Proton affinity difference

In most papers the experimental proton affinity difference between water and ammonia is taken as being equal to 37.5 kcalmol-1. Various calculated values of this gap have appeared. The PA values of ammonia and water were 205.6 and 168 or 168.6 kcalmol-1 respectively (APA = 37 or 37.6 kcalmol-1) according to Dixon and Lias26 the work of Defrees and McLean27 led to similar calculations of P/HN1I3) = 204.0 and PA (H30) = 165.1, with A PA = 38.9 kcalmol-1, although Pople and coworkers28 predicted a A PA = 39.4 kcalmol-1. 

Tjctam N-Proton Affinity O-Proton Affinity Difference Favoring N... 

This question was explored in some detail by ab initio calculations pairing formic acid with methyleneimine, in which the N atom participates in a double bond. As illustrated in Fig. 6.21, the two configurations examined place the N atom either syn or anti with respect to the carboxyl group. The calculations were carried out with a 4-31G basis set at the SCF level. (These results were later confirmed at the correlated MP2 level with a 6-3IG basis set .) The protonation energies of HCOO and NHCHj at the SCF/4-31G level of theory are 360 and 230 kcal/mol, respectively. When adjusted by vibrational and other terms, the calculated values of AH(300 K) are 352.6 and 221.9 kcal/mol, only a little bit larger than the experimental quantities of 345.2 and 214.3 kcal/mol, respectively. Most important for our purposes, the theoretical overestimation is equal to 7 kcal/mol in both cases consequently, the calculated protonation enthalpy difference of 130.7 kcal/mol is very close indeed to the experimental value of 130.9 kcal/mol. And it is this proton affinity difference which is the key element in considering the question of neutral versus ion pairs. 

It is particularly notable that when the frequency is plotted as a relative red shift, Av /v, and the acidity/basicity of A and B is taken as a normalized proton affinity difference, it is possible for a single function to fit the spectral data for a range of different acids and bases. A central conclusion of these results matches nicely with what was found from both ab initio calculations and from gas-phase measurements the system shifts smoothly from a neutral to an ion pair as the proton affinities of the partners change. 

It might also be pointed out that correlation diagrams of this t5q>e have relevance in solution work as well, even though the vibrational bands are much broader. The maximal shift in the spectral band for complexes of pyridines with carboxylic acids occurs for a proton affinity difference of 109 kcal/mol pairing of pyridine N-oxides with these acids peaks at 102 kcal/moF . When plotted as relative frequency change versus NPAD, the functions have minima at values of the latter parameter somewhat less negative than the —0.23 noted above. 

NPAD Normalized Proton Affinity Difference. A measure of the relative proton... 

Szulejkc J E and McMahon T B 1991 A pulsed electron beam, variable temperature, high pressure mass spectrometrio re-evaluation of the proton affinity difference between 2-methylpropene and ammcnia Int. J. Mass Spectrom. Ion Proc. 109 279-94... 

Proton affinity differences are more meaningful and may be measured more easily than proton affinities themselves. When we deal with pK values, we are actually comparing the proton affinity differences between water and a base A" (or between water and a base B). It is understood that the comparison is done in an aqueous solution. 

The second relationship is of particular interest because extended compilations of pa are accessible in the form of PA (proton affinities in the gas phase) and (acid-base dissociation constants in aqueous solution). With this in mind, the proton-affinity difference becomes... 

Bond energies in any electronegativity class with fixed Epa can be further modulated by the donor/acceptor proton-affinity difference (Apa), a quantity already seen to represent the reaction energy of Figure 15, A E, as well as its empirical counterparts, APA in the gas phase... 

Various distinct sites exist on one crystal plane and each of them may have a different proton affinity different crystal planes of one sorbent may have different sites different samples of one nominally identical sorbent may display different dominating crystal planes. 

Adduct formation with NH4" occurs with substances where the proton affinity differs little from that of NHg and can be used to confirm the molecular mass interpretation. In these cases the formation and addition of higher (NH3), clusters is observed with instruments with threshold pressures of... 

Figure 1 Comparison of calculated — experimemal proton affinity differences (AA//pa, 298 K, kj mol ) with the 6-311G(d,p) basis. Experimental values are taken from Szulejko and McMahon " with the exception of CH2CO and CH2O taken from Traeger, and HNC and HCN calculated from the data in the compendium of Lias et al. Differences in proton affinities which result from calculated energies that are larger than experiment are colored blue negative errors are colored red...

The justification for employing proton affinities can be seen from inspection of Table 2.2. For example, ethanol has a substantially higher proton affinity than water, the difference being 85 kJ moC. In terms of gas-phase basicities, the difference is 86 kJ mol L which is almost identical to the proton affinity difference and well within the expected error margins for the experimental determinations of these quantities. Similar conclusions can be reached... 

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