Proton transfer enthalpy

On the other hand, Arnett and his coworkers have reported both the enthalpies of the protonation (AHJ and the hydrogen bond (AHf) for acid-base reactions. They calculated Hj by measuring the association constants for the proton transfer (ionization) in a number of bases by using FSO3H as the acid and determined Ai/j by calorimetric measurements of the heat of dissolution of P-FC6H4OH in various hydrogen bond acceptors, including sulphoxides, in They have also tried to correlate and... 

Equilibrium measurements measure the relative AG, and thermochemical studies generally are interested in enthalpy values, AH. The enthalpy can be obtained from AG by using the relation AG = AH — TAS. The entropy of proton transfer can either be estimated, reliably calculated using electronic structure calculations, or can be measured directly by using a Van t Hoff approach. Measuring the quantity AS requires a variable temperature study. 

Alternatively, the translational energy threshold for endothermic proton transfer from MH+ to R can be measured using a flowing afterglow triple quadrupole instrument.127 These data define the proton affinity of M, relative to that of R. Thus, the PA of cyclopropenylidene was found to exceed that of ammonia by 23.3 1.8 kcal/mol (Table 6).128 In order to obtain absolute proton affinities, the enthalpies of formation of both the base and the conjugate acid must be known from other measurements (Eq. 9). Numerous reference compounds with known absolute PA are available.124... 

The methods summarized by Eqs. 8 and 9 have both been applied to halocarbenes. The proton affinities obtained by ICR bracketing125 (Table 6) are consistently lower than those derived from enthalpies of formation (Table 7). The case of dichlorocarbene, with a difference in AH of 15 kcal/mol), is particularly disturbing and has been analysed in some detail.134 Notably, the PA of CCl2 from an earlier bracketing experiment126 was closer to the enthalpy-derived PA. The discrepant results from similar experiments125126 indicate that HCClJ is not a good substrate for proton transfer studies. 

Figure 2.3. The enthalpy dependence (—AE, kcal/mol) as a function of the solvent reorganization energy for the rate of proton transfer when Ea — 1.0 kcal/mol, Eq = 1.0 kcal/mol, AQ — 0.1 A and Gig = 200 cm-1. Rates are normalized to the maximum rate constant for proton transfer. Larger graph Es = 2.0 kcal/mol. Smaller graph Es = 7.0kca/mol.

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

TABLE 2.2. Solvent Dependence of the Substituent Effects upon Enthalpy Change for Proton Transfer a... 

Figure 2.4. The normalized rate constants for proton transfer as a function of the negative enthalpy change (—AFkcal/mol) for the solvent butanenitrile. Experimental data = squares. The BH model = solid curve with Es — 1.5kcal/mol, Ea — l.Okcal/mol, q = 200 cm-1, and T — 298 K. 

Figure 2.7. The rate constants for proton transfer as a function of the negative enthalpy change for the three nitrile solvents as a function of Es and otg with the remaining parameters held constant as specified in Figure 2.5. Butanenitrile = squares with Es = 8.0kcal/mol and g>q = 195 cm-1. Propanenitrile = triangles with Es — 12.0kcal/mol and mq = 179cm-1. Acetonitrile = circles with Es — 17.0kcal/mol and cdq = 164 cm-1.

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. 

Ab initio MO calculations were carried out on the hydrolysis of CH3CI, with explicit consideration of up to 13 water solvent molecules. The treatments were at the HF/3-21G,HF/6-31G,HF/6-31 G orMP2/6-31 G levels. Forn > 3 three important stationary points were detected in the course of the reaction. Calculations for n = 13 at the HF/6-31 G level reproduced the experimental activation enthalpy and the secondary deuterium KIE. The proton transfer from the attacking water to the water cluster occurs after the transition state, in which O-C is 1.975 A and C-Cl is 2.500 A. 

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.

All of this suggests that the ion association explanation may be applied here to an essentially bimolecular (or associative) phenomenon. Considering the difference between hydroxide and any other reagent in water, apart from its basicity, one concludes that its mobility must play an important part. Whereas all the other reagents must be in a suitable position within the solvation shell before they can enter the complex, the hydroxide ion, by means of a Grotthus chain proton transfer, can be transmitted to any position where it is needed while the complex becomes activated. It can therefore be looked upon as an unsaturatable ion aggregate with hydroxide fully delocalized about the complex. Consequently, we do not observe any departure from the first-order dependence upon hydroxide concentration. This contribution to the reactivity will appear in the activation entropy rather than in the enthalpy term. 

Theory helps the experimentalists in many ways this volume is on chemical shift calculations, but the other ways in which theoretical chemistry guides NMR studies of catalysis should not be overlooked. Indeed, further theoretical work on two of the cations discussed above has helped us understand why some carbenium ions persist indefinitely in zeolite solid acids as stable species at 298 K, and others do not (25). The three classes of carbenium ions we were most concerned with, the indanyl cation, the dimethylcyclopentenyl cation, and the pentamethylbenzenium cation (Scheme 1), could all be formally generated by protonation of an olefin. We actually synthesized them in the zeolites by other routes, but we suspected that the simplest parent olefins" of these cations must be very basic hydrocarbons, otherwise the carbenium ions might just transfer protons back to the conjugate base site on the zeolite. Experimental values were not available for any of the parent olefins shown below, so we calculated the proton affinities (enthalpies) by first determining the... 

Enthalpies of activation, transition-state geometries, and primary semi-classical (without tunneling) kinetic isotope effects (KIEs) have been calculated for 11 bimolecu-lar identity proton-transfer reactions, four intramolecular proton transfers, four nonidentity proton-transfer reactions, 11 identity hydride transfers, and two 1,2-intramole-cular hydride shifts at the HF/6-311+G, MP2/6-311+G, and B3LYP/6-311+-1-G levels.134 It has been found that the KIEs are systematically smaller for hydride transfers than for proton transfers. The differences between proton and hydride transfers have been rationalized by modeling the central C H- C- unit of a proton-transfer transition state as a four-electron, three-centre (4-e 3-c) system and the same unit of a hydride-transfer transition state as a 2-e 3-c system. 

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