Acids proton affinity

Base Conjugale Acid Proton Affinity (experimental, kcal/mol) Proton Affinity (AM1, kcal/mol)... 

In principle, the equilibrium approach can be used to measure any of the thermochemical properties listed above. However, in practice, it is most commonly used for the determination of gas-phase acidities, proton affinities, and electron affinities. In addition, equilibrium measurements are used for measuring ion affinities, including halide (F, Cl ) and metal ion (alkali and transition metal) affinities. 

It is possible to detemiine the equilibrium constant, K, for the bimolecular reaction involving gas-phase ions and neutral molecules in the ion source of a mass spectrometer [18]. These measurements have generally focused on tln-ee properties, proton affinity (or gas-phase basicity) [19, 20], gas-phase acidity [H] and solvation enthalpies (and free energies) [22, 23] ... 

Many of the inorganic oxoacids are strong (i.e. have negative PX3 values) in aqueous solution. But, as we have seen, use of a solvent with a lower proton affinity than water (for example pure ethanoic (acetic) acid makes it possible to differentiate between the strengths of these acids and measure pX values. The order of strength of some typical oxoacids is then found to be (for H X -> H , X- + H") ... 

In the pure acid the dihydrogen sulphate has a proton affinity, so that... 

ChloricfVII) acid is one of the strongest acids known, and it behaves as such even when dissolved in solvents with poor proton affinity thus it can be used as an acid in pure ethanoic acid as a solvent ... 

Additional gas-phase reactivity data, such as gas-phase acidities of alcohols [41], proton affinities of alcohols and ethers [41], and proton affinities of carbonyl compounds [42] could equally well be described by similar equations. 

Fundamental enthalpies of gas-phase reactions such as proton affinities or gas-phase acidities can be correlated with the values of the Inductive and the polarizability effect. 

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

The higher the proton affinity, the weaker the Brpnsted acid. 

Because of this, the thermochemistry of many physical processes can be described in different ways. Thus, the ionization energy of neutral A is the same as the electron affinity of A, the proton affinity of B is also the gas-phase acidity of BH, and the gas-phase acidity of AH is the same as the proton affinity of A. ... 

Thermodynamically it can be stated, if the differences of solvation of the compounds X" and HX between two solvents are neglected, that the difference in the pK values of compound HX in the two solvents is completely determined by the difference in the proton affinities of the two solvents80 hence a comparison of the pjfifj, values of various compounds in the solvents 1,2-DCE, m-cresol, acetic acid, pyridine and water is worth considering (see Table 4.5)80. 

These models refer to reactions with the simplest nucleophile, H, both under neutral conditions and in the protonated form. Chemical reactivity can be strongly altered by catalytic effects acid/base catalysis is of particular importance. We regard the studies on ga phase acidities and on proton affinities discussed in the above sections to bear special significance for quantitative modelling of acid/base catalysis in the future. 

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. 

Despite a wide range of the modifiers used, there is a clear symbasis between the calculated proton affinities and the experimental stability indices Kst (Figure 3). The decrease in concentration of nitric acid in the interlayer space will give rise to a poorer stability of GICs, if the proposed stabilization mechanism is valid. To verify this assumption, the treatment of graphite nitrate with 25-fold excess of acetic acid, compared to that used in the above procedure, was carried out in a glass column. Most of the molecules of nitric acid are probably removed from the interlayer space under such a treatment. It turns out that this procedure reduces the expansion... 

The acidic and adsorptive properties of the samples in gas phase were evaluated in a microcalorimeter of Tian-Calvet type (C80, Setaram) linked to a volumetric line. For the estimation of the acidic properties, NH3 (pKa = 9.24, proton affinity in gas phase = 857.7 kJ.mol-1, kinetic diameter = 0.375 nm) and pyridine (pKa = 5.19, proton affinity in gas phase = 922.2 kJ.mol-1, kinetic diameter = 0.533 nm) were chosen as basic probe molecules. Different VOC s such as propionaldehyde, 2-butanone and acetonitrile were used in gas phase in order to check the adsorption capacities of the samples. 

R = H, X = S, A = Et3N and Py). In solution the former is mainly in an ionic form the latter exists as a complex. The basicity of the amine is assumed to be important. Triethylamine is a stronger base than pyridine and the ionic form is stabilized. When the proton affinity is weak, the basicity in relation to the B(III) atom, a Lewis acid, plays an important role. This involves an equilibrium shift toward the complex. This assumption is confirmed by an easy displacement of pyridine by triethylamine. The reverse process demands more severe conditions. In the NMR spectra of the triethylamine complex the signal is shifted from 22 to 42 ppm as pyridine is added. The absence of signals of two separate forms is evidence in favor of their fast interconversion. The chemical shift of the signal in 3IP spectra is 22 ppm (EtOH), 26 ppm (Py, DMFA), and 42 ppm (EtOH, Py) for complexes with triethylamine and pyridine. 

It is interesting to note that even a "saturated" molecule such as CH4 has a significant attraction for a proton. This demonstrates clearly that even a pair of electrons that is shared in a bond can be a binding site for H+. In general, the more acidic (or less basic) a compound is, the lower the value for its proton affinity. For example, the proton affinity for NH3 is 866kJ/mol, whereas that for PH3 is 774, in keeping with the fact that PH3 is the weaker base. 

The data shown in Tables 9.2 and 9.3 reveal some interesting facts. We know that HI is a strong acid in aqueous solution. However, the proton affinity of 1(g)- is 1314kJ/mol, whereas that of H20 is 686kJ/ mol. Therefore, for the reaction... 

Fig. 6 Normalized quadrupole coupling constants, Cq/Cq(0) as a function of proton affinity of adsorbed probe molecules. Cq(0) is the quadrupole coupling constant of the unloaded acidic zeolite (assumed to be loaded with N2 molecules, proton affinity of494 kJ/mol). Data are taken from DFT calculations of Ehresmann [234] and Koller [232], and NMR measurements of Jiao [233], and Marthala [235]...

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