Pyridine nuclear magnetic resonance spectrum

Infrared spectroscopy is an important technique for studying acidity. Acidic OH groups can be studied directly. Probe molecules such as pyridine may be used to study both Bronsted and Lewis acidity since two forms of adsorbed probes are easily distinguished by their infrared spectra. Quantitative infrared spectroscopy may be performed by measuring the spectrum of acidic OH or probes adsorbed on thin, self-supporting wafers of the acidic solid. Other spectroscopic methods which may provide information in specific cases include Fourier Transform Raman spectroscopy, electron spin resonance spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy. 

Some of the important differences in the nuclear magnetic resonance spectra of the /3-n-furanose form in various solvents are shown in Table I. As expected, the chemical shifts are somewhat altered on changing the solvent however, new bands appear with pyridine. In methyl sulfoxide, the mutarotation is very slow, and only one form is seen soon after dissolution. In anhydrous pyridine, the mutarotation is a little faster, so that, in a few minutes, the a- and /S-n-furanose forms are present in equal amounts. With the addition of deuterium oxide, equilibrium between the three forms is rapidly reached. This last spectrum is essentially the same as that of the crude product mentioned earlier. The small values for Jz 4 indicate that all three forms are in fixed conformations and are, therefore, not acyclic. The spectrum of the a-D-pyranose form is closely related to that of (24). The two furanose forms show, overall, a similar spectrum, except that the proton at C-1 of the )3-d anomer shows a relatively large coupling, J = 3.25, of unknown origin. The spectra of many five-membered heterocyclic compounds are anomalous, and not yet fuUy understood. 

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