Condensed-phase proton transfer

The first atomically resolved direct PT reaction in a biological system was suggested to be found in ferredoxin I (Edl) of Azotohactervindandii [113]. In proteins, PT serves as a rapid means to transport charge in systems such as bacteriorhodopsin [114,115] and cytochrome-c oxidase [116, 117]. PT also participates in numerous enzymatic reactions [118] including liver alcohol dehydrogenase [119-122] and dihydrofolate reductase [123-125]. 

measures progress aiong the reaction coordinate, and A, = 0.35 corresponds to the -COOH-water-[3Fe-4S] state, whereas A, = 1.05 is -COQ- -water-[3Fe-4S]H+. The fluctuation bars indicate energy variations from simulations started at the transition point [k Rs 0.55). 

The optimized morphing parameters differ for each protein. However, a detailed analysis shows that the PESs are always morphed into minima (Hno oh hno) where Rj.jo equals the average N-O distance in the X-ray structure, Tqh = 1.93 

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Lobaugh J and Voth G A 1994 A path integral study of electronic polarization and nonlinear coupling effects in condensed phase proton transfer reactions J. Chem. Phys. 100 3039... 

Whenever discrepancies arise, as they commonly do, between gas-phase and condensed-phase proton-transfer thermodynamics, the source may be assigned unequivocally to solvent effects. These factors may in turn be analysed into contributions from the solvation of the neutral basic species (NH3 and B) and the corresponding ions (NH4 and BH ), by means of the simple thermodynamic cycle... 

To formulate the basic model, we consider the transfer of a proton from a donor AHZ,+1 to an acceptor B 2 in the bulk of the solution. For reactions in the condensed phase, at any fixed distance R between the reactants, the transition probability per unit time W(R) may be introduced. Therefore, we will consider first the transition of the proton at a fixed distance R and then we will discuss the dependence of the transition probability on the distance between the reactants. 

The effects of transfer of atoms by tunneling may play an essential role in a number of phenomena involving the transfer of atoms and atomic groups in the condensed phase. One may expect that these effects may exist not only in the proton transfer reactions considered above but also in such processes as the diffusion of hydrogen atoms and other light ions (e.g., Li+) in liquids, tunnel inversion and isomerization in some molecules, quantum diffusion of defects and light atoms in the electrode at cathodic incorporation of the ions, ion transfer across the liquid/solid interface, and low-temperature chemical reactions. 

The fundamental approach to a proton transfer process, which is crucial to mimic many chemical and biological reactions, has relied deeply on studies of excited-state intramolecular proton transfer (ESIPT) reactions in the condensed phase. 

Elsaesser TH, Bakker HJ (2002) Ultrafast hydrogen bonding dynamics and proton transfer processes in the condensed phase. Springer, Heidelberg... 

Theorists have been addressing the issue of tunneling as the predominant reaction mode in proton-transfer reactions for more than 20 years [10]. However, from an experimental perspective, there have been few significant advances relating to tunneling as a predominant reaction mode in the condensed phase at ambient temperature [20, 21]. In part, this is because design of experiments to test the predictions of the various theoretical formulations has been exceedingly difficult. 

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. 

Quinone oximes and nitrosoarenols are related as tautomers, i.e. by the transfer of a proton from an oxygen at one end of the molecule to that at the other (equation 37). While both members of a given pair of so-related isomers can be discussed separately (see, e.g., our earlier reviews on nitroso compounds and phenols ) there are no calorimetric measurements on the two forms separately and so discussions have admittedly been inclusive—or very often sometimes, evasive—as to the proper description of these compounds. Indeed, while quantitative discussions of tautomer stabilities have been conducted for condensed phase and gaseous acetylacetone and ethyl acetoacetate, there are no definitive studies for any pair of quinone oximes and nitrosoarenols. In any case. Table 4 summarizes the enthalpy of formation data for these pairs of species. 

In this article, we present an ab initio approach, suitable for condensed phase simulations, that combines Hartree-Fock molecular orbital theory and modem valence bond theory which is termed as MOVB to describe the potential energy surface (PES) for reactive systems. We first provide a briefreview of the block-localized wave function (BLW) method that is used to define diabatic electronic states. Then, the MOVB model is presented in association with combined QM/MM simulations. The method is demonstrated by model proton transfer reactions in the gas phase and solution as well as a model Sn2 reaction in water. 

Kinetic Acidities in the Condensed Phase. For very weak acids, it is not always possible to establish proton-transfer equilibria in solution because the carbanions are too basic to be stable in the solvent system or the rate of establishing the equilibrium is too slow. In these cases, workers have turned to kinetic methods that rely on the assumption of a Brpnsted correlation between the rate of proton transfer and the acidity of the hydrocarbon. In other words, log k for isotope exchange is linearly related to the pK of the hydrocarbon (Eq. 13). The a value takes into account the fact that factors that stabilize a carbanion generally are only partially realized at the transition state for proton transfer (there is only partial charge development at that point) so the rate is less sensitive to structural effects than the pAT. As a result, a values are expected to be between zero and one. Once the correlation in Eq. 13 is established for species of known pK, the relationship can be used with kinetic data to extrapolate to values for species of unknown pAT. 

The early work on gas-phase acidities and basicities (Brauman and Blair, 1970 Brauman et al., 1971), obtained by probing the preferred direction of proton transfer reactions in the gas phase, established a benchmark in physical organic chemistry. The results pointed out, for example, that polarizability arguments could satisfactorily account for the smooth trends observed within a homologous series, unlike the -values in condensed media where solvation effects may reverse the relative orders of acidities and basicities. 

S. Bratos, J.-C. Leicknam, G. Gallot, and H. Ratajczak, Ultrafast Hydrogen Bond Dynamics and Proton Transfer Processes in the Condensed Phase, T. Elsaesser and H.J. Bakker (eds.), Kluwer Academic, Dordrecht, 2002, p. 5. 

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