Proton affinity definition

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. 

While a great deal of research has been done on condensations of oxometalates in aqueous solutions, there has been little such work in the field of thiometalates. The condensation reactions subsequent to protonation of thiometalate ions take place at lower pH than those of the oxoanions, since the proton affinity of S is appreciably lower than that of O. Investigation of condensation behavior of thiometalate ions is more difficult, since various complex decomposition processes are also involved. For example, no definite condensation product has yet been isolated from an aqueous solution containing MoS2-. 

This section will use gas-phase thermochemical data from Appendices 6 for molecules and 7 for radicals. These data include ionization energy (IE), electron affinity (EA), proton affinity (PA), gas-phase basicity (GB) and gas-phase acidity. Definitions of these parameters are given in Table 1.5. Some values of gas-phase basicities are given in Table 1.6. 

This technique is especially valuable in cases where equilibrium measurement is hampered due to low volatility (peptides, zwitterions, salts etc.). Gas phase acidity (GA) and basicity (GB), and proton affinity (PA) are defined in the absolute sense through the following definitions ... 

The heats of adduct formation with many Lewis acids have been measured experimentally in the gas phase using the technique of ion cyclotron resonance [10]. The absolute proton affinity (PA) of a gaseous base molecule has a precise thermodynamic definition in relation to the negative of the enthalpy variation for a hypothetieal reaction of attachment of an isolated proton to a molecule M in the gas phase. 

Another method to access aeidie groups on activated carbons is direct potentio-metric titration, but it suffers from very slow exchange equilibria in solution [113] and the lack of definite endpoints. The latter faet reflects the spread of dissociation constants of most surface funetionalities on carbons caused by interactions between neighboring groups and effeets resulting from the limited size of the graphene sheets. Assuming a continuous, rather than discrete, distribution of pK values, Contescu et al. [114] showed that potentiometric titration data could be transformed to proton affinity distribution (PAD) eurves and that the results were in... 

The absolute proton affinity (PA) of a base molecule in the gas phase has a precise thermodynamic definition in relation to the enthalpy variation of the hypothetical reaction... 

Proton affinity and pK measure the acidity of a molecule. Proton affinity depends only on the molecule itself, while pK values depend on the surrounding molecules. pK values may be defined for any solvent, and reveal how important the solvent is in practice. Although pKj values are not unique for the molecule, they are more useful than proton affinities. This depends also on the fact that pK is related to the free energy, which depends on both enthalpy and entropy. The free energy difference may be obtained via the acidity constant, if we use the definition of Eqnation 9.7 together with Equation 5.88. 

Bronsted s definition acknowledges the existence of acid-base reactions in the gas phase, in which the complicating effects of solvation are absent. The acidity of a substance B is then measured by the proton affinity PA of the molecule, i.e. the variation of enthalpy involved in the reaction ... 

We will see later that other reactions can be in competition with Reactions (3.2) and (3.3). Reaction (3.3) is possible only if the proton affinity of the analyte (affinity of the molecule for the proton) is higher than that of the reagent gas. By definition, the proton affinity of a molecule M is the variation of enthalpy associated with the reaction MH+ M + H+ in the gas phase. Table 3.1 taken from Harrison s book supplies proton affinity values for families of common organic compounds."... 

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