CONVENTIONAL AND DOUBLE HYBRID DFT, CR–CCSD(T), CASSCF AND MR–CISD IN THE PES EXPLORATION OF THERMAL REACTION 2NO+O2→2NO2: THEORY VS. EXPERIMENT

O.B. Gadzhiev, S. K. Ignatov, A.G. Razuvaev 

N.I. Lobachevsky State University of Nizhny Novgorod, 
Russia, Nizhny Novgorod, 23 Gagarin Avenue, 603950. E-mail: euriscomail@mail.ru

We reported the quantum chemical study of the singlet potential energy surface for the reaction between molecular oxygen and two nitric oxide molecules. Conventional density functional theory (B3LYP, TPSS, VSXC, BP86, PBE) and double hybrid functionals B2–PLYP and B2K–PLYP (for thermochemistry and thermochemical kinetics), coupled cluster (CCSD(T)) as well as completely renormalized (CR–CCSD(T)) and the CASSCF (active space up to (26,16)) theory levels were used for the PES exploration. We considered elementary steps involving various N2O4 isomers (cis-cis-, cis-trans-, trans-trans-ONOONO, cis- and trans-ONONO2, O2NNO2), as well as Coupe type quasi–aromatic hexagonal ring intermediate (NO2•O2N).
T1-test values in CCSD calculations show high contribution of non-dynamic electronic correlation. Thus, they lead to the qualitative incorrect stability order of N2O4 isomers. The results of CCSD(T), CR–CCSD(T) and all DFT calculations were in a  qualitative agreement. Electronic structure of NO2•O2N at CASSCF (in two active spaces (26,16) and (2,2)) and MR–CISD (2,2) with g3 correction theory levels was found to be under crucial influence of non-dynamic electronic correlation. 
Overall the activation enthalpy of the reaction 2NO+O22NO2 at absolute zero ΔH≠(0K), evaluated at B3LYP/6-311+G(2d) theory levels, corrected by the value of the reaction enthalpy, equals to –6.5 kJ/mol respectively. This agrees with the experimental value Еa of –4.5 kJ/mol at Т=298–600К and  Е0 kJ/mol  at Т650К. 

This work was partially supported by the Russian Foundation for Basic Research (Project 07-03-00390)