Multi-Dimensional Solid State NMR - Methods Development and Application to Materials

Abstract

In recent years, the scope of application of solid-state NMR to different areas has been witnessing a significant increase. Primarily, this is due to the possibility of routine use of proton NMR spectroscopy at ultrafast magic angle spinning (MAS). However, even at 60 kHz MAS frequencies, residual line-widths in proton spectra pose a challenge in identifying the closely spaced resonances. In this context, editing experiments that distinguish resonances based on the attached heteronuclei would help in simplifying the appearance of a spectrum and also help in assignment. Two new editing methods have been proposed, one is to obtain the carbon bonded protons and the other is to eliminate the protons attached to carbon atom. The cross-polarization - reverse cross-polarization method has been utilized for editing proton spectra by identifying protons attached exclusively to a nucleus such as carbon or nitrogen. A second approach uses proton detected cross-polarization polarization-inversion and eliminates signals of protons attached to a carbon atom. The intensity oscillations observed in this experiment can also be used to estimate the dipolar coupling between proton and carbon. The utility of the above methods has been illustrated for the case of several samples. Two proton detected three dimensional experiments have also been proposed for assigning resonances and also for getting the proximity and structural information, namely. the 3D SOCP-HSQC which is based on second-order cross-polarization mechanism and the other carried out using 1H double quantum coherences. The proposed sequences have significant advantages which have been demonstrated on several amino acids and peptides. Solid-state NMR is also being increasingly used for the study of novel materials of significant application potential. In the thesis, investigations on newly proposed fast ionic conducting materials have been presented. 23Na and 29Si solid-state NMR investigations have been carried out on sodium strontium silicate ion conductor, Sr0.55Na0.45SiO2.775 and the results provide the first experimental evidence to show that different synthesis conditions induce multiple devitrified phases. The surprising observation of about an order of magnitude higher ionic conductivity achieved in devitrified samples is attributed to the growth of the crystalline δ-Na2Si2O5 phase within the amorphous Na2Si2O5 phase domains, identified using NMR. Together with XRD and conductivity measurement data, the study leads to the identification of the chemical phase composition and an understanding of the composition-property-structure correlation in this material.