Applications Of Multiple Quantum Methods In NMR For Determination Of Dipolar Couplings And Chiral Discrimination
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This thesis is about excitation, detection, properties and applications of multiple quantum coherences applied to different dipolar coupled spin systems. Major focus of the work is on spectral simplification, measurement of residual dipolar couplings and discrimination of enantiomers in chiral aligning media. The first chapter gives a brief account on the fundamentals of nuclear magnetic resonance spectroscopy and multiple quantum coherences. This includes a description of product operator and polarization operator formalisms of pulses and evolution of magnetization. Subsequently a detailed account of two dimensional multiple quantum – single quantum (MQ-SQ) correlation experiments is given. Demonstration of the homonuclear MQ-SQ pulse sequence on a weakly coupled spin system and analysis of the spectrum obtained are also discussed. Homo-nuclear multiple quantum studies carried out to obtain relative the signs of the couplings have been reported in the initial part of the second chapter. The technique has been applied on doubly labeled acetonitrile (13CH313C15N) aligned in a liquid crystalline medium. Special situations like ambiguity in the determination of relative signs of the couplings from the appearance of two dimensional MQ-SQ spectra and the explanation for the same are also discussed. Homo-nuclear MQ experiments on indistinguishable spins, like protons in a methyl group of 13CH313C15N oriented in liquid crystal, and distinguishable spins, like the two carbons in the same molecule, have been carried out. Different directions of approach in which these results need to be analyzed have been discussed. Subsequent part of the chapter is about the correlation of connected MQ-SQ coherences. These experiments are significant in reducing the cross-peaks further from the MQ-SQ spectra. This concept is extended for the discrimination of optical enantiomers dissolved in chiral aligning medium made of poly-Γ-benzyl-L-glutamate (PBLG) and CDCl3. In molecules of Chemical and biological interest one encounters several nuclei such as, 1H, 13C, 15N and 19F. It will be of general interest to determine magnitudes and relative signs of the couplings among these coupled nuclei by NMR experiments. Utilization of hetero-nuclear MQ Experiments in solving such problems is discussed in the third Chapter. Hetero-nuclear MQ experiments were carried out on dipolar coupled 13CH313C15N, with the aim of obtaining the values and signs of various hetero-nuclear couplings in the molecule. The splitting of transitions in the spectra of oriented molecules is always influenced by the sum of dipolar and scalar couplings. Hence precise determination of dipolar couplings requires the knowledge of scalar couplings. To determine the J couplings, experiments were carried out on the same molecule in isotropic medium. When many coupled nuclei are involved one has to carry out several experiments to derive all the spectral parameters. In circumventing this problem heteronuclear multiple quantum experiments involving more than two nuclei as active spins are advantageous. This reduces the number of experiments and thereby reducing the total experimental time. Second part of this chapter demonstrates how a triple resonance triple quantum experiment can provide majority of the couplings from a given coupled system. The feasibility of the experiment is demonstrated even for molecules containing natural abundant isotopes. Application of multiple quantum j-resolved technique for chiral discrimination and obtaining complete one dimensional spectrum of each enantiomer from their racemic mixture is discussed in the fourth chapter. The two dimensional experiment consists of a selective double quantum excitation period followed by selective refocusing during indirect time domain, isotropic mixing and nonselective detection of SQ transitions. Hence this pulse sequence is named as DQSERF-COSY (Double Quantum Selective Refocused Correlation Spectroscopy). The experiment exploits the existence of different intra-methyl couplings between the enantiomers dissolved in chiral liquid crystal medium to separate the one dimensional spectra of each enantiomer in different cross sections. This is possible due to the fact that all the nuclei in any one of the enantiomers are coupled among themselves and there is no inter molecular interaction between the two enantiomers. Also one can extract all the couplings between protons in each enantiomer, which can subsequently be utilized for determination of the residual dipolar couplings, structure and orientation parameters.