Salt Cluster With Surface Defect Shows Anomalous Acid-Base Chemistry

The acid displacement reaction of formic acid in sea-salt aerosols to release hydrogen chloride is an enigma in atmospheric chemistry: a weak acid transfers a proton to the conjugate base of a strong acid, reversing solution-phase acid-base chemistry. To shed light on this question, we compared the reactivity of formic acid with Na₁₃Cl₁₂ ⁺ and Na₁₄Cl₁₃ ⁺ under ultra-high vacuum conditions in a mass spectrometer. No reaction was observed with Na₁₄Cl₁₃ ⁺, a known magic cluster that corresponds to a cubic section of the crystal lattice. Na₁₃Cl₁₂ ⁺, which corresponds to the cubic section with a surface defect, reacts readily with release of hydrogen chloride and incorporation of formate into the salt structure. Quantum chemical calculations show that the reaction is substantially endothermic for the magic cluster. The surface defect, however, makes it exothermic. The reason lies in the induced fit of the formate ion in the Na₁₃Cl₁₁(HCOO)⁺ cluster. Its oxygen atoms interact with five sodium ions. In sum, these electrostatic interactions more than offset the 49 ± 13 kJ mol^-1 difference in gas-phase acidity between hydrogen chloride and formic acid.