Proton theoretical models

Our present views on the electronic structure of atoms are based on a variety of experimental results and theoretical models which are fully discussed in many elementary texts. In summary, an atom comprises a central, massive, positively charged nucleus surrounded by a more tenuous envelope of negative electrons. The nucleus is composed of neutrons ( n) and protons ([p, i.e. H ) of approximately equal mass tightly bound by the force field of mesons. The number of protons (2) is called the atomic number and this, together with the number of neutrons (A ), gives the atomic mass number of the nuclide (A = N + Z). An element consists of atoms all of which have the same number of protons (2) and this number determines the position of the element in the periodic table (H. G. J. Moseley, 191.3). Isotopes of an element all have the same value of 2 but differ in the number of neutrons in their nuclei. The charge on the electron (e ) is equal in size but opposite in sign to that of the proton and the ratio of their masses is 1/1836.1527. 

It is not an exaggeration to say that electrospray has introduced a new era, not only for the analytical mass spectroscopist, but also for the more physically oriented researcher interested in physical measurements involving the above ions, which are of such great importance in condensed-phase ion chemistry. In particular, gas-phase ions produced by electrospray allow, for the first time, thermochemical measurements involving ions of biochemical significance such as protonated peptides, deprotonated nucleotides, and metal ion complexes with peptides and proteins. It is to be expected that such data will be of importance in the development of theoretical modeling of the state of these systems in the condensed phase.34,35... 

Since the rate for the tunneling of a proton is strongly dependent on barrier width, it is necessary that the molecular systems to be studied constrain the distance of proton transfer. Also, since the various theoretical models make predictions as to how the rate of proton transfer should vary with a change in free energy for reaction as well as how the rate constant should vary with solvent, it is desirable to study molecular systems where both the driving force for the reaction and the solvent can be varied widely. 

The following discussion begins by presenting an in-depth view of the mechanism for the photochemical reduction of benzophenone by N, iV-dimethyl-aniline. This discussion is followed by a presentation of the theoretical models describing the parameters controlling the dynamics of proton-transfer processes. A survey of our experimental studies is then presented, followed by a discussion of these results within the context of other proton-transfer studies. 

In 1967, Dogonadze, Kuznetsov, and Levich began the development of a theoretical model that would account for the full quantum nature of the transferring proton [10, 18, 52, 53]. In contrast to the model based on transition state theory where the quantum properties of the proton are an ad hoc addition to the model,... 

In recent years, there have been numerous studies examining the dynamics of proton transfer within the context of recently developed theoretical models. Reactions in the gas phase, in the solution phase, and in matrices have been examined [59-72]. Few of these studies, however, have addressed the issue of how the rate of proton transfer correlates with the thermodynamic driving force, which is an important correlation for discerning the validity of the various theoretical models. However, there have been two series of investigations by Kelley and co-workers [70, 71], and by Pines et al. [65, 66] that have sought to elucidate the role of solvent dynamics on the rate of proton 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. 

A comparison of calculated and measured proton affinities (basicities) of nitrogen bases relative to the proton affinity of ammonia as a standard is provided in Table 6-17. The calculations correspond to the usual theoretical models, and the experimental data derive from equilibrium measurements in the gas phase. The data span a large range the proton affinity of the strongest base examined, quinuclidine, is some 27 kcal/mol greater than that of the weakest base, ammonia. 

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