Spectroscopy of Hydrothermal Reactions. 8. Kinetics of Aqueous NH(4)XCN (X = O, S) and OCS by Flow Reactor-Infrared Spectroscopy.

The kinetics and pathway of hydrothermolysis of 1 m NH(4)SCN to CO(2), NH(3), and H(2)S were determined at 543-573 K and 275 bar by the use of FTIR spectroscopy and a Pt/Ir flow reactor with diamond windows or a 316 stainless steel flow reactor with sapphire windows. The rates of SCN(-) loss and CO(2) formation were the same. The reaction is (pseudo) second-order with E(a) = 113 +/- 11 kg/mol and ln(A, kg/(mol.s)) = 21 +/- 2. DeltaS() = -84 J/(mol.K), which suggests a bimolecular, rate-determining, initial decomposition step for NH(4)SCN. A reaction scheme is proposed in which OCS and a monothiocarbamate species are undetected intermediates. The absence of OCS is explained by the rapid hydrothermolysis rate of OCS to CO(2) and H(2)S which was determined by IR spectroscopy at 393-423 K under 275 bar to be E(a) = 44 +/- 5 kJ/mol and ln(A/s) = 13 +/- 1 for OCS. The resulting rate is about 10(3) times faster than the hydrothermolysis rate of NH(4)SCN at 543 K. The results are compared to the equivalent reaction for NH(4)OCN. NH(4)OCN reacts about 3 x 10(3) times faster than NH(4)SCN at 543 K. The trend in the rates is consistent with the charge distribution and the trend in the bond distances, which resulted from ab initio quantum mechanical calculations at the HF/N311G//HF/N31G level in the OCN(-) and SCN(-) ions and the proposed carbamate and monothiocarbamate intermediates.