Structure and IR spectroscopic behaviour of 2,7-dichloro-1,8-bis(dimethylamino)naphthalene and its protonated form

X-ray diffraction and IR spectroscopic studies on the proton sponge 2,7-dichloro-1,8-bis(dimethylamino)naphthalene and its adduct with HBr were performed. It was shown that the presence of the chlorine atoms in positions 2 and 7 leads to some change of conformation with respect to the free molecule. The main factor determining the conformation remains the repulsion of the nitrogen lone electron pairs. However, the repulsion between the methyl groups and chlorine atoms causes the dimethylamino groups to be slightly less twisted compared with unsubstituted dimethylaminonaphthalene. The protonation leads to the formation of a symmetrical cation (symmetry plane passing through the C9—C10 axis) with the N…N distance equal to 2.561(3) Å. This corresponds to the situation where the barrier to the proton transfer should be very low. As in other short [NHN]+ bridges, one observes the IR absorption band at about 500 cm−1, assigned to the transition between the split O+ → O− vibrational levels. A very high frequency isotopic ratio (νH/νD = 1.8) is observed for this transition.