J . Phys. Chem. 1990, 94, 152-157
152
2H NMR Line-Shape Studies of Dimethylammonium Chloride, Bromide, and Iodidet C.I. Ratcliffe Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR6 (Received: March 17, 1989; In Final Form: May 22, 1989)
2H NMR powder line shapes have been obtained as a function of temperature for the dimethylammonium salts (CH3)2ND2tX(X = CI, Br, I) and (CD&NH2CI 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 104.5O (Br) and 104.6' (I) were obtained. Low-temperature spectra of (CD,),NH,Cl show, as expected, methyl group reorientation in agreement with previous work. The P-a-phase transition is marked by distinct line-shape changes and occurs at 309 K in (CH3),ND2CI and at 317 K in (CD3)2NH2C1. In the a-phase chloride the lineshapes correspond to rapid n-fold reorientation ( n expected to be 4 from the symmetry of the crystal) about an axis (C,) 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 8-phase chloride above 260 K is more complex, and a model involving C2flips and n-fold reorientation among four inequivalentsites about the (7, 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.
Introduction Dimethylammonium chloride, (CH3)2NH2CI,is known to exist in three crystalline forms:' 313 K
~ - P - Y
260 K
I
;
supermob,
The P-phase can be supercooled so that in some instances, in ~ , ~y-phase does not form. particular in the absence of m o i s t ~ r e ,the Single-crystal X-ray diffraction studies show that the @-phase is orthorhombic (Ibam) with one molecule per asymmetric unit.2 The two methyl groups are equivalent but the two N-H bonds are not. The a-phase is tetragonal (14/mmm), with one molecule per asymmetric unit! It shows disorder (either static or dynamic) about a 4-fold axis passing through the (CH3)2NH2+ion parallel to the C.-C direction. The authors considered different models to describe this disorder in the least-squares refinement. Their simplest model (I) has ordered CI- ions and a 4-fold disorder of the (CH3)2NH2+ion. However, from infrared studies they concluded that the two N-H's on the molecule are inequivalent, and since model I has equivalent N-H's. they then developed a complicated model (IIIB), in which the Cl's are also disordered. If the disorder in IIIB is dynamic, cooperative motions of ions are required. I H NMR line shapes and second-moment s t ~ d i e indicated s~~~~~ rapid CH3 group reorientation above 200 K. Further motion was evident for the high-temperature range of the P-phase and for the a-phase. It was concluded that these higher temperature motions involved reorientation of the whole ion about its C2axis and about the crystal axis parallel to the C-C vector (the latter being a 4-fold axis in the a-phase). The y-phase gave rigid lattice second moments at l IO K,3*5whereas another set of results,6 which presumably represent the 8-phase, gave second moments consistent with methyl reorientation even at 4 K. The low second moment for the latter case might be explained by tunneling effects; the barrier in the 8-phase is low enough for appreciable reduction of the second moment due to tunneling, whereas in the y-phase it is too high. Activation energies for the various motions have been determined from 'H NMR spin-lattice relaxation ( T , )studies3 (Table I). An unusual frequency dependence of TI was observed in the high-temperature range of the &phase, associated with the whole-ion motions. The T , results also provided the most convincing evidence for two types of whole-ion motion in the a-phase. Just above the P a transition the T , values fall on the high-
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'Published as NRCC No. 30646. 0022-3654/90/2094-0152$02.50/0
TABLE I: Activation Energies and Correlation Time Constants for Motions in the Three Halide Salts of (CH3)*NH2+Determined from 'H T , Measure~nents~~'" phase motion E,,b kJ/mol T ~ ' , XIO-I4 s C3 (CH,) 14.69 26.7 Y-CI C3 (CH3) 11.08 (12.84) 24.8 p-Cl a-CI C, cation 41.5 1.6
P-Br a-Br
C, cation C3 (CH3) C3 (CH,)
cation I
C3 (CH3) cation
22.9 17.0 (20.66) 18.5 62 16.9 (18.55) 56.9
3.7 6.1 0.026 1.2
0.019
Results for chloride ref 3, for bromide and iodide ref 7. *Barrier heights from INS study* in parentheses. temperature slope (w07,
