Gas Phase Basicity and Proton Affinity

Not all ionization methods rely on unimolecular conditions as strictly as El does. Chemical ionization (Cl, Chap. 7), for example, makes use of reactive collisions between ions generated from a reactant gas and the neutral analyte to achieve its ionization by some bimolecular process such as proton transfer. The question which reactant ion can protonate a given analyte can be answered from gas phase basicity (GB) or proton affinity (PA) data. Proton transfer, and thus the relative proton affinities of the reactants, also play an important role in many ion-neutral conplex-mediated reactions (Chap. 6.12). In the last decade, proton transfer reaction (PTR) MS has emerged as a tool for analyzing volatile organic compounds (VOCs) in air. Therefore, PTR-MS is interesting for analytical work concerning environmental issues and in occupational health and safety (Chap. 7.3). 

we deal with proton affinity and gas phase basicity as thermodynamic quantities. Consider the following gas phase reaction of a (basic) molecule, B  

The tendency of B to accept a proton is then quantitatively described by 

For the experimental determination of GBs and PAs refer to Chap. 9.18.3. Some representative PA and GB values are collected in Table 2.6. 

Porter, C.J. Beynon, J.H. Ast, T. The Modem Mass Spectrometer. A Complete Chemical Laboratory. Org. Mass Spec-trom. 1981,16, 101-114. 

The methods for the determination of GBs and PAs make use of their relation to Keq (Eq. 2.40) and the shift of upon change of [AH] or B, respectively. [101,103] Basically, the value of GB or PA is bracketed by measuring with a series of several reference bases ranging from lower to higher GB than the unknown. There are two methods we should address in brief, a detailed treatment of the topic being beyond the scope of the present book, however. 

The kinetic method [102,104-106] compares the relative rates of the competitive dissociations of a proton-bound adduct [A-H-B] formed by admitting a mixture of A and B to a Cl ion source. [104,105] There, the proton-bound adduct [A-H-B] is generated amongst other products such as [AH] and [BH]. Using standard tandem MS techniques, e.g., MIKES, the cluster ion [A-H-Bref] is selected and allowed to undergo metastable decomposition  

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An updated compilation of gas-phase basicities and proton affinities has been published (98MI3). 

Hunter, E. P. L. Lias, S. Evaluated gas phase basicities and proton affinities of molecules An update. I. Phys. Chem. Ref. Data 1998,27,413-457. 

In the first place, this chapter deals with the fundamentals of gas phase ion chemistry, i.e., with ionization, excitation, ion thermochemistry, ion lifetimes, and reaction rates of ion dissociation. The final sections are devoted to more practical aspects of gas phase ion chemistry such as the determination of ionization and appearance energies or of gas phase basicities and proton affinities. 

Lias, S.G. Liebman, J.F. Levin, R.D. Evaluated Gas Phase Basicities and Proton Affinities of Molecules Heats of Formation of Protonated Molecules. J. Phys. Chem. Ref Data 1984, 75,695-808. 

The influence of cyclopropyl on the gas phase stability of carbocations as measured by ion cyclotron resonance is shown in Table 14, along with data for some reference compounds. The results are given as gas phase basicities, GB, and proton affinities, PA, defined as AG° and AH°, respectively, for dissociation of the protonated molecule, as in equation 11. In addition hydride affinities D(BH H ) for some cations defined as — AH° for equation 18 are included. For the gas phase basicities and proton affinities the products B are alkenes, amines, nitriles or carbonyl compounds, and thus for these values the stability of the cation is compared to a derivative where the substituent is conjugated with a carbon-carbon or carbon-oxygen double bond, or a nitrogen lone pair, whereas for hydride affinities the products are saturated. 

The gas phase basicity and proton affinity are intimately related entities being defined by the same hypothetical reaction ... 

Data Compilations. Gas Phase Basicities and Proton Affinities. 

TABLE 6.6 Gas-Phase Basicity and Proton Affinities for Nitrogen Bases... 

E. P. L. Hunter, S. G. Lias., Evaluated gas phase basicities and proton affinities of molecules An Update. J. Phys. Chem. Rtf. Data, 1998, 27,413 E. P. L. Hunter, S. G. Lias., Proton Affinity and Basicity Data, in NIST Chemistry Web Book, Standard Reference Database, 2005, No. 69 (eds W. G. MaUard and P. J. Linstrom), National Institutes of Standards and Technology, Gaithersburg, MD (http // webbook.nisLgov/chemistry, accessed May 2012). 

A.G. Harrison, Gas-phase basicities and proton affinities of amino acids and peptides, Mass Spectrom. Rev. 16 (1997), pp. 201-217. 

Harrison, A. (1997) The Gas-phase Basicities and Proton Affinities of Amino Acids and Peptides. Mass Spectrom. Rev. 16 201-217. 

Lias SG, Liebman JF, Levin RD (1984) Evaluated gas phase basicities and proton affinities of molecules heats of formation of protonated molecules. J Phys Chem Ref Data 13 695. doi 10.1063/1.555719... 

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