Elucidation of crystal growth, structural characterization, thermal properties, and molecular dynamics using NMR near...

This discussion explores various facets of recently elucidated characteristics of [N(CH₃)₄]₂CoCl₄ single crystals. Phase transitions were identified at 196, 276, and 293 K, and thermodynamic stability was observed up to 640 K. With increasing temperature, the crystal structures of phases III, II, and I were monoclinic, orthorhombic, and orthorhombic, respectively. In addition, the nuclear magnetic resonance (NMR) chemical shifts of ¹H, ¹³C, and ¹⁴N were analyzed to elucidate the structural geometry near T_C. In phases I and II, two ¹³C NMR signals were observed that may be attributed to the different N(1)(CH₃)₄ and N(2)(CH₃)₄ sites, whereas, in phase III, four ¹³C NMR signals were observed that may be attributed to the six different N(CH₃)₄ sites. By contrast, because the spin of ¹⁴N is I = 1, two signals are generally expected, however, in phase I, four signals were observed owing to the different N(1)(CH₃)₄ and N(2)(CH₃)₄ sites. In phase III, 12 signals were observed because six different N sites were present. The results of the ¹³C and ¹⁴N NMR indicate that the transition from phase I to phase III involves a decrease in the symmetry of the crystal structure. The temperature dependences of the relaxation times T₁ and T_1ρ in the laboratory and rotating frame for¹H enabled the estimation of the activation energies for the molecular motion of the N(CH₃)₄ groups.