<i>S</i>‑Nitrosation of Aminothiones

Nitrosation reactions span a diverse range of applications, from biochemistry to industrially important processes. This study examines nitrosation of aminothiones in acidic solutions and re-evaluates currently accepted diffusion limits and the true nature of the nitrosating agent for nitrous acid initiated reactions. Experimental measurements from stopped-flow UV/vis spectrophotometry afforded derivation of equilibrium constants and reaction enthalpies. Apparent <i>K</i>_eq corresponds to 559–382 M^–2 for thioacetamide (TA, 15–25 °C) and 12600–5590 M^–2 for thiourea (TU, 15–35 °C), whereas the reaction enthalpies amount to −27.10 ± 0.05 kJ for TA and −29.30 ± 0.05 kJ for TU. Theoretical calculations via a thermochemical cycle agree well with reaction free energies from experiments, with errors of −2–4 kJ using solvation method SMD in conjunction with hybrid meta exchange–correlation functional M05-2X and high-accuracy multistep method CBS-QB3 for gas-phase calculations. The kinetic rates increase with acidity at activation energies of 54.9 (TA) and 66.1 kJ·mol^–1 (TU) for the same temperature range, confirming activation-controlled reactions. At pH 1 and below, the main decomposition pathway for the <i>S</i>-nitroso species leads to formation of nitric oxide