Vitamin C: an experimental and theoretical study on the gas-phase structure and ion energetics of protonated ascorbic acid

In order to investigate the gas-phase mechanisms of the acid catalyzed degradation of ascorbic acid (AA) to furan, we undertook a mass spectrometric (ESI/TQ/MS) and theoretical investigation at the B3LYP/6-31 + G(d,p) level of theory. The gaseous reactant species, the protonated AA, [C6 H8 O6 ]H(+) , were generated by electrospray ionization of a 10(-3)  M H2 O/CH3 OH (1 : 1) AA solution. In order to structurally characterize the gaseous [C6 H8 O6 ]H(+) ionic reactants, we estimated the proton affinity and the gas-phase basicity of AA by the extended Cooks's kinetic method and by computational methods at the B3LYP/6-31 + G(d,p) level of theory. As expected, computational results identify the carbonyl oxygen atom (O2) of AA as the preferred protonation site. From the experimental proton affinity of 875.0 ± 12 kJ mol(-1) and protonation entropy ΔSp 108.9 ± 2 J mol(-1)  K(-1) , a gas-phase basicity value of AA of 842.5 ± 12 kJ mol(-1) at 298 K was obtained, which is in agreement with the value issuing from quantum mechanical computations. Copyright © 2016 John Wiley & Sons, Ltd.