Developments and applications in solid-state NMR spectroscopy

Analytical chemistry

Cutajar, Marica

University:University of Glasgow

关键词: Analytical chemistry;   

Others  :  http://theses.gla.ac.uk/74247/1/10800639.pdf

来源: University of Glasgow

【 摘 要 】

Recently there has been a lot of interest in using [2]H (I = 1) MAS NMR as an alternative method to 'static' [2]H quadrupolar-echo NMR for studying dynamics on the us timescale in solids. This thesis discusses two novel observations that increase the usefulness of the MAS approach. The spinning sidebands observed in [2]H MAS NMR spectra may be very strongly broadened if us-timescale dynamics are present. In Chapter 4, it will be shown that the usefulness of this phenomenon for the study of motion is significantly enhanced by the fact that this motional broadening is either absent or greatly reduced in the corresponding [2]H double-quantum MAS spectrum. A comparison of linewidths in the single- and double-quantum dimensions in a two-dimensional [2]H double-quantum MAS NMR experiment provides a very sensitive test for the presence of dynamics. Various simple deuterated materials, organic and inorganic, as well as a synthetic polymer will be used as examples to show how results drawn from the [2]H double-quantum MAS NMR experiment yield information about the dynamics of the system. [2]H quadrupolar-echo NMR studies of static solids are often hampered by the presence of very long spin-lattice (T[1]) relaxation times, up to several minutes for immobile deuterons. In Chapter 5, therefore, it will be shown that this problem is often not present in [2]H MAS NMR studies, where the phenomenon of rotational resonance, governed by spin diffusion can lead to the observation of a common T[1] relaxation time for all deuterons in the solid state. Thus, if rapidly relaxing methyl group (or similar) deuterons are present, even highly immobile deuterons can exhibit T[1] relaxation times of less than 1 s under MAS conditions, greatly facilitating data acquisition. To illustrate this phenomenon, experimental examples using [2]H NMR of deuterated materials will be shown and discussed. In the last chapter, solid-state MAS NMR spectroscopy is used to further investigate structure and framework topology of aluminophosphates. This is done by exploiting scalar (I) couplings across the Al-O-P bonds to obtain a heteronuclear correlation (HETCOR) between [27]Al and [31]P which allows us to obtain spectra that show through-bond connectivities between the two respective nuclei. We show that using a MQMAS experiment in combination with a refocused INEPT HETCOR (MQ-J-HETCOR) experiment we can obtain important information about AlPOs that x-ray diffraction techniques fails to yield. In addition to two-dimensional NMR experiments, 'first-principles' calculations are used to help and confirm the assignment of the spectra obtained.