A Deuterium NMR Study of some Ammonium Tetraphenylborates

Abstract

Abstract Deuterium nmr has been used to study the rotational dynamics of the ammonium ion in ammonium tetraphenylborate and of the cation in a series of methylammonium tetraphenylborates. The results of variable temperature 2H relaxation measurements (T1Z and T1Q) on the parent compound, ND4B(C6H5 )4 , lead to a low activation energy for reorientation of the ND4+ ion, 5.0 ± 0.5 kJ mol-1 , indicative of weak cation-anion interactions. Related studies of the tetramethyl-d12 ammonium salt indicate rapid jumps of the whole cation (τ ≪ 10-5S) over the complete temperature range investigated, 137 to 385 K, with an activation energy of 14.1 + 0.5 kJ mol-1 . Deuterium nmr lineshape studies of the methylammonium salt indicate rapid reorientation of the CD3 and ND3 groups about their C3 axes; however, the motion of the C - N axis is restricted to large amplitude librations. Similar studies of the dimethylammonium salt indicate methyl group jumps at 245 K; at temperatures above 320 K the cation also undergoes two-site jumps about the C2 axis. In the case of the trimethylammonium salt, the nmr lineshapes indicate methyl group jumps superimposed on C3 rotations of the complete cation about the N - H bond axis over the temperature interval 185-348 K. In all the tetraphenylborate salts studied, the 2H nmr lineshapes indicate substantial librational motion of the cation at room temperature.