shift reagent

https://doi.org/10.1351/goldbook.08207
Paramagnetic substance that induces an additional change of the NMR resonance frequency of a nucleus near any site in a molecule to which the substance binds.
Source:
PAC, 2022, 94, 353. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)) on page 502 [Terms] [Paper]