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, T1, means the nucleus sheds energy fast, so something nearby takes it efficiently. Relaxation needs magnetic fields that flicker near the nucleus's own frequency. Unpaired electrons make strong flickering fields, so a nucleus near a paramagnetic centre relaxes fast. A carbon with protons attached relaxes faster than one without. In a small molecule, a group that spins faster than the rest relaxes more slowly, not faster.
Two stereochemical assignments fit the shifts, and only one puts two protons close in space. Irradiate one proton: an enhancement on the other, the nuclear Overhauser effect, shows they sit close through space, usually within about 5 angstroms. 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. At the coalescence temperature, where the two lines just merge, the exchange rate is about 2.22 times their separation in hertz, measured when they were fully apart. The Eyring equation turns that rate and temperature into the free energy barrier.
If an experiment fails at one field strength, try another. Peak separation in hertz scales with the field, so a higher field can resolve what overlaps at a lower one. There is a second reason. For a medium-sized molecule the NOE can pass through zero, because its sign depends on how fast the molecule tumbles compared with the spectrometer frequency. At a different field the enhancement can reappear, or a rotating-frame experiment (ROESY) can be used instead.
Relaxation tracks surroundings, cross-relaxation measures distance, and broadening with temperature means exchange.
2 · Watch
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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.