Real life example

The spectra follow a system under chemical exchange at various exchange rates. The effect of going from slow to fast exchange are clearly visible, going from two lines over broadening and shifting to a finally single, coalesced and sharper signal again.

Note, that in fast exchange the signal is averaged in a population dependent manner!

For labile protons : even if we did not supress the water:

the labile protons in fast exchange would not be observable= due to the excess of H2O the chemical shift for the averaged shift is = basically the water chemical shift.

Even in intermediate exchange, labile protons would not be observable as they broaden out.

Implication for kinetics:

NMR can follow interconversions/ chemical exchange kinetics in the 1-100 s-1 range. Using the chemical shift separation of converting signals, a series of spectra at varying T, we can obtain the coalescence temperature , which can be correlated with thermodynamic parameters via kc= 2.22 Dn (at Tc)

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Summary?

• Why is water a problem for NMR

• Why do we work in water, not D2O

• What is the chemical shift time scale

• What is the difference between excitation/ non-excitation water supression techniques

• Do we need 2D-NMR

• Do we see all protons in amino acids /nucleotides

• What is / was the preferred pH for NMR in water

• What kinetic range can NMR characterize

Think about:

• Explain the temperature dependance of the HN-chemical shifts
  or
  

• Explain the pH sensitivity of the HN-chemical shifts