Quantum Chemistry Calculations on the Mechanism of Isoquinoline Ring-Opening and Denitrogenation in Supercritical Water

Computational studies at the M06/6-311G(d,p) and M06-2X/6-311+G(d,p) levels were performed to explore the detailed mechanism of isoquinoline ring-opening and denitrogenation in a supercritical water system. Three reaction paths with the same product, 2-(2-oxoethyl) benzaldehyde, were supported by the computational results. The rate-limiting step in the major degradation reaction is an addition reaction at the N position. H2O is added to both the 1C–2N double bond (1C–2N addition reaction) and the 2N–3C double bond (2N–3C addition reaction) of the isoquinoline molecule, where the oxygen of H2O is added to the carbon atom. The energy barrier of the 1C–2N addition reaction is 52.7 kcal/mol, while that of 2N–3C addition (from Path 6) is 60.1 kcal/mol. From catalysis by two water molecules, the barrier of 1C–2N addition (Reaction (1)) is reduced to 27.5 kcal/mol. Catalysis from water molecule clusters is shown to considerably affect the process of isoquinoline ring-opening and denitrogenation, as indicated by ...