Proton transfer, hydrogen bonds bond vibrations

The EVB method proposed by Warshel [6] is an elegant and computationally very efficient method of describing the entire BO surface, thus allowing treatment of chemical reactions such as proton transfer in hydrogen bonds. It can also be used in vibrational analysis. In conjunction with the environment described at the molecular mechanics level it was the first QM/MM method. Vibrational analyses of hydrogen bonded systems and of enzymatic catalysis have a lot in common. 

Kiefer PM, Hynes JT (2002) Nonlinear free energy relations for adiabatic proton transfer reactions in a polar environment. II. Inclusion of the hydrogen bond vibration. J Phys Chem... 

In aqueous solution the exchange process H20 + D20 2HD0 occurs very rapidly via proton transfer. Consequently, in H20—D20 mixtures the rare earth has an average number of O—H bonds in its solvation sphere, the number being proportional to the H20/D20 ratio. The major quenching of a rare-earth ion in solution is due to the hydrogen vibrations about it and is proportional to the number of these bonds. 

In contrast to the subsystem representation, the adiabatic basis depends on the environmental coordinates. As such, one obtains a physically intuitive description in terms of classical trajectories along Born-Oppenheimer surfaces. A variety of systems have been studied using QCL dynamics in this basis. These include the reaction rate and the kinetic isotope effect of proton transfer in a polar condensed phase solvent and a cluster [29-33], vibrational energy relaxation of a hydrogen bonded complex in a polar liquid [34], photodissociation of F2 [35], dynamical analysis of vibrational frequency shifts in a Xe fluid [36], and the spin-boson model [37,38], which is of particular importance as exact quantum results are available for comparison. 

Abstract The problem of the low-barrier hydrogen bond in protonated naphthalene proton sponges is reviewed. Experimental data related to the infra-red and NMR spectra are presented, and the isotope effects are discussed. An unusual potential for the proton motion that leads to a reverse anharmonicity was shown The potential energy curve becomes much steeper than in the case of the harmonic potential. The isotopic ratio, i.e., vH/VD (v-stretching vibration frequency), reaches values above 2. The MP2 calculations reproduce the potential energy curve and the vibrational H/D levels quite well. A critical review of contemporary theoretical approaches to the barrier height for the proton transfer in the simplest homoconjugated ions is also presented. 

Occasionally, potential energy curves for bond stretching vibrations can be unusually flat as well. One special case is of particular importance in hydrogen-bonded complexes. At infinite intermolecular distance two states are possible in which the proton is either bound to molecule A or to molecule B. The two states are related by a proton transfer process ... 

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