Anion proton affinities

Accurate anion proton affinities are now available from theoretical calculations and this enables one to estimate the proton affinity as well as the relative energy of hypothetical carbanion configurations, as, for example, the cyclopropyl anion in the pyramidal and the planar C2y configuration. 

In mass spectrometry, the main types of anions arising from ionization of a neutral molecule M are deprotonated ions, [M — H]. Key energies associated with the formation and stability of these gas-phase anions are the anion proton affinity (APA) and the electron affinity (EA). 

Anion Proton Affinity. This enthalpy change measures the ease of losing a proton from a compound [Eq. (3)] and is related to the inherent acidity of a gas-phase molecule. It is usually defined at 298 K as follows ... 

TABLE 7.9 Examples of Electron Affinities (kj/mol) and Anion Proton Affinities (kJ/mol)... 

The anion proton affinity (gas phase acidity) is generalized by Reactions 7.44 and 7.45. The fundamental concept of anion proton affinity (acidity) is well defined in the gas phase (69,75,103,110,111). 

The addition of Reactions 7.44 and 7.45 is simply Reaction 7.38, and thus the energetics for the abstraction of a proton from the sample molecule [M] by the reactive anion [C] can be calculated by comparing the gas-phase acidities of the sample molecule to the gas-phase acidities of [C-l-H] that is, A/7 (Reaction 7.38) = PA (sample) — PA (C-l-H). If the reactive anion has an anion proton affinity (gas-phase acidity) greater than that of the sample molecule, then the NICl reaction can take place. For example, if the reactive anion were Cl ([C-l-H] = HCl) and the sample molecule were toluene ([M] = C6H5CH3), then A/7 (Reaction 7.38) = 1593 — 1395 = -1-198 kJ/mol. The reaction would be endothermic and would not occur. However, if the reactive anion were NH2 ([C-l-H] = NH3) and the sample molecule were toluene [h.H (Reaction 7.38) = 1593 — 1689 = -96 kJ/mol], the reaction would be exothermic and would occur. If the reaction is strongly exothermic, there is substantial excess energy in the [M—Hj anion and fragmentation could occur or the electron could detach. 

CT) complex with absorption maxima at 470 and 550nm, was produced. These species were formed only in polar solvents with relatively high proton affinity. The data suggested an intermolecular proton transfer, from electronically excited TNB to the solvent forming the anion... 

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

In Chapter 7, it was shown how the enthalpy of decomposition of an ammonium salt can be used to calculate the proton affinity of the anion. The proton affinity is a gas-phase property (as is electron affinity) that gives the intrinsic basidty of a species. The reaction of H+ with a base B can be shown as... 

Figure 9.3 shows the relationship between ionic radius and proton affinity in a graphical way for monatomic ions having a — 1 charge. It is clear that to a good approximation there is a correlation between the size of the anion and its proton affinity. While this is in no way a detailed study, it is clear that the smaller (and thus harder] the negative ion (with the same type of structure) the more strongly it binds a proton. 

Although it is not surprising that anions have a rather high affinity for protons, it is also found that neutral molecules bind protons with the release of energy. Table 9.3 shows the proton affinities for some neutral molecules having simple structures. 

The theoretical study of the structure of propene was then used as a model to calculate the effect of the structure on the proton affinity, and later to predict the acidity of similar systems such as cycloalkenes46. Deformation of the CCC angle as a function of the stability of the anion was probed, and the results were in agreement with the acidities of the hydrogens of propene. The allylic protons were found to be more acidic than the vinylic ones, which is in contrast to the results of Grundler47. 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

Note that the standard enthalpy of this reaction, Aacid77°(AH), is equal to the proton affinity of the anion, PA(A ). As shown in figure 4.5, this quantity can be related to PA(A) by using the adiabatic ionization energy of AH and the adiabatic electron affinity of A. The result is also expressed by equation 4.28 (derived from equations 4.4 and4.9), where A = (TT g - o)ah+ ( 298 o)ah and A = ( 298 o )a- - ( 298— o )a These thermal corrections are often smaller than the usual experimental uncertainties of proton affinity data (ca. 4 kJ mol-1). 

Such a dual reactivity toward protons depends on the difference between proton affinity to an electron and the first ionization potential of an anion-radical. This difference may not be very strong. The fate of the competition between directions a and b in Scheme 1.10 also depends on relative stability of the reaction products. It is reasonable to illustrate the duality with two extreme examples from real synthetic practice. 

The use of formic acid, acetic acid and ammonium formate rather than triflu-oroacetic acid can substantially increase sensitivity because their proton affinities are lower than that of the TFA anion - though TFA is often used in the analysis of peptides. It is always advisable to keep the level of acid additives to less than 0.1% v/v, and preferably 0.03-0.05% v/v, in the final eluent. Triethylamine or ammonium hydroxide can be used successfully in negative mode because they promote deprotonation of acidic species. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

Proton Affinities and Intrinsic Basicities of Azole Anions ... 

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