Solvent effect on ultra-fast intramolecular proton tranfer rate by means of spin-lattice relaxation

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The NMR spin-lattice relaxation was applied to analyze the potential energy surfaces or the geometries of intramolecular hydrogen bonds as well as the proton transfer rates.I. Solvent effect on hydrogen-bond structure.The O-H and N-H bond distances respectively for dibenzoylmethane (DBM) and N, N'-diphenyl-6-aminofulvene-1-aldimine (PALDIM) were determined in several solvents by the measurements of the ^1H and the ^<15>N spin-lattice relaxation times caused by the magnetic dipolar interaction with the ^<17>O nucleus or the ^<15>N-H proton. The NH bond distance of PALDIM increased in 0.05 A in acetonitrile than that in carbon tetrachloride. In the case of the OH distance of DBM, 0.04 A increase in DMSO was observed whereas no significant change was observed in CCI4 and acetonitrile. These trend is different from that predicted by a quantum mechanical calculation regarding solvent as dielectric continuum. The interaction between the hydrogen -bonding proton and the electron donor site of … More the solvent molecule is probably important to elongation of the X-H distances.II. Proton transfer rate constant of benzoic acid dimmerThe OH proton spin-lattice relaxation times caused by the magnetic dipolar interaction with the ^<17>O nucleus were determined in hydrocarbon mixture with wide range viscosity. In the high viscosity solvents the observed relaxation rates was different from those calculated only considering the reorientational motion of the dimmer as the fluctuation of the magnetic interaction. The difference was attributed to the contribution of the proton jumping between the two oxygen atoms. The estimated rate was 5x10^9 s^<-1> at 298 K and was much slower than that expected in the crystalline state in spite of the 60 - 80 kJ mol^<-1> of the driving force for the proton transfer in the crystal. The present result is rather better coincidence with the tunnel splitting by an abinitio MO calculation result for formic acid dimmer, ca. 3x 10_8 s^<-1> if one considers a stabilization of the charge delocalization on the carboxyl group in the benzoic acid by the resonance effect. Less