Abstract: 2H NMR spectra of polycrystalline organic solids are dominated by quadrupolar coupling. Intramolecular origin of this coupling makes 2H NMR line shape is strongly sensitive to the mode and the rate of the molecular motion and allows determination of the reorientational mechanism of deuterated organic fragments or the molecule as a whole within a time scale of 10-3-10-7 s. When any anisotropic motion becomes fast in 2H NMR time scale, τC <10-7 s , the line shape exhibits no further sensitivity to the rate of this process. The information about the molecule reorientation and the time scale can be obtained from an analysis of the 2H NMR spin-lattice relaxation time T1. Unique capability of deuterium (2H) solid-state NMR to probe the dynamics in solid materials in a broad range of characteristic times has found the numerous applications for probing molecular structure and details of molecular motion in solid organic and inorganic materials. It is used for characterization of dynamics in crystalline and amorphous polymers, biopolymers, liquid crystalline materials, for analysis of mobility in proton conducting materials, for characterization of adsorbates in zeolite and other microporous materials, as well as for probing dynamics of the linkers in metal-organic frameworks and covalent organic frameworks. In this paper the basics of deuterium solid state NMR are shortly introduced for characterization of molecular motion in solids. Further some examples of 2H NMR application for characterization of molecular dynamics are provided. In particular, a rotational dynamics in solid tert-butyl alcohol has been established. It is shown that the molecule is immobile as a whole, while methyl groups perform fast rotation around successive C-C and C-O bonds, rotation of t-butyl fragment (around C-O bond) is faster than the rotation of methyl groups around C-C bonds by one order of magnitude. At 103-123 K reorientation of both the methyl groups and the butyl group occurs by 3-site jump (JJ) mechanism. Reorientation mechanism changes within a temperature range of 123-173 K. At 173-243 K reorientation of the methyl groups occurs by 3-site jump mechanism, while the butyl fragment rotates by free diffusion mechanism (JD mechanism).1 For t-BuOH adsorbed in H-ZSM-5 zeolite two axes rotation of the methyl groups is retained. However, the rotation of butyl fragment occurs faster than in the solid state and the activation energy of the rotation decreases. For t-BuOH in silicalite-1, two additional modes of the alcohol motion were identified: translational diffusion by jumps from one channel intersection site to another one and a large amplitude wobbling of the molecule at channel intersection site.2 Peculiarities of adsorption and translational diffusion of n-octane in the channel system of zeolite H-ZSM-5 have been established. n-Octane is adsorbed at low temperature exclusively in the straight channels of the zeolite. Below 373 K 2H NMR line shape (Q2 = 18 kHz, =0 for n-octane- d18 is accounted by the molecules diffusing along the straight channel. At T=373K evolution of 2H NMR line shape occurs. The line shape for adsorbed n-octane-d18 is dramatically changed. Two signals with notable values of asymmetry parameters ( Q21 = 18 kHz, 1=0.33;and Q22 = 22 kHz, 2 =0.8) are appeared instead of a single signal at T<373 K. The two signals correspond to translational diffusion of n-octane along the zigzag and straight channel. So heating at 373 K results to a redistribution of n-octane molecules, formerly located (after adsorption) exclusively in the straight channels, over the zig-zag and the straight channels.3 An example for characterization of dynamics in solid proton conducting materials is given for anhydrous 12-tungstophosphoric acid. At T< 423 K 2H NMR line shape for the deuterated acid corresponds to immobile protons (deuterons), τC>10-3 s. At T>423 K the line shape corresponds to a superposition of two signals from isotropic and anisotropic proton motion. Isotropic motion is accounted for by proton migration over the surface of the Keggin anion by hopping between neighboring oxygen atoms. Anisotropic motion represents the exchange of a deuteron between the two possible orientations of the OD bond at a bridged oxygen of the Keggin anion. Analysis of the rate constants have shown that the activation energies for both motions are of similar values but the rate of anisotropic motion is one order of magnitude faster that the isotropic motion.4 The examples for characterization of the framework mobility with 2H NMR in metal-organic frameworks are provided for MIL-47(V),5 MIL-53 (Al,Cr),5-6 UiO-66( Zr)7 and [Zn2(bdc)2(dabco)].8 We show that the reorientation of the phenylene fragments of terephthalate linkers in the studied MOFs occurs by π-flipping of the plane of the phenylene ring with the rate of 104-108 Hz. The rate of the rotation is sensitive to the phase transition between “large pore” and “narrow pore” phases in “breathing” MOF MIL-53. It is also demonstrated that the presence of guest molecules in the pores of MOFs strongly influences the rates and activation energies for the linker fragment rotational motions. So monitoring the rotation of linker by 2H NMR could be a good sensor for the presence of guest molecules and phase transitions in MOFs. References 1. A. M. Nishchenko, D. I. Kolokolov and A. G. Stepanov, J. Phys. Chem. A, 2011, 115, 7428−7436. 2. A. M. Nishchenko, D. I. Kolokolov, A. A. Gabrienko and A. G. Stepanov, J. Phys. Chem. C, 2012, 116, 8956–8963. 3. A. G. Stepanov, A. A. Shubin, M. V. Luzgin, H. Jobic and A. Tuel, J. Phys. Chem. B, 1998, 102, 10860−10870. 4. D. I. Kolokolov, M. S. Kazantsev, M. V. Luzgin, H. Jobic and A. G. Stepanov, ChemPhysChem, 2013, 14, 1783−1786. 5. D. I. Kolokolov, H. Jobic, A. G. Stepanov, V. Guillerm, T. Devic, C. Serre and G. Férey, Angew. Chem., Int. Ed., 2010, 49, 4791−4794. 6. D. I. Kolokolov, A. G. Stepanov and H. Jobic, J. Phys. Chem. C, 2014, 118, 15978−15984. 7. D. I. Kolokolov, A. G. Stepanov, V. Guillerm, C. Serre, B. Frick and H. Jobic, J. Phys. Chem. C, 2012, 116, 12131−12136. 8. A. E. Khudozhitkov, D. I. Kolokolov, A. G. Stepanov, V. A. 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Cite: Stepanov A.G.
Characterization of Molecular Mobility in Solid and Microporous Materials by Deuterium NMR
2016 International Workshop on Solid State Nuclear Magnetic Resonance (IWSSNMR) 19-21 Aug 2016