H MAS NMR Study of Local Structure and Dynamics of Water Molecule in (±)-[Co(en)₃]Cl₃·nD₂O

Abstract

Dependence of the local structure and dynamic behavior of hydration water in (±)-[Co(en)3]Cl3·nD2O crystal (0 ≤ n ≤ 4) on the dehydration process was studied by means of 1H magic-angle sample spinning (MAS) NMR technique. The line shape in the 1H MAS NMR spectrum depends strongly on the water content (n). For n < 1.5, the spectrum consists mainly of two peaks whose relative intensities change with n, suggesting that the hydration water forms two different types of domainlike structures at room temperature. In addition, the full width at half-maximum of each component of the doublet was less than 1 ppm at room temperature, implying that the water moves rapidly enough to reduce the intermolecular 1H−1H dipole interaction. On cooling, the resonance line broadens and its line width exceeds 10 ppm below 200 K where the components of the doublets cannot be discerned. This aspect suggests that the water molecule undergoes slower motion than the MAS rate (5.0−7.0 kHz). The temperature dependence of the line width was analyzed by assuming the Davidson−Cole's spectral density. The activation energy (E a) for the molecular motion of the water depends drastically on the water content: E a assumes the constant value of 24 kJ mol-1 for n ≤ 2.1, but it decreases continuously for n > 2.1 and reaches 18 kJ mol-1 at n = 4. This finding suggests that the molecular motion of hydration water in the pore, which averages out the dipolar interaction between a trace amount of HDO and the protons on the pore wall, changes from whole molecular translational jump to proton migration through hydrogen bond accompanied by molecular reorientation.

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

“…The Cole-Davidson spectral density function (Eq. 5) was developed in studies of dielectric relaxation [26] and has been applied to NMR as well as dielectric relaxation [27-29]. $${\mathrm{normalJ}}_{\mathrm{CD}}(\mathrm{\omega })=\frac{1}{\mathrm{normal\omega }}\frac{\sin (\mathrm{\beta }·\arctan (\mathrm{\omega \tau }))}{{(1+{(\mathrm{\omega \tau })}^2)}^{\mathrm{\beta }/2}}$$where β characterizes the distribution of correlation times.…”

Polymorphism and Pseudopolymorphism of the [Ni(4-Methylpyridine)4(NCS)2] Werner Complex, the Compound that Led to the Concept of “Organic Zeolites”

“…Examples of such architectures include 1D, − 2D, ,− and 3D − ,− host species and host frameworks. Robust microporous architectures built upon discrete units are less common. − ,− Most of these discrete structures either comprise cumbersome macrocycles, − or make use of interactions stronger than van der Waals, such as intermolecular hydrogen bonds 44,45,145,146 or secondary 3D coordination assembly …”

“…Robust microporous architectures built upon discrete units are less common. [44][45][46][47][48][136][137][138][139][140][141][142][143][144][145][146][147][148] Most of these discrete structures either comprise cumbersome macrocycles, [136][137][138][139][140][141][142][143] or make use of interactions stronger than van der Waals, such as intermolecular hydrogen bonds 44,45,145,146 or secondary 3D coordination assembly. 48 Structure of β-[Ni(4-MePy) 4 (NCS) 2 ]*(C 6 H 6 ) and Flexibility of the β Matrix.…”

Electron spin–lattice relaxation of nitroxyl radicals in temperature ranges that span glassy solutions to low-viscosity liquids

“…For β = 1, J CD (ω) reduces to J BPP (ω). The Cole-Davidson spectral density model has been applied to NMR and dielectric relaxation [19][20][21], in which the spectral density of solvent molecules is monitored. However, in an EPR experiment the solute motion is monitored.…”