[NHN]+ hydrogen bonding in protonated 1,8-bis(dimethylamino)-2,7-dimethoxynaphthalene. X-ray diffraction, infrared, and theoretical ab initio and DFT studies.

Structural (X-ray diffraction), infrared spectroscopic, and theoretical MP2 and DFT studies on the HBr and DBr adducts of 1,8-bis(dimethylamino)2,7-dimethoxynaphthalene ((CH3O)2.DMAN) were performed. This particular proton sponge has been chosen for its strong basicity and display of the buttressing effect influencing the hydrogen bond dynamics and properties. The studies revealed a symmetric, planar DMAN.H+ cation with a short (NHN)+ hydrogen bond of 2.567(3) A. The X-ray diffraction results suggest that the proton is in the central position in the bridge, while the calculations show two potential energy minima with the zero point energy level close to the top of the barrier. The infrared spectra display an (NHN)+ band at 488 cm(-1) and an (NDN)+ band at 235 cm(-1), respectively. It gives the isotopic ratio of 2.08, the highest value reported to date. Such a result suggests a peculiar shape of the potential for the proton motion, characterized by an extremely high positive anharmonicity. The calculations reproduce this particular potential, yielding an ISR value displaying a very good agreement with the experimental one. The anharmonic frequencies, however, show the discrepancy between the observed and calculated transitions.