Evaluated Gas Phase Basicities and Proton Affinities of Molecules; Heats of Formation of Protonated Molecules Cite as: Journal of Physical and Chemical Reference Data 13, 695 (1984); https://doi.org/10.1063/1.555719 Published Online: 15 October 2009 Sharon G. Lias, Joel F. Liebman, and Rhoda D. Levin ARTICLES YOU MAY BE INTERESTED IN Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update Journal of Physical and Chemical Reference Data 27, 413 (1998); https:// doi.org/10.1063/1.556018 Density-functional thermochemistry. III. 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National Bureau of Standards, Gaithersburg, MD 20899 The available data on gas phase basicities and proton affinities of molecules are compiled and evaluated. Tables giving the molecules ordered (1) according to proton affinity and (2) according to empirical formula, sorted alphabetically are provided. The heats of formation of the molecules and the corresponding protonated species are also listed. Key words: basicity, heats of formation, ion-molecule reaction, proton affinity, proton transfer. Contents 1. Introduction......................................................... 696 3.2.4. Propylene .... ............. .... ............. 700 1.1. Definitions................................................ 696 3.2.5. Formaldehyde........................... 701 1.2. Proton Affinities and Gas Phase 3.2.6. Water ......................................... 701 Basicities: An Overview.......................... 696 3.2.7. Ethylene.... ........ ........................ 702 2. Types of Experiment Leading to Proton 3.2.8. Carbon monoxide ..................... 702 Affinity/Gas Basicity Data.. ....... ....... ..... 696 3.2.9. Carbon dioxide ......................... 702 2.1. Absolute Proton Affinity Values from 3.2.10. Oxygen atom............................. 702 Ionization Thresholds.............................. 697 3.2.11. Molecular oxygen...... ....... ........ 702 2.2. Proton Affinity Values from Thermoche­ 3.3. Summary of Thermochemical Data on mical Information Derived from Hydride Comparison Standards for Proton Affinity Transfer Equilibrium .Constant Determi- Scale ......................................................... 703 nations .......... ............. .......... ....... .............. 697 4. Experimental Error Limits.... .......... ................... 704 2.3. Relative Gas Basicity Values from Proton 5. Explanation of the Tables ................................... 704 Transfer Equilibrium Constants ....... ....... 697 5.1. Explanation of Table 1 ............................ 704 2.4. Relative Gas Basicity/Proton Affinity S.1.1. List of Symbols Used in Table 1 ...... 707 Values from "Bracketing"....................... 697 5.2. Explanation of Table 2 ............................ 708 2.5. Other Sources of Proton Affinity/Gas 5.2.1. Conventions Used..................... 708 Basicity Data ...... ............. .............. ........... 698 5.2.2. Heats of Formation of Neutral 3. Description of the Evaluation... .............. ........... 698 Molecules........................... ....... 708 3.1. Thermodynamic Ladders........................ 698 5.2.3. References ................................. 710 3.1.1. Entropy Changes...................... 698 6. Literature Coverage........... ................................. 710 3.1.2. Temperature.............................. 698 7. References ............................................................ 711 3.2. Assignment of Absolute Values to the Scale; Reference Standards ....... .............. 699 3.2.1. Ammonia....... .......... .................. 699 List of Tables 3.2.2. Ketene....................................... 700 3.2.3. Isobutene ................................... 700 1. Gas phase basicities and proton affinities .......... 713 Annotated references to Table 1 ........................ 772 2. Proton affinities and heats of formation of @ 1984 by the U.S. Secretary of Commerce on behalf of the United States. molecules and corresponding protonated This copyright is assigned to the American Institute of Physics and the American Chemical Society. species ................................................................... 782 Reprints available from ACS; see Reprint List at back of issue. References to Table 2 ......................................... 806 0047-2689/84/030695-99/$12.00 695 J. Phys. Chern. Ref. Data, Vol. 13, No.3, 1984 696 LIAS, LIEBMAN, AND LEVIN 1. Introduction change of reaction, which represents the relative gas phase basicities of compounds M and N; if the entropy 1.1. Definitions change of the reaction is determined, or can be reliably estimated, a value for the enthalpy change of reaction, The gas phase basicity and proton affinity of a or relative proton affinity, is obtained. Thus, the molecule, M, are both defined in terms of the extensive scales of data presented here based on hypothetical reaction: equilibrium constant measurements provide relative gas phase basicities and relative proton affinities, but do· not directly give any information about absolute values of (1) proton affinities, i.e., about the actual enthalpy changes of reaction 1 for the various compounds. Absolute The gas phase basicity is the negative of the free energy values must be assigned based on some comparison chan.ge ~ssociated with this reaction, while the proton standard incorporated in the thermochemical ladder for affimty IS the negative of the corresponding enthalpy which heats of formation of both M and MH+ are change. independently available. These species will necessa~ily be limited to those for which an MH+ ion of known structure can be generated in a mass spectrometer, so 1.2. Proton Affinities and Gas Phase Basicities: that a reliable independent gas phase heat of formation An Overview will be known. Thus, it must be emphasized that for data derived Proton transfer reactions are of considerable from equilibrium constant measurements, absolute importance in chemistry. Acid-base reactions have been values for proton affinities cited by authors depend on ~tudied extensively in solution for decades, but it is only the proton affinity value selected for a comparison m the past twenty years or so that experimental standard, and these may vary considerably from year to techniques have been devised to permit the quantitative year, and from paper to paper. (For example, proton study of the· thermochemistry of proton transfer affinities varying from 202 kcallmol to 210 kcallmol r~actions in the gas phase. Particularly in the years have been cited for ammonia, which is often used as a Slllce 1971, when the first gas phase ion-molecule comparison standard for scales of proton affinities). equilibrium studies on proton transfer appeared, there Although researchers working in the field are usually has been a burgeoning of data jn the literature, and acutely aware of these arbitrary fluctuations in absolute consequently, a great interest in using information from values assigned to gas phase proton affinities, workers such gas phase studies to distinguish between the in related fields who seek to use these data often fall chemical effects of solvation and the effects of intrinsic unwarily into the trap of using proton affinities from the molecular properties. To date, although several literature without realizing that the experimental results excellent reviews of this active field of research have l 7 actually lead only to relative thermochemical appeared - , and two unevaluated compilations of data relationships, and that the absolute values assigned to are ~vai1able8-9, there was, until the current publication, proton affinities may have changed because of no s1D~le reference which presented a comprehensive ~nhsequent re-evaluations of the thermochemistry of a collectIOn of data on gas phase proton atlinities primary standard. The current publication seeks to evaluated for internal consistency. provide a complete and internally-consistent set of gas The vast majority of proton affinities presented phase proton affinity values based mainly on the vast here ::Ire hased on measurements of the equilibrium body of data generat.eu by cquiIlbriuUl constant constants of gas phase proton transfer reactions: measurements, and to assign absolute values to· the resulting proton affinity scale using the best current (2) information about the thermochemistry of positive ions, imposing the requirement of internal consistency. where: -RT In Keq = ..6.GRn === A.HRn T..6.SRn (3) and the equilibrium constant for reaction 2 is obtained 2. Types of Experiment Leading to from a mass spectrometric observation of the relative Proton Affinity/Gas Basicity Data abundances of equilibrated ions, MH-j- and