Nuclear magnetic resonance signal averaging

The behavior associated with hindered rotation is seen in the nuclear magnetic resonance spectrum of methyl 5-acetamido-5-deoxy-2,3,4-tri-O-methyl-a-D-xylopyranoside (210) (see Fig. S). - In this compound, widely separated signals for the two anomeric protons of the rotamers A and B appear especially well at lower field. The ratio of forms B to A is about 1.5 to 1 at 110° (upper curve), the signals coalesce, as a result of more-rapid rotations, into one signal (which is the average of the separate signals). 

Nuclear magnetic resonance spectroscopy gives precise information on complexation in solution. Equilibrium is rapidly established on an NMR time scale, hence only an average spectrum is observed and it is difficult to determine the spectrum of a pure complex. When complexation of a sugar or polyol with a diamagnetic ion occurs, all of the signals shift downfield. Equation (11.1) allows the variation of the shielding constant Ao- of the proton to be calculated when the nucleus is subjected to an electric field E whose projection on the C-H bond is... 

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