Arginine/2,5-Dihydroxybenzoic Acid Clusters:  An Experimental and Computational Study of the Gas-Phase and Solid-State Systems^†

The 1:1 adduct of arginine with 2,5-dihydroxybenzoic acid (DHB) has been studied in the gas phase and in the solid state. Experimentally, the ionization energy (IE) of the 1:1 cluster was determined by wavelength-dependent laser ionization of clusters formed by seeding DHB and arginine into a supersonic jet expansion. Ionization laser power studies performed at several discrete wavelengths established the upper and lower limits for the 1:1 cluster IE and dissociation energy. Subsequent one-color scanned-wavelength laser ionization studies allowed an experimental establishment of the 1:1 cluster IE of 7.193 eV. A combination of molecular dynamics/simulated annealing calculations on the 1:1 cluster followed by density functional theory geometry optimizations using reasonably large basis sets yielded 15 distinct minima on the potential energy surface, all within 5.2 kcal/mol in energy at the B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G** level. The Boltzmann-averaged IE at the same level is 7.11−7.14 eV, in excellent agreement with experiment. Cocrystals of arginine and DHB have been grown, and the crystal structure has been solved. The dominant intermolecular interaction in the cocrystal is a double hydrogen bond (salt bridge) between the guanidinium group of arginine and the (deprotonated) carboxylate group of DHB. This is exactly the same interaction that is found in the lowest-energy structure of the gas-phase 1:1 adduct. The electronic structure of the solid-state cocrystal has been modeled using a cluster approach.