Gas-phase structures and thermochemical properties of protonated 5-HMF isomers

The gas-phase ion chemistry of protonated 5-hydroxymethyl-2-furaldehyde (5-HMF), the main product of hexose sugar dehydration reaction, was investigated by means of mass spectrometric and theoretical calculations. Calculations at the B3LYP-D3/6-311++G** and the G3 (MP2, CCSD(T)) levels of theory indicated the carbonyl oxygen as the 5-HMF preferred protonation site. The Cooks' extended kinetic method was used to measure the unknown proton affinity (PA) and gas-phase basicity (GB) of 5-HMF. The experimentally measured values, PA = 207.3 kcal mol(-1) and GB = 200.0 kcal mol(-1), were found to be in good agreement with those calculated by means of G3 (MP2, CCSD(T)) composite methods. Among the seven different 5-HMF optimized protomers, the theoretically predicted potential energy surface of [5-HMF]H+ ions indicated that the carbonyl oxygen protonated most stable species, as well as each one of the four ring-protonated less stable isomers, were trapped in deep potential wells and, at least in principle, could be formed from the gas phase acid-catalyzed hexose sugar decomposition. (C) 2019 Elsevier B.V. All rights reserved.