Proton Dynamics in Hydrogen‐bonded Crystals

First Observation of Ultrafast Intramolecular Proton Transfer Rate between Electronic Ground States in Solution

“…Thus, the observation of the PT rates between electronic ground states in solution is indispensable for understanding how a solvent affects the PT rate. Nevertheless, experimentally observed quantitative information has not been obtained to date for the PT rates of this type in solution; experimental information is only available for the gas and solid states, which has been obtained by methods such as microwave, infrared spectra, nuclear magnetic relaxation, and inelastic neutron scattering. − …”

“…Measurements of nuclear magnetic relaxations have been applied to determine the PT rates in a solid because the proton jumps accompanying the PTs cause fluctuations in the magnetic dipolar couplings, which are sometimes the primary source of magnetic relaxations of nuclei with 1/2 spins such as protons because the couplings depend on the inverse of the cube of the distance between the nuclear magnetic moments. − In contrast, molecular rotation is usually the main cause of the fluctuation contributing to nuclear magnetic relaxations in solution. , However, the fluctuation caused by the PT should contribute to the spin-lattice relaxation when the time scale of the proton jump approaches that of the molecular rotation. Picosecond-order PTs are expected to occur in low-PT-barrier hydrogen-bond systems.…”

Application of Nuclear Magnetic Relaxation To Elucidate Proton Location and Dynamics in N···H···O Hydrogen Bonds

“…Here, we use 15 N spin–lattice relaxation attributed to the 15 N– 1 H magnetic dipolar interaction as a direct tool for observation of the proton (hydrogen) location and dynamics, i.e., the PT rate. The 15 N– 1 H magnetic dipolar interaction in the solid state has often been applied to analyze the structures of hydrogen-bond systems including nitrogen atom(s) because of the inverse cubic dependence of the magnitude of the local magnetic field by the nuclear spins, such as protons, near the 15 N nucleus. − In solution, on the other hand, the dynamics of the molecules, e.g., the reorientational motion, causes the 15 N– 1 H dipolar interaction to fluctuate, so that the 15 N spin–lattice relaxation time attributed to the 15 N– 1 H magnetic dipolar interaction, T 1 dd ( N H), depends on the correlation time of the reorientation of the NH vector. , The T 1 dd ( N H) value is also dependent on the PT dynamics, because the PT, i.e., the proton jump, causes the NH distance to fluctuate. The contribution of the PT dynamics to the spin–lattice relaxation induced by the magnetic dipolar interaction in solution has already been discussed in detail. , In the present paper, we attempt to determine the proton location and its dynamics, i.e., the PT rates, for the chosen Schiff bases in dichloromethane and acetonitrile solutions from the 15 N spin–lattice relaxation times, taking into account the NH reorientational motions.…”