TemperatureNMR

Temperature-NMR, commonly referred to as variable-temperature NMR (VT-NMR), is NMR spectroscopy performed at controlled sample temperatures that differ from ambient conditions. It is used to investigate temperature-dependent processes such as molecular dynamics, conformational exchange, phase behavior, and reaction kinetics.

VT control is provided by a variable-temperature accessory integrated with the NMR probe. It heats or cools the sample by circulating gas or through a contact heating/cooling element. Temperature is monitored with a sensor near the sample and calibrated against a reference. Typical ranges run from about -100 to +100 °C, depending on solvent and hardware. Equilibration after each temperature change is required. Solvent choice matters, as freezing, boiling, or viscosity changes can affect spectra and relaxation properties.

Observing spectra over a temperature series reveals changes in chemical shifts, peak widths, and line shapes due to altered dynamics. For exchange processes, temperature affects rate constants; line-shape analysis or exchange spectroscopy (EXSY) can yield kinetic parameters and activation energies (Arrhenius behavior). VT-NMR is widely used to study conformational equilibria, tautomerism, ligand binding, and the dynamics of small molecules, polymers, and biomolecules.

Limitations include the need for stable and uniform temperature; artifacts can arise from nonuniform heating, solvent loss, or sample degradation at high temperatures. Low temperatures may cause crystallization or poor solubility. Not all nuclei or experiments benefit from temperature variation, and interpretation must consider temperature-induced changes in solvent shifts, viscosity, or relaxation properties.