Adsorption of Glyoxal (CHOCHO) and Its UV Photolysis Products on the Surface of Atmospheric Ice Nanoparticles. DFT and Density Functional Tight-Binding Study

The structures, energies, harmonic vibrational frequencies, and thermodynamic parameters of the water clusters (H2O)48, (H2O)72, and (H2O)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 CH2O, HCOOH, H2O2, CO, CO2 and radicals CHO, HO, HO2) 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

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

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

Mechanism Of Nitric Oxide Oxidation Reaction (2No + O2 → 2No2) Revisited

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. © 2011 American Chemical Society

Adsorption Of Glyoxal (Chocho) And Its Uv Photolysis Products On The Surface Of Atmospheric Ice Nanoparticles. Dft And Density Functional Tight-Binding Study

The structures, energies, harmonic vibrational frequencies, and thermodynamic parameters of the water clusters (H2O)48, (H2O)72, and (H2O)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 CH2O, HCOOH, H 2O2, CO, CO2 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\u27s atmosphere. © 2014 American Chemical Society

Quantum Chemical Study Of The Initial Step Of Ozone Addition To The Double Bond Of Ethylene

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. © 2012 American Chemical Society

Gas-Phase Reactions Regarding GaN Crystal Growth in a Carbon-Based Transport System: A Quantum Chemical Study

To elucidate an experimentally observed increase of GaN crystal growth rate in the Ga­(l)-graphite-NH₃(g) system (Jacobs, K. et al. <i>J. Cryst. Growth</i> <b>2010</b>, <i>312</i>, 750–755), a gas-phase chemical model of the chemical vapor deposition (CVD) process was developed in a quantum chemical study. The reaction mechanisms in a gas-phase Ga/HCN/NH₃ system were predicted using density functional theory (DFT) and post-Hartree–Fock methods including conventional DFT (B3LYP/cc-pVTZ) and coupled cluster (CCSD/cc-pVTZ) theory levels. Activation and reaction energies were refined with a CCSD­(T)/aug-cc-pVTZ//B3LYP/cc-pVTZ composite approach. A relatively modern variant of the coupled cluster theory (ROCCL method) in conjunction with the aug-cc-pVDZ basis set for H, N, and aug-cc-pV­(D+d)­Z for Ga atoms) was employed to investigate bond cleavage reaction pathways. Reactions in Ga­(²P) + NH₃ and Ga­(²P) + HCN gas-phase systems and reactivity of products of the Ga­(²P) + HCN reaction (HGaCN and HGaNC) with both Ga (atomic vapor) and NH₃ (reactive diluent gas) were included in the model of the chemical transport. Elementary steps involving newly reported cyclic (HGaCNGa) containing intermediates were considered. The role of HCN as a chemical transport reagent in GaN CVD was established unambiguously. A comparative study of minimum energy pathways (MEPs) for reactions in the Ga/NH₃ and Ga/HCN/NH₃ systems supported the experimental observation of the GaN deposition rate increase

Adsorption of Glyoxal (CHOCHO) and Its UV Photolysis Products on the Surface of Atmospheric Ice Nanoparticles. DFT and Density Functional Tight-Binding Study

The structures, energies, harmonic vibrational frequencies, and thermodynamic parameters of the water clusters (H₂O)₄₈, (H₂O)₇₂, and (H₂O)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-<i>cis</i>- and s-<i>trans</i>-glyoxal molecules as well as its sunlight UV photolysis products (molecules CH₂O, HCOOH, H₂O₂, CO, CO₂ and radicals CHO, HO, HO₂) 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

Adsorption and Diffusion of Hydrogen on the Surface of the Pt₂₄ Subnanoparticle. A DFT Study

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