Probing Multiple Reaction Pathways in Solid-State Acid−Base Chemistry by Isothermal Solid-State Calorimetry

Abstract

Isothermal solid-state calorimetry (ISSC) was applied to directly quantify reaction enthalpies in the solid state for acid−base reactions involving 4-(dimethylamino)pyridine (DMAP) and benzoic (BzH) or salicylic acid (SalH) at 25 °C. Solid-state reactions between DMAP and excess acid, with a small amount of added liquid, led to rapid, quantitative conversion to new crystalline phases, accompanied by well-defined calorimetric signals. Depending on the reactant molar ratio in the calorimetric cell, salt or salt cocrystals were obtained, with the BzH system exhibiting a more complex reactivity landscape, including a novel multicomponent salt cocrystal (DMAPH⁺)₂Bz⁻₂(BzH)₃. Reaction enthalpies determined directly by calorimetry were consistent across different synthetic routes and agreed well with values derived from thermodynamic cycles and periodic DFT calculations. The results demonstrated that ISSC enabled direct determination of solid-state reaction enthalpies and provided valuable thermodynamic insight into competing reaction outcomes in this rather elaborate acid−base reaction system.