Proton magnetic relaxation and internal rotations in tetramethylammonium cadmium chloride

Abstract

Proton spin–lattice relaxation times (T1) and second moments (M2), at 14.7 MHz, have been investigated for tetramethylammonium cadmium chloride. Discontinuities in T1 at 104 and 119 K indicate the occurrence of crystallographic transformations. It is proposed that, in the low temperature phase below 104 K, the correlation time for hindered motion of one of the methyl groups is somewhat different from that of the other three. Using this model, the calculated T1 and M2 values are in satisfactory agreement with the experimental data. For the low temperature phase, the activation energies of the tetramethylammonium ion, 2.0±0.3 and 1.6±0.3 kcal/mole (8.4±1.3 and 6.7±1.3 kJ/mole), are considerably lower than the values 5.5–11 kcal/mole observed for the tetramethylammonium halides. In the high temperature phase, the activation energy decreases even further to 0.7±0.2 kcal/mole (2.9±0.8 kJ/mole). This decrease is in accordance with other order–disorder phase transitions involving tetrahedral ions.

NQR and X-ray Studies of [N(CH3)4]3M2X9 and (CH3NH3)3M2X9 (M = Sb,Bi; X = Cl,Br)

“…Such NMR results are known in literature e.g. for [N(CH 3 ) 4 ]X (X = CI, Br, I [16]; X = CdCl 3 [17]). The disappearance of 81 Br NQR lines around 100 K is due to the onset of these cation reorientations.…”

“…in Cs 3 Bi 2 Br 9 , [3], and the third one is built up of isolated M 2 X| e anions as found in [N(CH 3 ) 4 ] 3 Sb 2 Br 9 [4]. We tried to investigate the structures of A 3 M 2 X 9 in more detailed X-ray diffraction experiments and by observing NQR * Presented at the Xlth International Symposium on Nuclear Quadrupole Resonance Spectroscopy, London, United Kingdom, July [15][16][17][18][19]1991. Reprint requests to Dr. H. Ishihara, Dept.…”

1H NMR study of internal motions and quantum rotational tunneling in (CH3)4NGeCl3

“…Similar behavior has been observed in (CH 3 ) 4 NCdCl 3 , but at much lower temperatures. [22] The observed plateau second moment of about 2.8 G 2 in the HT region suggests that both TMA and methyl group reorientations are active down to 160 K and below which, motions of both the groups start freezing. Our results show no resolved plateau in second moment, indicating that the two motions do not have widely differing correlation frequencies.…”

Proton magnetic relaxation in (TMA)2HgBr4 and (TMA)2HgI4

“…The crystal structure of TMACdCI3 permits the possibility of one of the four methyl groups to have a different correlation time compared to that of the other three, on the basis of which occurrence of a T lmin corresponding to the TMA ion on the low temperature side was accounted for (Tsang and Utton 1976). The crystal structure of (TMA)2HgI4 does not lend support to a similar possibility.…”