Symmetry and exchange

1 Explain why the deuterium-decoupled spectrum of cyclohexane- n contains only a sharp singlet at room temperature but two singlets at 170K. 

Answer One would expect the ring to take up the most stable chair conformation, so the lone proton will be found either in an equatorial position (a) or in an axial one (b). The two forms will be present at effectively the same concentrations because any isotope effect on the conformational preference should be too small to be significant. All the couplings to deuterium have been removed by decoupling, so the proton resonance should be a singlet. 

J Explain the temperature dependence of the H spectrum of azetidine. Both spectra were acquired with decoupling of the NH proton. 

0 2 At room temperature, the carbonyl region of the 20 MHz 13C spectrum of this A/-glucoside shows a single resonance at [ 170 ppm for C, a single resonance for C+ at 

E3 2 ie 19F spectrum of IF5 at 200K contains a doublet (4F) and a quintet (IF) at higher temperatures, the spectrum collapses to a 5F singlet. What is the structure of IF5 and what is happening to the spectrum on warming  

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More than any other spectroscopic technique, NMR allows us to determine the symmetry of a molecule, and observe the often-unexpected chemistry that is occurring in a solution. These symmetry and exchange effects are explored in Chapter 3. 

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