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Frozen Rotation of the Ammonium Group Observed for the First Time in Liquid-Phase NMR Spectra.

Hindered rotation of the -NH3+ group in a sterically crowded molecular environment was frozen on the NMR time scale. These effects were observed for solutions of 1,2,3,4-tetrabromotriptycyl-9-ammonium tetrafluoroborate in tetrahydrofuran with and without excess of HBF4. In the absence of acid, the hindered rotation is accompanied by a number of other effects originating in part from practically nonremovable traces of water in the solvent. Because of these additional effects, an effective rate constant of the hindered rotation could be evaluated at only one temperature for the neutral solution of the salt. For one of the acidified samples, which was prepared with special precautions against moisture, these complications are practically nonexistent at low and intermediate temperatures. For the latter sample, the Arrhenius parameters of the hindered rotation could be determined from the rotation rates evaluated in the range 194-226 K.