H-2 NMR LINE-SHAPE STUDIES OF DIMETHYLAMMONIUM CHLORIDE, BROMIDE, AND IODIDE

2H NMR powder line shapes have been obtained as a function of temperature for the dimethylammonium salts (CH3)2ND2+X- (X = Cl, Br, I) and (CD3)2NH2Cl in order to study the motions occurring at room temperature and above. The cations in the bromide and iodide salts undergo activated 180° flips around the molecular C2 pseudosymmetry axis. DND bond angles of 104.5° (Br) and 104.6° (I) were obtained. Low-temperature spectra of (CD3)2NH2Cl show, as expected, methyl group reorientation in agreement with previous work. The β-α-phase transition is marked by distinct line-shape changes and occurs at 309 K in (CH3)2ND2Cl and at 317 K in (CD3)2NH2Cl. In the α-phase chloride the lineshapes correspond to rapid n-fold reorientation (n expected to be 4 from the symmetry of the crystal) about an axis (Cn) parallel to the C⋯C vector. 180° flips about the C2 axis could also occur without affecting the line shape. A CNC bond angle of 112.3° was obtained. Motion in the β-phase chloride above 260 K is more complex, and a model involving C2 flips and n-fold reorientation among four inequivalent sites about the Cn axis is proposed. In this model the equilibrium site determined in the X-ray structure is much more highly populated than the other three sites. The static ND2 quadrupole coupling constants show increases attributed to the effects of decreasing hydrogen bond strengths in the order chloride, bromide, iodide.