NMR After Assignment: Relaxation, Distance and Exchange
Once peaks are assigned, the same experiment reports on motion and distance. Relaxation times track how a molecule tumbles, cross-relaxation between nearby nuclei measures separation in space, and a line that broadens with temperature is a process happening on the timescale of the measurement.
What a learner can do afterwards
- Explains what a short longitudinal relaxation time says about the environment of a nucleus
- Uses a through-space enhancement to decide between two stereochemical assignments
- Reads a set of variable-temperature spectra and extracts the barrier to an exchange process
- Says why an experiment that fails at one field strength can succeed at another
1 · Read
Once peaks are assigned, the same experiment reports on motion and surroundings. A short longitudinal relaxation time means the nucleus sheds energy fast, which says something nearby takes it: a mobile group, a big slowly tumbling molecule, or a paramagnetic neighbour.
Two stereochemical assignments fit the shifts, and only one puts two protons close in space. Irradiate one proton: an enhancement on the other proves closeness through space. That cross-relaxation picks the assignment no shift could settle.
A line that broadens with temperature is a process happening on the measurement timescale. Cool the sample and the signal splits into its exchanging partners, and the coalescence point yields the barrier to the exchange.
If an experiment fails at one field strength, try another. Peak separation scales with the field, so a higher field can resolve what overlaps at a lower one.
Relaxation tracks surroundings, cross-relaxation measures distance, and broadening with temperature means exchange.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.