New Hydrolysis Rate Constants Reveal Longest Cyanide Persistence in Cool, Neutral Waters

Abstract

Hydrogen cyanide (HCN) is a key molecule in prebiotic chemistry, and its availability in water is limited by hydrolysis. In this work, we estimate the hydrolysis rates of \ce{HCN}, particularly the errors associated with the rates as a function of temperature, pH and in the presence of salts containing sulfite, sulfide and phosphate. For pure water, we find an acid-catalyzed hydrolysis rate constant at ${\rm 0 ^{\circ}C}$ of $\ln (k^+_{273}/1\,{\rm M^{-1} s^{-1}}) = -12.7 \pm 1.12$ with an activation energy of $67.1 \, {\rm kJ \, mol^{-1}}$. We find a base-catalyzed rate constant of $\ln (k_{273}^-/1\,{\rm M^{-1} s^{-1}}) = -9.0 \pm 1.27$ at ${\rm 0^{\circ}C}$ with an activation energy of $89.5 \, {\rm kJ \, mol^{-1}}$. These values are consistent with estimates from the literature within our uncertainties at ${\rm pH} > 8$ but diverge from literature values at lower pH. In the presence of salts, hydrolysis is accelerated under acidic conditions to 6--14$\times$ the acid-catalyzed rate without salts. However, at ${\rm pH \gtrsim 8}$ hydrolysis rates become $2.5\times$ times slower with sulfite and sulfide. These results demonstrate the importance of estimating uncertainties associated with rates when constraining the maximum concentrations of prebiotic molecules attainable in natural waters on the Earth and other planets.