Alumnos

We have directly characterized the metal centre local environment in several representative microporous zinc phosphites and phosphates by natural abundance 67Zn solid-state NMR at an ultrahigh magnetic field of 21.1 T. This work has demonstrated

that observation of solid-state 67Zn wide-line NMR spectra of Zn-containing microporous materials at natural abundance is feasible at very high magnetic fields. The observed spectra are sensitive to the Zn local geometry and mostly dominated by the second-order quadrupolar interaction with 67

The NMR interaction parameters were calculated theoretically to reproduce the experimental results and, in some cases, to assist in spectral assessments. For three materials investigated, since their unit cell sizes are relatively large (>1,000 Å

Zn quadrupolar coupling constant ranging from 2.4 to 9.0 MHz.

3

) the CASTEP calculation cannot be carried out simply due to the limitation of the computational resources available to us. Instead, molecular model cluster approach was used. The EFG parameters of the clusters were computed by using Gaussian 09 program. The calculated CQ

The Gaussian calculation results of a model cluster bearing the general properties of zinc phosphites indicate that the geometric parameters around Zn centres such as Zn– O bond length, O–Zn–O and P–O–Zn angle all contribute to the C

values generally are reasonably close to the experimental results.

Q, but the Zn–O bond

length is the most dominant factor. Among all the materials investigated, there are only three whose unit cell sizes are suitable for the CASTEP calculation. As reported by many researchers,41-47

In short, this work demonstrated that it is now possible to study Zn centre environments in porous materials by

when the CASTEP calculations using the crystal structure available in the literature do not reproduce the experimental results correctly, geometry optimization can significantly improve the agreements between computed and observed value. This is exemplified by ZnPO-Li-ABW.

67

abundance. It should also be pointed out that the sensitivity could still be an issue even at 21.1 T. The extremely low Zn concentration resulting from the very low density may prevent one from obtaining the spectra with very high quality within a reasonable period of time. Combination of 67Zn NMR data and theoretical calculations has the potential to be used as a tool to characterize Zn containing materials with low Zn concentrations. For example, this approach can be expanded to examine other important types of Zn- containing porous materials such as metal-organic frameworks (MOFs) and zinc imidazolate frameworks (ZIFs).

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Chapter 4 Solid-state

O NMR Spectroscopic Studies of the