Nuclear magnetic resonance conformational interconversion

After a short qualitative introduction to the principles of dynamic nuclear magnetic resonance spectroscopy , the proposed interconversion processes for cycloheptanes and cycloheptenes are explained in detail. According to calculations, the most favourable conformation for cycloheptanes seems to be the twist chair cycloheptenes prefer the chair form. Possible conformational processes for chair and boat forms are discussed and illustrated. 

Other workers have studied conformational and configurational equilibria by nuclear magnetic resonance spectroscopy. Horton and coworkers295ribopyranose tetraacetate exists in acetone-d6 at room temperature in the CE and CA conformations, in rapid equilibrium, in the ratio of 9 11. At —84°, the ratio of the CE and CA conformers was 2 1 at —60°, the rate of interconversion of the two conformers was found to be 130 times per second. The equilibrium... 

P. Bucci, M. Longeri, C. A. Veracini, L. Lunazzi, Nematic phase nuclear magnetic resonance investigations of rotational isomerism. HI. Conformational preferences and interconversion barrier of 2,2 -bithienyl, J. Am. Chem. Soc., 96, 1305-1309 (1974). 

The n-site Bloch-McConnell equations describe the evolution of nuclear spin magnetization in the laboratory or rotating frames of reference for molecules subject to chemical or conformational interconversions between n species with distinct NMR chemical shifts. Trott and Palmer used perturbation theory to approximate the largest eigenvalue of the Bloch-McConnell equations and obtain analytical expressions for the rotating-frame relaxation rate constant and for the laboratory frame resonance frequency and transverse relaxation rate constant. The perturbation treatment is valid whenever the population of one site is dominant. The new results are generally applicable to investigations of kinetic processes by NMR spectroscopy. 

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