DFT studies of proton transfer of 2, 4-dihydroxybenzoic acid derivatives

In this work, the proton transfer and photophysical properties of 2,4-dihydroxybenzoic acid and its chlorinated and brominated derivatives were studied using Density Functional Theory (DFT) to understand the proton transfer mechanism of these molecules as matrices in matrix-assisted laser desorption-ionization (MALDI). Structures, IR, UV-Vis, and fluorescence spectra were obtained together with the energies of frontier orbitals from DFT and time-dependent DFT calculations. In addition, the proton affinity (PA) and gas phase acidity (GPA) of the molecules were determined to understand the heavy atom substitution effect. Upon substitution of Cl and Br, the UV-Vis absorption at ~330 nm increases with heavy atom substitutions. While no apparent change in the OH bond strength at the ground state, it is substantially weakened upon electronic excitation at both the first singlet and triplet excited states under heavy atom substitution. These results demonstrate that both the UV-Vis absorption and intersystem crossing play important roles in facilitating proton transfer in heavy atom substitution of 2,4-dihydroxybenzoic acid.