---
title: "NMR After Assignment: Relaxation, Distance and Exchange"
description: "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 l"
canonical: https://lightmysky.com/learn/science/nmr-after-assignment-relaxation-distance-and-exchange-mt_dCV2EbNlF2
source: https://lightmysky.com/learn/science/nmr-after-assignment-relaxation-distance-and-exchange-mt_dCV2EbNlF2.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Chemistry · Ages 22 to 23
Page: https://lightmysky.com/learn/science/nmr-after-assignment-relaxation-distance-and-exchange-mt_dCV2EbNlF2

## Ready when they can

- 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

## Lesson: What NMR says after assignment

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.

**Example.** 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.

**Tip.** 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.

**Recap.** Relaxation tracks surroundings, cross-relaxation measures distance, and broadening with temperature means exchange.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Structure Elucidation from a Complete Data Set](https://lightmysky.com/learn/science/structure-elucidation-from-a-complete-data-set-mt_O2uungksMZ)
- [Reading a Proton NMR Spectrum: Shift, Integration and Multiplicity](https://lightmysky.com/learn/science/reading-a-proton-nmr-spectrum-shift-integration-and-multiplicity-mt_WgxqHHf-YL)

## Opens up

- [Reducible Representations and Predicting Infrared and Raman Activity](https://lightmysky.com/learn/science/reducible-representations-and-predicting-infrared-and-raman-activity-mt_DP4JfkS6c0)