A computational study of the thermodynamic properties of sinapic and ferulic acids and their corresponding radical cations

The gas-phase basicities and gas-phase proton affinities of sinapic acid and ferulic acid, which are common matrices used in matrix assisted laser desorption/ionization mass spectroscopy, have been calculated using density functional theory at the B3LYP/6-311+G(2df,p)//B3LYP/6-31G** level. Respectively, the GBs of the two acids are 869.0 and 862.4 kJ/mol while the gas-phase PAs of the same systems are 903.4 and 896.0 kJ/mol. Our results indicate that the protonation in these systems takes place on the carboxylic sites. In addition, the GAs of the radical cations of these acids has also been calculated. The calculated GAs of SA and FA radical cations are 899.8 and 889.6 kJ/mol, respectively. Our results indicate that deprotonation in the two radical cations takes place on the phenol sites. We also provide the first estimates of the vertical ionization potentials (IPs) of the same systems at the B3LYP/6-311++G(2df,p)//B3LYP/6-31+G** level using the lowest energy structure found for each acid. The calculated IPs of SA and FA are 7.54 and 7.82 eV, respectively.