Referencing of Spectrum Collected Unlocked

Chemical Shift Referencing of Spectrum Collected in Unlocked Experiment

In some situations, especially for deuteron spectrum (H2 or D) when it's collected unlocked with protonated solvent, care must be taken for chemical shift referencing. Here we focus on H2 spectrum collected unlocked as an example.

Note the following when collecting NMR data unlocked.

Limit the duration as brief as possible. When unlocked, field drifting effect leads to severe resonance broadening over hours.
Make sure lock is turned OFF in the lock panel and also set alock=n before data collection. If lock is turned on but the lock signal (from deuterated solvent) cannot be located, the field will drift/move steadily, leading to bizarre multiplicity in all peaks.
Normal shimming with H2 signal is not possible. For shimming, you can (1) shim with an identical but deuterated solvent with the same volume before inserting the sample of interest, or (2) shim with a strong H1 signal.
Care must be taken to reference the spectrum. DO NOT rely on the default referencing.

For chemical shift referencing of the H2 spectrum (or other spectra) collected unlocked, you can use the correct referencing of the H1 spectrum from the same sample collected immediately before or after the H2 data collection. In the following, we assume the H2 data is collected under exp2, and the H1 data are collected under exp1 (or any other, separate experiment).

1. Collect a H1 spectrum with very low gain and flip angle. Enter gain=0 pw=1 nt=1 go, process the data. Reference the spectrum using the strong H1 resonances from the solvent. Save the data for later use.
2. Collect the H2 data and process the spectrum.
3. To use the H1 chemical shift under exp1 to reference the H2 data in exp2, enter mref(1,2). If this step confirms one of the small peaks in the H2 spectrum is the natural abundance H2 signal from the solvent, it is best to directly use it to reference the H2 spectrum at this point.

Updated, July 2019, H. Zhou