4 years ago

Formation of (HCOO–)(H2SO4) Anion Clusters: Violation of Gas-Phase Acidity Predictions

Formation of (HCOO–)(H2SO4) Anion Clusters: Violation of Gas-Phase Acidity Predictions
Xue-Bin Wang, Marat Valiev, Gao-Lei Hou
Sulfuric acid is commonly known to be a strong acid and, by all counts, should readily donate its proton to formate, which has much higher proton affinity. This conventional wisdom is challenged in this work, where temperature-dependent negative ion photoelectron spectroscopy and theoretical studies demonstrate the existence of the (HCOO)(H2SO4) pair at an energy slightly below the conventional (HCOOH)(HSO4) structure. Analysis of quantum-mechanical calculations indicates that a large proton affinity difference (∼36 kcal/mol), favoring proton transfer to formate, is offset by the gain in intermolecular interaction energy between HCOO and H2SO4 through the electron delocalization and formation of two strong hydrogen bonds. However, this stabilization comes with a severe entropic penalty, requiring the two species in the precise alignment. As a result, the population of (HCOO)(H2SO4) drops significantly at higher temperatures, rendering (HCOOH)(HSO4) to be the dominant species. This phenomenon is consistent with the photoelectron data, which shows depletion in the spectra assigned to (HCOO)(H2SO4), and has also been verified by ab initio molecular dynamics (AIMD) simulations.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b05964

DOI: 10.1021/jacs.7b05964

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