Effects of Counterion and Solvent on Proton Location and Proton Transfer Dynamics of N-H•••N Hydrogen Bond of Monoprotonated 1,8-Bis(dimethylamino)naphthalene

The proton location and proton transfer (PT) 2 dynamics of a hydrogen bond are under the influence of the static and dynamical properties of the solvent and counterions. In the present study, the N-H distances were determined for salts of 1,8-bis(dimethylamino)naphthalene, DMANH(+)X(-) (X- = BPh4-, ClO4-, and Cl-), in acetonitrile (AN) solution, and DMANH(+)Br(-) in water by observing the N-15 spin-lattice relaxation caused by the N-15-H-1 magnetic dipolar coupling under assumption that the PT time was shorter than the NH reorientation time (similar to 10(-11) s). The obtained N-H distances decreased in the following order: DMANH(+)BPh(4)(-) > DMANH(+)ClO(4)(-) > DMANH(+)Br(-)/H2O > DMANH(+)Cl(-), indicating that interactions with the environment affect the PT potentials. To understand the results at the molecular level, Car-Parrinello molecular dynamics simulations were performed for DMANH(+), DMANH(+) in water, and DMANH(+)-Cl- ion-pair in AN. The results of simulation suggest that (1) the N-H distance decreases in the presence of a solvent and counterion; (2) the PT time is probably similar to 10(-12) s, which confirms the above assumption used for the NMR relaxation data analyses; and (3) fluctuation of the interactions with the solvent or counterion has a significant role in PT. Quantum nuclear effects on the hydrogen bond were also examined.