Thermochemistry of reactive nitrogen oxide species and reaction enthalpies for decomposition of ONOO− and ONOOH

The standard enthalpies of formation ( Δ H f 298 ∘ ) of a set of 16 biologically relevant reactive nitrogen oxide species were calculated using CBS-Q, CBS-QB3, CBS-APNO, G3 and G3X levels of theory. The G3 method gives the smallest mean absolute deviation of 5.1 kJ mol−1, compared with available experimental ( Δ H f 298 ∘ ) values. Enthalpies of three reactions related to decomposition of ONOO− and ONOOH were also examined with various levels of theory. Large discrepancies are found for thermochemical results involving ONOO− using the CBS-APNO and CBS-RAD methods. This is due to the use of inadequate geometry predicted by the QCISD theory.