Abstract
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.
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