Theoretical Investigation of the Proton Affinity and Gas-Phase Basicity of Neutral x,y-Dihydroxybenzoic Acid and its Derivatives

Proton affinities (PA), gas-phase basicities (GB) and acidities (GA), which are some of the important physical properties of a matrix in matrix-assisted laser desorption ionization mass spectrometry, have been calculated using density functional theory (DFT) for a number of dihydroxybenzoic (DHB) acid isomers and derivatives. The theoretical PA and gas-phase basicity (GB) values for the neutral x,y-DHB acids, ionic radicals, Na+ and K+ salts as well as oxygen- and hydrogen-bridged dimers of x,y-DHB have been calculated. Analysis of the computational data indicates that there are lower PA/GB values for the anionic dimers compared to the PA/GB values for the electrically neutral oxygen-bridged dimers. The PA/GB values for the neutral and radical cations are larger than the neutral monomers and the PA/GB values for the radical anions are slightly lower than the anionic class of isomers. The PA/GB values for the salts (x, y-DHB−Na/K+) are significantly higher (100–150 kJ mol−1) than the neutral x,y-DHB acids. The above theoretical results are in agreement with experimental values obtained by Fourier transorm ion cyclotron resonance mass spectrometry employing a thermokinetic method. Correlation of experimentally and theoretically predicted values suggests that this theoretical calculation method could be used to derive information on different matrices.