Title

Quantum Chemical Study Of Trimolecular Reaction Mechanism Between Nitric Oxide And Oxygen In The Gas Phase

Abstract

Singlet and triplet potential energy surfaces of the reaction between molecular oxygen and two nitric oxide(II) molecules were studied by quantum chemical methods (coupled cluster, CASSCF, and density functional theory: B3LYP, TPSS, VSXC, BP86, PBE, B2-PLYP, B2K-PLYP). Elementary steps involving various N2O4 isomers (cyclic, cis-cis-, cis-trans-, trans-trans-ONOONO, cis- and trans-ONONO2, O2NNO 2) were considered, as well as weakly bound molecular clusters preceding formation of O2NNO2, and Coupe-type quasi-aromatic hexagonal ring intermediate NO2-O2N. We found that activation energy strongly depends on the conformation of ONOONO peroxide, which is formed barrierlessly. The best agreements with experimental values were achieved by the B3LYP functional with aug-pc3 basis set. The lowest transition state (TS) energies correspond to the following reaction channel: 2NO + O2 (0 kJ/mol) - cis-cis-ONOONO (-45 kJ/mol) → TS1 -NO 2-O2N (-90 kJ/mol) → TS2 -cis-ONONO2 (-133 kJ/mol)→ TS3 → trans-ONONO2 (-144 kJ/mol) → TS4 → O2NNO2 (-193 kJ/mol). A valley ridge inflection (VRI) point is located on the minimum energy path (MEP) connecting NO 2-O2N and cis-ONONO2. The energy landscape between NO2-O2N and CC-TS2 can be classified as a downhill valley-pitchfork VRI bifurcation according to a recent classification of bifurcation events [Quapp, W. J. Mol. Struct. 2004, 95, 695-696]. The first and second transition states correspond to barrier heights of 10.6 and 37.0 kJ/mol, respectively. These values lead to the negative temperature dependence of the rate constant. The apparent activation enthalpy of the overall reaction was calculated to be △rH ‡(0) = -4.5 kJ/mol, in perfect agreement with the experimental value. © 2009 American Chemical Society.

Publication Date

8-13-2009

Publication Title

Journal of Physical Chemistry A

Volume

113

Issue

32

Number of Pages

9092-9101

Document Type

Article

Personal Identifier

scopus

DOI Link

https://doi.org/10.1021/jp900484s

Socpus ID

68549092621 (Scopus)

Source API URL

https://api.elsevier.com/content/abstract/scopus_id/68549092621

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