Reorientation of DABCO (1,4-diazabicyclo[2.2.2]octane) in halogen-bonded molecular complex DABCO-2(C6F5I)

Abstract

Temperature dependence of 1H NMR spin-lattice relaxation time is investigated of a 1:2 molecular complex of DABCO (1,4-diazabicyclo[2.2.2]octane) and iodopentafluorobenzene formed by the halogen-bonding between nitrogen and iodine atoms. Besides the known phase transition at 230 K from polar (space group Pc) to nonpolar (space group P21/c) phases, another first-order phase transition was suggested for the first time at 129 K. The correlation time of reorientational motion of DABCO molecule is determined as a function of temperature. The activation energies Ea of the motion were estimated to be 20 kJ mol−1 and 11 kJ mol−1 for the crystallographically nonequivalent DABCO molecules in the intermediate-temperature phase.