Variable Temperature to Improve NMR Resolution

Many millions of dollars have been spent on high field NMR magnets to improve both sensitivity and chemical shift dispersion.  Many younger NMR users have had the good fortune to use only high field spectrometers where chemical shift resolution is often not an issue.  These users are not familiar with some of the "tricks" used to improve resolution which were needed on lower field instruments where chemical shift resolution was frequently a problem.  With the current helium shortage and the increasing popularity of low field permanent magnet spectrometers, these "tricks" will again become more and more common.  Among them are; the use of paramagnetic chemical shift reagents, the use of aromatic solvents or solvent mixtures and the use of variable temperature.  In this post, I would like to demonstrate the incredible power of simply changing the temperature at which the NMR data are collected.

The ¹H chemical shift is a sensitive parameter related to the conformation of a molecule.  In solution, small molecules may adopt a number of conformations whose populations depend on the potential energy profile.  Furthermore, the molecules are often in fast exchange between the available conformations and the observed chemical shift is the weighted average chemical shift of all of the conformations present.  As the temperature is changed, the populations of conformations are altered and the observed average chemical shift value may change.  The changes in chemical shifts at different temperatures are often enough to resolve resonance which may have overlapped with one another at room temperature.

The chemical shift of exchangeable protons ( -OH, -NH or NH₂) depends dramatically on the degree of both inter-molecular and intra-molecular hydrogen bonding.  When molecules with exchangeable protons are dissolved in aprotic solvents, one is often able to observe the exchangeable protons as well as their associated J couplings.  Such is the case with sucrose dissolved in DMSO-d₆ where all of the -OH protons can easily be observed.  When the temperature is changed, the populations of available conformations change and the degree of intra-molecular hydrogen bonding is affected with dramatic changes in the chemical shifts of the -OH resonances.  The figure below shows the anomeric and -OH region of the 500 MHz ¹H NMR spectrum of sucrose in DMSO-d₆ collected as a function of temperature.

All of the protons can be assigned with standard 2D NMR methods.  As the temperature is increased, the anomeric proton (1) moves to higher frequencies while the -OH protons (2-9) all move to lower frequencies to different extents.  Note that the resonances in the highlighted region of the spectrum at 21°C are overlapped with one another but at higher temperatures are fully resolved.  The resolution has increased by simply increasing the temperature.