Gas-phase basicities of the isomeric dihydroxybenzoic acids and gas-phase acidities of their radical cations

The thermochemical acid/base properties of the six dihydroxybenzoic acids (x,y-DHB) as prototypical matrices used in matrix-assisted laser desorption/ionization (MALDI) have been investigated. The ground-state gas-phase basicities (GB) of the six DHB isomers and the gas-phase acidities (Delta G(acid)) of the corresponding radical cations ([x,y-DHB](.+)) have been determined by Fourier-transform ion cyclotron resonance mass spectrometry employing the thermokinetic method. The gas-phase basicities vary from 814 kJ mol(-1) for the least basic isomer, 3,5-DHB, to 831 kJ mol(-1) for the most basic isomer, 2,4-DHB. The obtained gas-phase acidities of the corresponding radical cations vary from 815 kJ mol-l for the most acidic species, 3,4-DHB to 858 kJ mol(-1) for the least acidic one, 2,5-DHB. The results indicate that ground-state proton transfer from the matrix radical cations to the analyte may play a role in the ionization process of MALDI, whereas proton transfer from protonated matrix molecules can be excluded. (J Am Soc Mass Spectrom 2000, 11, 544-552) (C) 2000 American Society for Mass Spectrometry.