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·NH₄]⁺). The IR-photon induced fragmentation of [Fru·NH₄]⁺ 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.