Identification of D‐Fructose Dehydration Products by Infrared Multiphoton Dissociation Mass Spectrometry: The Spectral Signature of An Elusive 5‐Hydroxymethylfurfural Isomer

A combined Electrospray Infrared MultiPhoton Dissociation Mass Spectrometry (ESI‐IRMPD‐MS) and Density Functional Theory (DFT) investigation has allowed to elucidate the structural features of the species arising from the triple dehydration of D‐fructose in the gas phase. The experimental workflow involves measuring and comparing the IRMPD spectra of different ionic populations: protonated 5‐hydroxymethylfurfural [ HMF ·H] + and the ionic species coming from the triple dehydration of the ammonium‐D–fructose complex ([ Fru ·NH 4 ] + ). The IR‐photon induced fragmentation of [ Fru ·NH 4 ] + reveals the coexistence of two ionic populations, which arise from of two independent not intercrossing fragmentation pathways of the ionic precursor. One population exhibits an IRMPD spectrum matching with the ([ HMF ·H] + ) one and corresponding to a carbonyl‐protonated structure. The second ionic product is its C2‐protonated protomer, which lies 75 kJ/mol above the global minimum. The presence of a less stable protomer is most likely due to its gas‐phase kinetic trapping. These findings contribute to a more refined understanding of gas‐phase carbohydrate dehydration and isomer formation at the molecular level.