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Free keywords:
trifluoromethanesulfonate; dimethylammonium; trifluoromethyl group; crystal structure; disorder; phase transitions
Abstract:
Dimethylammonium trifluoromethanesulfonate I was synthesized by reaction of trifluoromethanesulfonic acid with an excess of dimethylarnme. A temperature variable synchrotron measurement on the polycrystalline substance reveals that I passes through a phase transition below room temperature. The transition occurs in the temperature range of 282-285 K on heating and 272-280 K on cooling as determined by DSC. The room temperature phase crystallizes in space group Cmca (a = 11.031(6) angstrom, b = 18.466(14) angstrom, c = 8.173(9) angstrom, V - 1665(2) angstrom(3), Z = 8) and the low temperature phase in space group P 2(1)/c (a = 8.8717(18) angstrom, b = 8.0838(16) angstrom, c = 10.968(2) angstrom, beta = 92.128(4)degrees, V = 786.0(3) angstrom(3), Z = 4). The structures of both phases were determined by single crystal X-ray diffraction, but refinement did not yield satisfactory residuals for the low temperature phase because of twinning of the crystal. It was, therefore, independently solved from the synchrotron powder diffraction data using rigid body models of the constituent ions and ab-initio direct space methods. Both, the CF3 group and the SO3 group of the triflate ion, are rotationally disordered around the S-C bond, in the room temperature phase. In the low temperature phase, the triflate ion is well localized. Like in the alkali metal triflates, the triflate ions are arranged in double layers with the hydrophobic trifluoromethyl groups and the sulfonate groups, respectively, pointing towards each other. The dimethylammonium ion is located closer to the sulfonate group with contacts indicating hydrogen bonding. The packing in both phases is of the topological CsCl structure type.
