Variable temperature NMR spectra


Some types of dynamic NMR questions can be answered with little fuss.  For example, amides often show cis-trans isomerism such that two sets of broad peaks will be seen in the 1H spectrum at room temperature.  The spectrum simplifies if the sample is warmed a bit and the two sets of peaks coalesce.  For some amides, 40-50 C is sufficient to fix up the proton spectrum (though it may actually make the carbon spectrum worse, if the amide carbonyl signals broaden more but aren't yet coalesced).  If all we want is a simplified proton spectrum, we may not care what the sample temperature actually is or how different it is from the displayed temperature, as long as the solvent doesn't boil in the process.

However, for accurate determination of activation parameters, or kinetics experiments, the actual temperature must be known.  It can be quite challenging to get good quality spectra at temperatures far from ambient, especially if the acquisition times are long.  Variable temperature experiments also raise additional experimental safety considerations.  Bear the following in mind:

Troubleshooting problems during VT experiments:



Dave 7/23/09