By Paul M. Mayer, Christiane Aubry, John Holmes

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By combining these values, DfH for the ion is obtained. 3. 3 Plots of adiabatic ionization energy (IEa) vs the reciprocal of the number of atoms in the species (1=n) for some homologous series: (a) alkanes (RH), primary alkanols (ROH) and bromides (RBr); (b) di-n-alkyl ethers (ROR), cyclic ethers (cy-ROR) and alkylamines (RNH2). ß 2006 by Taylor & Francis Group, LLC. The lines have different slopes and these are in keeping with the relative abilities of the ionized functional group to localize the charge; the gentler the slope, the stronger the charge localization.

60 If a high-pressure ion source or ICR mass spectrometer is used, the equilibrium constant (k) for the process Aþ þ M $ A þ Mþ can be measured as a function of temperature and application of the Van’t Hoff equation {ÀRT log(k) ¼ DG ¼ DHÀTDS} then permits an accurate evaluation of the difference in ionization energy of A and M. Alternatively, if it is not possible to achieve equilibrium in the system, the method of bracketing is used. The ion of interest is allowed to undergo charge exchange with a series of molecules, chosen such that their IE lies close above and below that of the target molecule.

69 For example, with the series of carbocations CH3eþ, C2H5eþ, (CH3)2CHeþ and (CH3)3Ceþ (which represents sequential methyl substitution in the methyl cation) the plot of DfH(ion) versus ln n is an excellent straight line. 5, is the number of atoms in the ion. 5 Plots of DfH(ion) obtained by the sequential substitution of the functional groups, –CH3, –OCH3, –NH2 and –OH in the methyl cation as a function of ln(n). Note that only two values are known for –NH2 substitution, the open square (for three –NH2 groups) is estimated.

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