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, O2NNO2) 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 --> NO2.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 NO2.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 Delta(r)H(0) = -4.5 kJ/mol, in perfect agreement with the experimental value.

Intermediates Involved in the Oxidation of Nitrogen Monoxide: Photochemistry of the cis‐N2O2⋅O2 complex and of sym‐N2O4 in Solid Ne Matrices

“…This is close to the CCSD(T) relative energy reported previously (+5.7 kJ mol À1 ), [41] while the broken-symmetry (BS) DFT approach, [83] which is more appropriate to a diradicaloid system, [84] predict a reversed order of stability, (NO 2 ) 2 dimer > 2 NO 2 (À2.4 kJ mol…”

“…Furthermore, predicted barriers for the asymmetric NO 2 dimerisation varied substantially. [40,41,64] In contrast, the symmetric dimerisation to sym-N 2 O 4 was found experimentally and computationally to proceed barrier-free. [29] Former matrix-isolation studies related to this work were seriously hampered by the complex mixtures that were obtained by co-deposition of NO and O 2 in solid O 2 [21] and Ar, [36] and annealing of these deposits to about 30 K. [36,42] Under these conditions, diffusion of NO and O 2 within the solid matrices cannot be prevented and, in addition to various N 2 O 4 derivatives, different N 2 O 3 isomers were formed from the reaction NO + NO 2 .…”

“…[37,38] This controversy initiated some recent computational studies on its mechanism and of the N 2 O 4 potential-energy surface (PES). [39][40][41] The most detailed of these studies appeared very recently. [41] As one would expect from the known chemistry of nitrogen oxides, [42] these studies revealed a rather complex system and a remarkable large number of molecular N 2 O 4 species, which are found to be thermodynamically stable with respect to decomposition.…”

“…[39][40][41] The most detailed of these studies appeared very recently. [41] As one would expect from the known chemistry of nitrogen oxides, [42] these studies revealed a rather complex system and a remarkable large number of molecular N 2 O 4 species, which are found to be thermodynamically stable with respect to decomposition. Table 1 lists those species that may be envisioned and includes the ionic nitrosonium nitrate, NO + NO 3 À .…”

“…[39][40][41] Although such (NO 2 ) 2 dimers may rapidly dissociate in the gas phase to form 2 NO 2 free radicals, they might be trapped as a radical pair in low-temperature rare-gas matrices, because significant activation barriers have been predicted theoretically for their rearrangement into ONONO 2 and sym-N 2 O 4 . [40,41] Weakly bound (NO 2 ) 2 radical pairs probably play a crucial role also in the sym-N 2 O 4 -trans-ONONO 2 isomerisation, for which computational attempts to find a lowenergy pathway failed, [41,[59][60][61] or in the rapid gas-phase isotopic exchange reactions of isotopically labelled NO 2 [Eq. (5)]: [62] …”