Proton motion—quantum-dynamical

For the partially deuterated benzoic acid (C6D5COOH), the solid state H NMR spectrum is dominated by the intra-dimer H- H dipole-dipole interaction. In a single crystal, both tautomers A and B are characterised by a well-defined interproton vector with respect to the direction of the magnetic field (Fig. 1). Proton motion modulates the H- H dipole-dipole interactions, which in turn affects the H NMR lineshape and the spin-lattice relaxation time. It has been shown that spin-lattice relaxation times are sensitive to the proton dynamics over the temperature range from 10 K to 300 K, and at low temperatures incoherent quantum tunnelling characterises the proton dynamics. A dipolar splitting of about 16 kHz is observed at 20 K. From the orientation dependence of the dipolar splitting, the... 

S. Y. Kim and S. Hammes-Schiffer (2003) Molecular dynamics with quantum transitions for proton transfer Quantum treatment of hydrogen and donorac-ceptor motions. J. Chem. Phys. 119, pp. 4389-4398... 

For large molecular aggregates only classical mechanics can be applied to describe the nuclear motion of the particles through Newtonian-type equations of motion. This is an approximation, which may only fail if the motion of light particles such as protons needs to be accurately described. If the latter is the case, explicit nuclear quantum dynamics is required. Such extensions that include quantum effects [99, 100] or that incorporate full quantum dynamics have been explored [101],... 

This chapter describes a general theoretical formulation for PCET and summarizes the results of applications to a wide range of different types of PCET reactions in solution and enzymes. This theoretical formulation treats the active electrons and transferring proton(s) quantum mechanically and includes the interactions among the electrons, proton(s), and solvent or protein environment. Moreover, this formulation allows the inclusion of the proton donor-acceptor motion, explicit molecular solvent and protein, and dynamical effects. The theory described here is directly applicable to nonadiabatic PCET reactions accompanied by substantial solute charge redistribution and solvent reorganization. 

Computer simulations of excess proton conductivity in water have reached a powerful level [8,92,93,102]. Importantly, simulations extend to quantum-mechanical proton dynamic features, so that proton motion can be coupled to details of the molecular environmental dynamics. A recent feature article explored an analytical theory in order to rationalize these complex processes that involve interconversion of proton-bearing clusters and proton transfers [103]. With a simple two-state empirical valence bond model (see below for details), which implements in a classical way the above-mentioned idea of two limiting protonated structures, namely the 11502 and the H30 cluster, it was indeed observed that the two alternative sequences are equivalent with similar life times for both clusters, and that conversions between the two clusters are purely fluctuative. 

---