Database No. 69, Linstrom, P. J., and Mallard, W. G., Eds., March 2003, National Institute of Standards and Technology, Gaithersburg, MD 20899, <>.
  3. Do, K., Klein, T. P., Pommerening, C. A., Bachrach, S. M., and Sunderlin, L. S., J. Am. Chem. Soc. 120, 6093, 1998. []
  4. Kim, H.-T., Green, R. J., Qian, J., and Anderson, S. L., J. Chem. Phys. 112, 5717, 2000. []
  5. Park, S. T., Kim, S. K., and Kim, M. S., J. Chem. Phys. 114, 5568, 2001. []
  6. Hiraoka, K., Mizuno, T., Eguchi, D., Takao, T., and Ino, S., J. Chem. Phys. 116, 7574, 2002. []
  7. Oresmaa, L. O., Haukka, M., Vainiotalo, P., and Pakkanen, T. A., J. Org. Chem. 67, 8216, 2002. []
  8. Wang, F., Ma, S., Zhang, D., and Cooks, R. G., J. Phys. Chem. A 102, 2988, 1998. []
  9. Bouchoux, G., Gal, J.-F., Szulejko, J. E., McMahon, T. B., Tortajada, J., Luna, A., Yanez, M., and Mo, O., J. Phys. Chem. A 102, 9183, 1998. []
  10. van Beelen, E., Koblenz, T. A., Ingemann, S. and Hammerum, S., J. Phys. Chem. A 108, 2728, 2004. []

Proton Affinities and Gas-Phase Basicities

NameSynonymMol. form.FormulaCAS Reg. No.Mol. wt.Epa/kJ mol-1ΔbaseGº/kJ mol-1Note
Continued on next page...
Acenaphthene1,2-DihydroacenaphthyleneC12H10C12H1083-32-9154.207 851.7 821.0
AcetaldehydeEthanalC2H4OCH3CHO75-07-044.052 768.5 736.5
AcetamideEthanamideC2H5NOCH3CONH260-35-559.067 863.6 832.6
Acetic acidEthanoic acidC2H4O2CH3COOH64-19-760.052 783.7 752.8
Acetic anhydrideAcetyl acetateC4H6O3C4H6O3108-24-7102.089 844Ref. 9
Acetohydroxamic acidN-HydroxyacetamideC2H5NO2546-88-375.067 854.0 823.0
Acetone2-PropanoneC3H6O(CH3)2CO67-64-158.079 812 782.1
AcetonitrileMethyl cyanideC2H3NCH3CN75-05-841.052 779.2 748
AcetophenoneMethyl phenyl ketoneC8H8OC6H5C=OCH398-86-2120.149 861.1 829.3
4-AcetylanisoleC9H10O2100-06-1150.174 895.6 863.7
4-AcetylbenzonitrileC9H7NO1443-80-7145.158 826.8 795.0
AcetyleneEthyneC2H2HC≡CH74-86-226.037 641.4 616.7
4-Acetylphenyl acetateC10H10O313031-43-1178.184 853.2 821.3
4-AcetylthioanisoleC9H10OS1778-09-2166.239 888.2 856.3
AcridineDibenzo[b,e]pyridineC13H9N260-94-6179.217 972.6 940.7
Acrolein2-PropenalC3H4OCH2=CHCHO107-02-856.063 797.0 765.1
Acrylamide2-PropenamideC3H5NOC3H5NO79-06-171.078 870.7 839.8
AcrylonitrilePropenenitrileC3H3NCH2=CHCN107-13-153.063 784.7 753.7
Adenine1H-Purin-6-amineC5H5N5C5H5N573-24-5135.128 942.8 912.5

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