Acid-base strength proton affinity

One of the purest measures of acid-base strength, but one difficult to relate to solution reactions, is gas phase proton affinity ... 

Another quantitative measure of acidity is the proton affinity (PA), which was originally introduced to describe the basicity of molecules in their gaseous state. The PA is an intrinsic property of a molecule, defined in the absence of a solvent. Therefore, the PA seems to be a more appropriate parameter for the characterization of acidity or basicity (i.e., the acid or base strength) of surface OH groups than the pl[Pg.139]

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. 

The strength of an acid is determined by its ability to give up protons while the strength of a base is determined by its ability to take up protons. This strength is indicated by the dissociation or equilibrium constant, (pKfl), for the acid or base strong acids have a low affinity for protons, while weak acids have a higher affinity and only partially dissociate (e.g. HC1 (strong) and acetic acid (weak)). 

This can be construed as the competition between the bases HX and Y" for protons. It should be apparent that the more powerful HY is as a proton donor, the weaker will be Y as a proton acceptor. In other words, a strong acid will have a weak conjugate base and vice versa. It is often more convenient to rationalise the relative strengths of acids in terms of the proton affinities of their conjugate bases. We look first at acids in aqueous solution, and then at acid/base and other equilibria in non-aqueous polar solvents. 

The strength of solid acids and bases is characterized by proton affinity (PA). For a base B, PA is equal to the enthalpy of reaction B + H " -> BH in gas phase, where B is electrically neutral base and BH" is its protonated form. The methods have been developed to determine PA for various compounds by the combined application of different indicators, sorbents and IR spectroscopy methods [22]. 

Before processes of the type shown in Eq. (22) become general for the synthesis of hydro complexes of the platinum metals, it will be necessary to study this reaction more fully in order to be able to predict the direction of the possible equilibrium. This depends on the relative proton affinities of the conjugate bases, the strength of the metal-ligand bond, and possibly on the relative volatilities or solubilities of the acids. A further complication arises from the type of reaction shown in Eq. (23) which must also be considered when predicting the final result. 

So far all models used for the base protonation mechanism have given total barriers for decarboxylation of more than 40 kcal/mol. In all those models, the amino acids added were chosen mainly to affect the C-C bond strength, but a large fraction of the total barrier comes from the protonation cost. The next step is therefore to discuss models that focus on the region around 02 and include residues that might affect the proton affinity of 02. In the X-ray... 

Basic properties of the samples were studied by FTIR spectroscopy using CDCI3 adsorption at 20 °C. At the formation of H bonds, deuterochloroform behaves as a typical acid. A decrease in the frequency of vcd band of CDCI3 adsorbed relative to the value of physisorbed molecules (2265 cm-i) is caused by the formation of complexes with base sites. The strength of base site was determined by the band shift of the CD stretching vibrations that occurred under CDCb adsorption. The strength can be recalculated into the proton affinity (PA) scale using the formula (Paukshtis, 1992) ... 

In the earliest ab initio calculations, which for simplicity were based on isolated ZOH clusters, attempts were made to correlate the OH bond strengths from calculated proton affinities, with various structural parameters. The most frequently cited parameters are the nature of the sites and of the T(T ) atoms. A Inidged OH group was found more "acidic" than the terminal silanol group Several calculations showed an increasing "acidity" with increasing TOP... 

Lee et al. [174] have used microcalorimetry to measme the differential heats of adsorption of both a series of alkylamines and a series of substituted pyridines in H-ZSM-5. With few exceptions, the differential heats were approximately constant up to coverages close to the expected BrOnsted site concentration. The authors observed a strong correlation between gas-phase proton affinities and differential heats of adsorption of amine and pyridine bases but no useful correlation with aqueous base strengths, this contrast implying that the Hammett Ho value is probably not a meaningful description of zeohte acid strength. 

The calculated proton affinities at Tl, T3, and T4 sites are equivalent, indicating that these Bronsted sites have similar acid strengths. One can thus infer that the different acidic behaviors at the different sites (Marie et al. 2000, 2004) do not originate from different acid strengths of these OH sites. This leads to the suggestion that the MOR acidic properties are more related to the electronic structure of the base than to effects of the solid framework. 

Using a series of amines as probe molecuies, Parrilo et al. found a good correlation between heats of adsorption and gas phase proton affinities, but not with the proton transfer energies of those bases in aqueous phase [54-56]. These results indicate that the proton transfer dominates the interaction between the adsorbate and the acid sites. However, in a theoretical study published by Teraishi the fact that the heat of ammonia adsorption depends both on the proton affinity and the ammonium ion affinity was underlined [57]. With regard to the catalytic reaction in zeolites, the activity depends not only on the proton affinity but also on the stability of the cationic intermediate in the zeolite. The heat of ammonia adsorption, which includes the later effect, is thus in disagreement with proton affinity and provides a different measure of acidity, which is better suited to evaluate the acid strength of the zeolite in relation with its catalytic activity. 

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