Enthalpy of protonation

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

The importance of the H/B repulsion is also witnessed by the observation that the activation enthalpies of proton mobility in cubic perovskites with pentavalent B-site cations (I—V perovskites) are significantly higher than for perovskites with tet-ravalent B-site cations (II—IV perovskites). - ... 

Figure 9. Gas phase enthalpies of acidity versus solution phase enthalpies of proton transfer. Data are from Refs. 60 and 62. Both axes in kcal/mol.

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

T. Akrmoto, S. (1984) Mossbauer study on the high pressure phase ofiron(III) oxide. Solid State Commun. 50 97—100 Szczypa, J. Matysiak, J. Kosmilski, M. (1994) Standard enthalpies of proton adsorption on hematite in various solvent systems. Abstracts of 8 Int. Conf. Colloid Surf. Chem. Adelaide... 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

The experimental enthalpies of protonation and the formal enthalpies of protonation, RMgBr —RH, are fairly constant for structurally similar species (R = cycloalkyl, primary alkyl) and would be expected to be constant also for the primary cycloalkyhnethyl-magnesium bromides. For the two examples just discussed, the formal enthalpies of protonation that are calculated using the derived enthalpies of formation for the cyclopropyl-and cyclobutylmethyhnagnesium bromides are 262 and 235 kJmoU, respectively. The mean value is thus 248 kJmoU, which is very close to that expected for the formal protonation of other primary R groups. 

A more general method proposed by Forster is a thermodynamic calculation based on a cycle called after his name (Figure 4.12). If AH and AH are enthalpies of proton dissociation in the ground and the excited states, respectively, then... 

MO studies of aromatic nitration cast doubt on the existence of jt-complexes and electron-transfer complexes in liquid-phase nitrations.14 The enthalpy of protonation of aromatic substrates provides a very good index of substrate reactivity to nitration. Coulomb interaction between electrophile and substituent can be a special factor influencing regioselectivity. A detailed DFT study of the reaction of toluene with the nitronium ion has been reported.15 Calculated IR spectra for the Wheland intermediates suggest a classical SE2 mechanism. MO calculations of cationic localization energies for the interaction of monosubstituted benzenes with the nitronium ion correlate with observed product yields.16... 

Figure 47. Solvent dependence of the enthalpy of proton transfer between benzoic add and acetate anion in t-butyl alcohol + water mixtures Smh = 6 m AIT /2-3/J T (Avedikian and Morel, 1971).

Figure 15 Enthalpies of protonation (kcalmol" ) of phosphine ligands vs enthalpies of reaction (kcalmol" ) for the RhCl(CO) (PR3)2 system slope = 0.49 R = 0.96)...

The measure of basicity of phosphine hgands most widely quoted in the hterature is p/fa (aqueous), which is a measure of Bronsted basicity or proton affinity. A selection of pA a values for various phosphines is given in Table 4. AngeUci and coworkers have established a method for determining proton affinities from enthalpies of protonation using the extremely powerful acid CF3SO3H. The enthalpy values (A//hp) and pATa values are related by the empirical equation (3) ... 

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes formed by strong and weak bases with acid sites in zeolites. They examine the adsorption enthalpies of a series of strongly basic molecules such as alkylamines, pyridines and imines. These workers also performed studies of the adsorption properties of weak bases, including water, alcohols, thiols, olefins, aldehydes, ketones and nitriles. They report a poor correlation between the differential heats of adsorption on H-MFl zeolites and the enthalpies of protonation in aqueous solutions, but a much better correlation with gas-phase proton affinities [37]. 

The influence of the pre-treatment temperature on the acidic properties is a very important factor. For Bronsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density towards an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [40]. 

Hundeck et al. (1993) showed the ET analyzing racemates of DL-phenylalanine-methyl ester. Here, unspecific pig liver esterase cleaves both antipodes whereas a-chymotrypsin converts only the L-enantiomer (Fig. 10). Both reactions release protons that are easily detected by tris/HCl-buffer (enthalpy of protonation AH = -47 kj/mol). 

FIG. 3.104 Enthalpy of proton adsorption on (hydr)oxides as a function of their PZC. 

These estimates are then compatible with the following enthalpies of protonation ... 

Table 3.38. Enthalpy of Proton Dissociation for Some Hydrocarbons (Gas Phase)...

Sufficient data are provided to permit recalculation using the SIT and the TDB selected values [92GRE/FUG] for the first protonation constant (and enthalpy of protonation) for the sulphate ion. As in the original analysis, the initial concentrations, measured potential of the cell ... 

---