Study of coupled spin system using two dimensional NMR spectroscopy

Abstract

Chapter 1 contains a review of various 2D NMR techniques, their applications and an outline of this work. The theory of spin?echo multiple?quantum spectroscopy of strongly coupled spin systems and that of other 2D NMR experiments through density?matrix analysis have been outlined in Chapter 2. The first COSY experiment has been performed on an AA?BB? spin system oriented in liquid crystal. It is demonstrated that the COSY spectrum of oriented molecules in liquid crystal enables the identification of different symmetry classes, origin and connectivities of various transitions and some of the spectral parameters. A multiple?quantum pathway description has been introduced to describe various peaks of the COSY spectrum and to understand the energy?level diagram of coupled spin systems. These results are discussed in Chapter 3. COSY experiments of several oligopeptides have been performed and the complete resonance assignments and the J?connectivity map using a single COSY spectrum in each case have been described in Chapter 4. In Chapter 5, the effect of strong coupling on spin?echo multiple?quantum spectroscopy has been demonstrated. It is shown that the refocusing 180° pulse at the centre of the evolution period remarkably modifies the multiple?quantum spectrum, giving rise to new frequencies and new coherences, in the case of strongly coupled AB, ABX and AB? spin systems. The effect of selective and non?selective 180° pulses, in the case of ABX spin system, has been studied in detail. It has been noticed that strong coupling allows excitation and detection of multiple?quantum coherences of AB spins through pulses only on X spin. It has also been found that in the case of coupled spin systems, the study of multiple?quantum coherences helps in spectral analysis. The study of multiple?quantum coherences for the investigation of scalar?coupled networks in a molecule is described in Chapter 6. The 2D multiple?quantum spectrum of an AMX spin system has been calculated with flip?angle dependence of the mixing pulse, when the coupling between A and X spins is vanishingly small. It has been shown that the excitation and detection of triple?quantum coherence, 3?spin?1Q coherences, the double? and zero?quantum coherences involving the flipping of A and X spins through the common partner, M, are possible. The effect of strong coupling on multiple?quantum filtered COSY spectrum of an AB spin system is described in Chapter 7. The amplitudes of various multiple?quantum coherences of AB, AMX and ABX spin systems for different pulse schemes are listed in Appendix A. In Appendix B, the analytical expressions for the diagonal peaks of the ABX spin system and the study of strong coupling on diagonal peaks are presented. The study of relaxation of AX spin system using Fourier double?resonance spectroscopy is described in Appendix C. The FORTRAN program, which was used for the above?mentioned studies, is listed in Appendix D.