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

Abstract

The proton location and proton transfer (PT) 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 ¹⁵N spin–lattice relaxation caused by the ¹⁵N–¹H magnetic dipolar coupling under assumption that the PT time was shorter than the NH reorientation time (∼10^–11 s). The obtained N–H distances decreased in the following order: DMANH⁺BPh₄^– > DMANH⁺ClO₄^– > 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 ∼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