Fig. 4: The search engine enables the discovery of novel products in old data (HRMS spectra of previously unknown reaction products are indicated by dotted purple arrows).

a MEDUSA Search has registered widely known H-NHC and Ph-NHC ions, as well as a newly discovered [NHC-ethynyl]+ ion in a Pd/NHC-catalyzed Sonogashira reaction mixture. The isotopic distribution-informed search process allows the detection of previously unknown ethyl-NHC products; b MEDUSA Search registered unknown vinyl-NHC fragments in the Mizoroki-Heck reaction (Dipp – 1,3-diisopropylphenyl, Py – pyridine); c MEDUSA Search registered phenyl-substituted vinyl-NHC ions in the Pd-PEPPSI (Pyridine-Enhanced Precatalyst Preparation Stabilization and Initiation) catalytic hydrogenation reaction.