Low-temperature (120 K) structure and vibrational spectrum of protonated proton sponge: the adduct of 1,8-bis(dimethylamino)naphthalene (DMAN) with 4,5-dicyanoimidazole (DCI)

Low-temperature (120 K) studies of the structure of the DMAN.DCI adduct ind icate that in symmetrical [NHN](+) hydrogen bridge of 2.571 (1) Angstrom le ngth (2.579(2) Angstrom at room temperature) there is a disorder of the H-a tom occupying two positions at nitrogen atoms with a distance of 0.94(3) An gstrom. The comparison with the situation at room temperature seems to show a very low barrier for the proton transfer. The low-frequency vibrations w ith the participation of the whole N(CH3)(2) groups observed in Raman and i nelastic incoherent neutron scattering (IINS) spectra of about 100 cm(-1) e xcited at room temperature cause the fundamental level of the protonic mode to penetrate or exceed the barrier. The bending CNC vibrations of about 50 0 cm(-1) are strongly coupled with the protonic mode leading to Evans holes in the band ascribed to the 0(+) --> 0(-) transition. This hypothesis is c onsistent with the literature data relating to theoretical studies on H3NHN H3+, which show that the barrier for the proton transfer disappears at the hydrogen bond length of about 2.55 Angstrom. Copyright (C) 1999 John Wiley & Sons, Ltd.