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.

The reaction between molecular oxygen and two nitric oxide(II) molecules is studied with high-level ab initio wave function methods, including geometry optimizations with coupled cluster (CCSD(T,full)/cc-pCVTZ) and complete active space with second order perturbation theory levels (CASPT2/cc-pVDZ). The energy at the critical points was refined by calculations at the CCSD(T,full)/aug-cc-pCVTZ level. The controversies found in the previous theoretical studies are critically discussed and resolved. The best estimate of the activation energy is 6.47 kJ/mol.

The mechanisms of the initial step in chemical reaction between ozone and ethylene were studied by multireference perturbation theory methods (MRMP2, CASPT2, NEVPT2, and CIPT2) and density functional theory (OPW91, OPBE, and OTPSS functionals). Two possible reaction channels were considered: concerted addition through the symmetric transition state (Criegee mechanism) and stepwise addition by the biradical mechanism (DeMore mechanism). Predicted structures of intermediates and transition states, the energies of elementary steps, and activation barriers were reported. For the rate-determining steps of both mechanisms, the full geometry optimization of stationary points was performed at the CASPT2/cc-pVDZ theory level, and the potential energy surface profiles were constructed at the MRMP2/cc-pVTZ, NEVPT2/cc-pVDZ, and CIPT2/cc-pVDZ theory levels. The rate constants and their ratio for reaction channels calculated for both mechanisms demonstrate that the Criegee mechanism is predominant for this reaction. These results are also in agreement with the experimental data and previous computational results. The structure of DeMore prereactive complex is reported here for the first time at the CCSD(T)/cc-pVTZ and CASPT2/cc-pVDZ levels. Relative stability of the complexes and activation energies were refined by single-point energy calculations at the CCSD(T)-F12/VTZ-F12 level. The IR shifts of ozone bands due to formation of complexes are presented and discussed.

Recent experiments on the UV and electron beam irradiation of solid O 2 reveals a series of IR features near the valence antisymmetric vibration band of O 3 which are frequently interpreted as the formation of unusual O n allotropes in the forms of weak complexes or covalently bound molecules. In order to elucidate the question of the nature of the irradiation products, the structure, relative energies, and vibrational frequencies of various forms of O n (n = 1−6) in the singlet, triplet, and, in some cases, quintet states were studied using the CCSD(T) method up to the CCSD(T,full)/cc-pCVTZ and CCSD(T,FC)/aug-cc-pVTZ levels. The results of calculations demonstrate the existence of stable highly symmetric structures O 4 (D 3h ), O 4 (D 2d ), and O 6 (D 3d ) as well as the intermolecular complexes O 2 ·O 2 , O 2 ·O 3 , and O 3 ·O 3 in different conformations. The calculations show that the local minimum corresponding to the O 3 ···O complex is quite shallow and cannot explain the ν 3 band features close to 1040 cm −1 , as was proposed previously. For the ozone dimer, a new conformer was found which is more stable than the structure known to date. The effect of the ozone dimer on the registered IR spectra is discussed.

ABSTRACT:The structures, energies, harmonic vibrational frequencies, and thermodynamic parameters of the water clusters (H 2 O) 48 , (H 2 O) 72 , and (H 2 O) 270 were calculated using the standard DFT theory (BLYP/6-31++G(d,p) for small and medium clusters) and the modern tight-binding method SCC-DFTB (DFTBA and DFTB+). The adsorption and embedding of s-cis-and s-trans-glyoxal molecules as well as its sunlight UV photolysis products (molecules CH 2 O, HCOOH, H 2 O 2 , CO, CO 2 and radicals CHO, HO, HO 2 ) on nanosized ice clusters of up to 2.5 nm in diameter were studied within the above theoretical models. The structures of adsorption complexes on different sites of ice nanoparticles, the corresponding adsorption energies and thermodynamic parameters were estimated. We found that the DFTB method is a very promising tool for the calculations of structures and energies of ice nanoparticles, when compared to both DFT and semiempirical (PM3) methods. The obtained results are discussed in relation to the possible photolysis pathways, the reaction rates in the gas phase and in the adsorbed state, and the mechanisms of glyoxal photolysis catalyzed by the ice nanoparticles in the Earth's atmosphere.

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NHC-supported trihydrides Cp(NHC)RuH3 show excellent catalytic activity in the H/D exchange of pyridine and some other N-heterocycles under mild conditions and low catalyst loading.

Platinum and platinum based materials are of fundamental importance for modern and developed catalysts, fuel cells, sensors, hydrogen production and storage systems, and nanoelectronic devices. The subnanosize cluster Pt24 was considered as a model of the prospective catalytic system based on the oxide and carbide supported Pt nanoparticles (Pt NPs) or Pt NPs with soft spacers anchored to their surface. Structural, electronic, thermodynamic, and spectral properties of the adsorption complexes of molecular and atomic hydrogen on Pt NPs have been studied using the DFT method (the BLYP functional with the 6-31G­(p) basis for H and the CRENBS pseudopotential for Pt atoms). On this basis, the adsorption energies for molecular hydrogen at the Pt NPs along with the energies and activation energies of its dissociation were estimated and the pathways of activationless dissociative adsorption were found. The full map of adsorption energies of atomic hydrogen at the various surface regions of Pt24 was obtained. The structures of transition states for the rearrangements between the adsorption complexes were located, and the activation energies for surface migration were calculated. Additionally, several ways of subsurface diffusion of H atoms inside the Pt24 cluster were considered which allows estimating the diffusion parameters and the probability of the hydrogen spillover when the cluster surface is highly covered by ligands restricting the surface migration. The IR and Raman spectra of most favorable adsorption complexes were simulated to provide the possibility of an experimental validation of the results obtained.