Paramagnetic species added to a sample in a
nuclear magnetic resonance spectroscopy experiment to promote more rapid
spin-lattice relaxation and allow a faster pulse repetition rate.
Example:
Tris(acetylacetonato)chromium(III) (chromium(III) acetyl acetonate, Cr(acac)3) is typically used.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 702 (https://doi.org/10.1515/pac-2019-0203)