Manipulation of Spin Dynamics and Multifaceted Applications of NMR Spectroscopy to Small Molecules

Abstract

The investigations have been carried out in the thesis entitled as “Manipulation of Spin Dynamics and Multifaceted Applications of NMR Spectroscopy to Small Molecules” to address the challenging problems encountered in chemistry, as far as the small organic molecules is concerned, and to underline the importance of NMR spectroscopy by utilizing various one- and two- dimensional techniques. The first part of the thesis discusses the design and utility of a novel experimental technique, cited as, 2D PS-Clean-PE-SERF. The developed methodology is utilized to address the inherent issues of sensitivity of detection, the spectral complexity and the precise measurement of JHH couplings of a selected proton from the complex 1H-NMR spectrum. In addition to enhancement in the sensitivity of detection, the novel experiment results in the complete elimination of the intense uninformative axial peaks and prevents the evolution of undesired couplings arising from the uncoupled partners, which otherwise would have created confusion in the assignment of true coupled partners of the chosen proton. In the second part of the thesis, the investigations have been carried out to probe the weak molecular interactions in the synthesized N-bezoylanthranilamide and its derivatives, especially the participation of organic fluorine in the hydrogen bond (HB) in the fluorine substituted derivative, by the extensive utility of number of one- and two- dimensional NMR techniques. The detection of doublet for an NH proton with significantly large coupling strength (1hJFH) arising due to the polarization transfer between 19F and 1H mediated through HB and a correlated peak in the HOESY established their spatial proximity conducive for HB. The rare and unusual observation of the partial retention of 1hJFH instead of complete nullification in the high polarity solvent DMSO-d6 is interpreted as due to the existence of favorable cis conformers in the investigated molecules. The rivalry between N-H…X and >C=O…H-N types of HBs is perceived as the reason for unusual shielding in the NH resonance frequency and relatively small 1hJFH, contrary to previous reports. NMR experimental findings are further corroborated by the single crystal XRD and DFT studies. In the last part of the thesis, utilization of the residual chemical shift anisotropy as a parameter for the discrimination of enantiomers in the chiral aligning medium is explored. The method is based on the differential alignment tensor components for each diastereomer, and the extent of discrimination is determined by the inter-tensor angle (θ). The robustness and the broad applicability of the method has been demonstrated in the water-compatible polysaccharide alignment medium, which has remained unexplored for the RCSA measurement, and in the versatile PBLG aligning medium which is also demonstrated on the functional group deficient molecule.