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- Title
To flame‐seal or not to flame‐seal NMR tubes: The role of liquid–vapor equilibria on the accuracy of variable temperature experiments.
- Authors
Morrelli, Derek; Maitra, Santanu; Krishnan, V. V.
- Abstract
In NMR experiments, it is crucial to control the temperature of the sample, especially when measuring kinetic parameters. Usually, it takes 2 to 5 min for the temperature of the sample inside the NMR probe to stabilize at a fixed value set for the experiment. However, the NMR sample tubes are flame‐sealed in some cases, such as when working with volatile solvents, atmosphere‐sensitive samples, or calibration samples for long‐term use. When these samples are placed inside the NMR probe, the spectrometer controls the lower portion (liquid phase) of the NMR sample tube with a gas flow at a fixed temperature, while the upper portion (vapor) is at ambient temperature. This probe design creates a unique temperature gradient across the sample, leading to vapor pressure build‐up, particularly inside a sealed NMR tube. By analyzing the temperature‐dependent spectral line shape changes of a chemical exchange process, we report that under standard experimental conditions, the sample temperature can take up to 2 to 3 h (instead of minutes) to stabilize. The time scale of the liquid–vapor equilibrium process is much slower, with a half‐life exceeding 35 min, in contrast to the 2‐min duration required to obtain each spectrum. This phenomenon is exclusively due to the liquid–vapor equilibrium process of the flame‐sealed NMR tube and is not observable otherwise.
- Subjects
VAPOR-liquid equilibrium; TEMPERATURE control; CHEMICAL processes; TUBES; VAPOR pressure; GAS flow
- Publication
Magnetic Resonance in Chemistry, 2024, Vol 62, Issue 1, p19
- ISSN
0749-1581
- Publication type
Article
- DOI
10.1002/mrc.5411