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 ascorbic acid, [C6 H8 O6 ]H(+) , were generated by electrospray ionization (ESI) of a 10(-3) M H2 O/CH3 OH (1:1) ascorbic acid 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 ascorbic acid 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 (PA) of 875.0 ± 12 kJ mol(-1) and protonation entropy ∆Sp (108.9 ± 2 J mol(-1) ) a GB 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.