Adiabatic Proton Transfer

A formalism similar to that used for partially adiabatic proton transfer reactions was applied in the calculation of the transition probability. This model of the diffusion jump is similar to the model of the diffusion of light defects in solids which was first considered in Ref. 62. 

Kiefer PM, Hynes JT (2002) Nonlinear free energy relations for adiabatic proton transfer reactions in a polar environment. I. Fixed proton donor—acceptor separation. J Phys Chem A... 

Figure 5.12 The adiabatic proton-transfer potential for thf displacements from equilibrium toward one or the other F in F H F (the tick-mark separation is...

In the ensuing discussion, the energy dependence of the rate constants for proton transfer within a variety of substituted benzophenone-lV, /V-dimethylaniline contact radical ion pairs is examined only the data for the nitrile solvents are discussed. This functional relationship is examined within the context of theories for non-adiabatic proton transfer. Finally, these results are viewed from the perspective of other proton-transfer studies that examine the energy dependence of the rate constants. 

An interesting question then arises as to why the dynamics of proton transfer for the benzophenone-i V, /V-dimethylaniline contact radical IP falls within the nonadiabatic regime while that for the napthol photoacids-carboxylic base pairs in water falls in the adiabatic regime given that both systems are intermolecular. For the benzophenone-A, A-dimethylaniline contact radical IP, the presumed structure of the complex is that of a 7t-stacked system that constrains the distance between the two heavy atoms involved in the proton transfer, C and O, to a distance of 3.3A (Scheme 2.10) [20]. Conversely, for the napthol photoacids-carboxylic base pairs no such constraints are imposed so that there can be close approach of the two heavy atoms. The distance associated with the crossover between nonadiabatic and adiabatic proton transfer has yet to be clearly defined and will be system specific. However, from model calculations, distances in excess of 2.5 A appear to lead to the realm of nonadiabatic proton transfer. Thus, a factor determining whether a bimolecular proton-transfer process falls within the adiabatic or nonadiabatic regimes lies in the rate expression Eq. (6) where 4>(R), the distribution function for molecular species with distance, and k(R), the rate constant as a function of distance, determine the mode of transfer. 

In recent years, there have been many significant advances in our models for the dynamics for proton transfer. However, only a limited number of experimental studies have served to probe the validity of these models for bimolecular systems. The proton-transfer process within the benzophenone-AL A -di methyl aniline contact radical IP appears to be the first molecular system that clearly illustrates non-adiabatic proton transfer at ambient temperatures in the condensed phase. The studies of Pines and Fleming on napthol photoacids-carboxylic base pairs appear to provide evidence for adiabatic proton transfer. Clearly, from an experimental perspective, the examination of the predictions of the various theoretical models is still in the very early stages of development. 

D. Laria, G. Ciccotti, M. Ferrario, andR. Kapral,/. Chem. Phys., 97,378 (1992).Molecular Dynamics Study of Adiabatic Proton Transfer Reactions in Solution. 

An electronically adiabatic proton transfer reaction may be either vibrationally adiabatic or vibrationally non-adiabatic. Vibrationally adiabatic refers to the situation in which the proton responds instantaneously to the solvent, while vibrationally non-adiabatic refers to the opposite limit. The adiabatic proton vibrational wave functions are calculated if the Schrodinger equation is solved for fixed values of Zp. 

Borgis, D. and Hynes, J.T. (1991). Molecular-dynamics simulation for a model non-adiabatic proton transfer reaction in solution. J. Chem. Phys. 94, 3619-3628... 

Case Study 5.1 Mechanistic photochemistry - adiabatic proton transfer reactions of 2-naphthol and 4-hydroxyacetophenone... 

Adiabatic Proton Transfer Free Energy Relationship (FER)... 

Adiabatic Proton Transfer Kinetic Isotope Effects... 

Kieeee, P. M., Hynes, J. T. (2003) Kinetic isotope effects For adiabatic proton transfer reactions in a polar environment, J. Phys. Chem. A 107, 9022-9039. 

Reductive alkylation of N-methylacridinium (87) occurs when it is irradiated with carboxylic acid salts. The reaction is thought to proceed by electron transfer from the carboxylate to the excited acrldinium ring followed by decarboxylation of RCOO coupling of the alkyl radical produced with the acridinyl radical then gives (88). A very similar sequence probably occurs in a reaction proposed as a synthetic procedure for decarboxylation of carboxylic acids.In this case an aza-aromatic compound such as acridine is irradiated with a carboxylic acid in benzene in the presence of tert-butyl thiol. The authors propose that a hydrogen bonded acridine-acid complex is excited and that adiabatic proton transfer is followed by electron transfer. This produces RCOO which decarboxylates, and reduction of the alkyl radical then ensues. The major fate of the acridine is coupling to (89) if the reaction is perfonned in the absence of oxygen. 

For this form of the matrix element to be correct, a specific separation of dynamic time-scales should exist in the system. Namely, as the above expression (11) suggests, the interaction associated with mixing of the i-th pair of orbitals should be much weaker (and therefore slower) than that of other orbitals. Such is the case, for example, in the non-adiabatic proton transfer, where the transfer matrix element has exactly the same form[28] ... 

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