Dynamic¹H and²H NMR investigations of acetonitrile confined in porous silica

Abstract

In this work, the molecular dynamics of acetonitrile and acetonitrile-d 3 confined in silica pores of nominal diameter 6 and 20 nm are studied by high field 1 H and 2 H NMR, and the results are discussed with reference to the bulk substances. NMR line-shapes, spin-lattice relaxation times (T 1 ), spin-spin relaxation times (T 2 ) and diffusivities (D) are reported as a function of temperature. The line-shape and relaxation measurements clearly reveal a two-component system in the solid phase, consisting of a narrow line superimposed on a broad complex resonance. The melting process takes place over a certain temperature region, due to an appreciable pore size distribution. Below the melting region, the narrow line originates solely from the surface layer, while the broad line is attributed to the crystalline solid at the interior of the pore. The confining geometry restricts the mobility of the liquid probe molecules and gives rise to significantly shorter T 1 and T 2 relaxation times and reduced self-diffusion rates, as compared to the bulk substance. This observation is also reflected in increased activation energies of the reorientational and translational motions. True intragrain diffusivities were obtained for the probe molecules by using the short diffusion time model, and extrapolating to zero observation time.

Solid‐State NMR Investigations of Supported Ionic Liquid Phase Water‐Gas Shift Catalysts: Ionic Liquid Film Distribution vs. Catalyst Performance

“…Since the ionic liquid can affect the activation of catalysts, the amount of ionic liquid on the porous support is obviously a crucial parameter that determines the catalytic performance of these materials [7]. In recent years, solid-state NMR techniques proved to be suitable methods for the characterization of guest molecules inside porous silica [8][9][10][11][12][13][14][15][16][17][18][19][20]. The volumetric ratio between ionic liquid and total pore volume of the support is referred to as a IL in vol.-%.…”

Probing the phase transition behavior of acetonitrile confined in mesoporous silica by FT Raman spectroscopy

“…[50] The thickness of the non-freezable contact layer in the pores is about 0.90 nm (assuming the contact layer has the thickness of three molecules wileyonlinelibrary.com/journal/jrs stacked one on top of the other). [40] The effective pore radius is only 0.69 nm [(3.18/2) nm −0.90 nm]. The unit cell dimensions of a, b, c for the β phase of acetonitrile are 4.102, 8.244, and 7.970 Å, [31] which are close to the effective pore diameter of 1.4 nm or 14 Å.…”

“…Acetonitrile confined in mesoporous silica can form hydrogen bonds with hydroxyl groups on the surface of mesoporous pore walls. [40] The hydrogen bonding facilitates the formation of a non-freezing liquid contact layer of acetonitrile near the pore wall of mesoporous silica. In this layer, the mobility of guest molecule is restricted due to the interaction with the pore surface.…”

Stacking Structure of Confined 1‐Butanol in SBA‐15 Investigated by Solid‐State NMR Spectroscopy

“…[3,6,12,13] Compared to the bulk liquids, the dynamic and thermodynamic properties of liquids change markedly in a confined space. Previous works elaborated the pore size dependence on molecular dynamics, [6,12,14,15] diffusion, [15][16][17] melting/freezing point, [13,18] phase transition, [12,19,20] and stacking structure [21][22][23][24][25] of confined liquid molecules. The magnetization relaxation times extracted from NMR spectroscopy can provide unique and exact dynamic properties of confined liquids.…”

“…[6] The behavior of temperature-dependent magnetization relaxation times reveal that not only the translational motion but also the reorientation motion of molecules that are restricted in a confined space. [15] For the confined effects on the thermodynamics of liquids, although phase transition can be monitored by calorimetry or by its dynamic difference, direct observation of the exact molecular nature in the confined space is hardly realized. The phase transition and structure of water in a confined space has received much attention for interpreting many biological processes.…”