Gas phase structures and thermochemical parameters of protonated 5-hmf isomers

5-hydroxymethyl-2-furaldehyde (5-HMF) is produced in high yields from the acid catalyzed dehydration of carbohydrates. This reaction represents the key process of the cellulosic biomass conversion into furantype platform chemicals. [1] Recently, we have employed mass spectrometric techniques to investigate the acid-catalyzed D-glucose and D-fructose dehydration highlighting the formation in the gas-phase of a protonated 5-HMF isomeric mixed population[2-3]. Full scan and MSn mass spectra were acquired using a Bruker AmaZon SL ion trap (IT) and a Waters Ultima Q-TOF operating in the positive ion mode. Proton affinity (PA) and gas-phase basicity (GB) of 5-HMF were obtained by using the Wesdemiotis and Fenselau Cooks’s “extended” kinetic method. The PA and GB values as well as the geometries of protonated isomeric structures were fully optimized in the gas-phase at the B3LYP/6-311++G** level of theory. In order to clarify the stability and structures of protonated 5-HMF isomers, the geometries of six different protomers were optimized at the B3LYP/6-311++G** level of theory and the energetics of their dissociation compared with the experimentally observed mass spectrometric fragmentation. Moreover, the unknown PA and GB of 5-HMF were measured by the extended Cooks’s kinetic method along with theoretical calculations. The experimental PA value of 207.2 ± 3 kcal mol-1 is in excellent agreement with the value of 207.5 kcal mol-1 computed at the B3LYP/6-311++G** level of theory. The whole picture emerging from experimental and theoretical results allowed to attribute the structures of different protonated 5-HMF isomers to the m/z 127 ions generated by the acid catalyzed decomposition of hexose carbohydrates in the gas phase. The unknown proton affinity and gas phase basicity of 5-HMF were determined by the joint application of Cook's kinetic method and quantum mechanical calculations. Theoretical calculations identify the oxygen atom of the aldehydic group as the most basic site. The formation of less stable protonated 5-HMF isomers as final product of the acid catalyzed decomposition of D-fructose and D-glucose was demonstrated on the basis of energetic considerations. In this work the unexplored gas-phase ion chemistry of protonated 5-HMF, one of the top ten bio-based platform chemicals, was firstly investigated.