Pressure tuning between NH...N hydrogen-bonded ice analogue and NH...Br polar dabcoHBr complexes

At normal conditions 1,4-diazabicyclo2.2.2]octane hydrobromide C(6)H(13)N(2)](+.)Br(-) forms centrosymmetric crystals, space group Pm2, NH(+)...N hydrogen-bonded linear polycationic chains with disordered protons in the structure. As in H(2)O ice Ih, the protons in C(6)H(13)N(2)](+.)Br(-) crystals remain disordered at low temperatures. Above 0.4 GPa the C(6)H(13)N(2)](+.)Br(-) crystals transform into a new polar NH(+)...Br(-) hydrogen bonded complex, space group Cmc2. It has been crystallized in-situ in a diamond anvil cell and its structure determined by X-rays. The low-pressure triggering of this transformation indicates that it is a possible source of defects in the real structure at normal conditions, where, along with disproportionation defects, they can be responsible for anomalous dielectric properties, including relaxor-like behavior of NH...N hydrogen-bonded compounds.