Reaction of H2 with O2 in Excited Electronic States: Reaction Pathways and Rate Constants

Toward size-dependent thermodynamics of nanoparticles from quantum chemical calculations of small atomic clusters: a case study of (B2O3)n

We present a method for obtaining canonical partition functions and, accordingly, temperature-dependent thermodynamics of arbitrary-sized (nano) particles from electronic structure calculations of the corresponding small size atomic clusters. The guiding idea here is to extrapolate the basic properties underlying the thermochemistry of clusters (electronic energies, rotational constants, and vibrational frequencies) rather than the thermodynamic functions themselves. The thus obtained scaling dependences for these basic properties expressed in a simple analytical form provide an efficient tool for fast evaluation of the size-selected thermochemical data for particles of any nuclearity. To exemplify the performance of the methodology, neutral stoichiometric boron oxide clusters are considered. To this end, the geometry and various physical properties of the energetically lowest-lying (B₂O₃)_n (n = 1,…,8) structures are found using density functional theory and the authors' multistage hierarchical procedure customized for global optimization of quite large cluster structures. With these data and based on the physically consistent scaling regularities for the principal cluster properties, the size-selected thermodynamic functions of boron oxide particles in the gas phase, such as enthalpy, entropy, and specific heat capacity, are derived. The variation of these characteristics with increasing cluster size is discussed in detail as well. To facilitate handling of the temperature and size dependences we have found here in further chemical kinetic and equilibrium modeling, the tabulated thermodynamic functions of interest are fitted for n = 1,…,1000 to the standard seven-parameter Chemkin polynomials.

Reaction of the N Atom with Electronically Excited O2 Revisited: A Theoretical Study

The kinetics of the reaction of N with electronically excited O₂ (singlet a¹Δ_g and b¹Σ_g⁺ states), potentially relevant for NO_x formation in nonthermal air plasma, is theoretically studied using the multireference second-order perturbation theory. The corresponding thermodynamically and kinetically favored reaction pathways together with possible intersystem crossings are identified. It has been revealed that the energy barrier for the N + O₂(a¹Δ_g) → NO + O reaction is approximately twice the barrier height for the counterpart process with O₂(X³Σ_g^-). The molecular oxygen in the b¹Σ_g⁺ state, in turn, proved to be even less reactive to atomic nitrogen than O₂(a¹Δ_g). Appropriate thermal rate constants for specified reaction channels are calculated by the variational transition-state theory incorporating corrections for the tunneling effect, nonadiabatic transitions, and anharmonicity of vibrations for transition states and reactants. The corresponding three-parameter Arrhenius expressions for the broad temperature range (T = 300-4000 K) are reported. At last, post-transition-state molecular dynamics simulations indicate that the N + O₂(a¹Δ_g) reaction produces vibrationally much colder NO molecules than the N + O₂(X³Σ_g^-) process.

Rate constants for collision-induced emission of O2(a1Δg) with He, Ne, Ar, Kr, N2, CO2 and SF6 as collisional partners

Rate constants for singlet oxygen collision induced emission of the a1Δg-X3Σ-g transition at 1.27 μm were measured for CO2, N2, SF6, and rare gases as collisional partners. Photolysis of ozone by 266 nm laser radiation produced singlet oxygen. We performed direct measurements of pressure dependences of the 1.27 μm emission intensity for partner gases. The measured rate constants kMa-X in the units of 10-24 cm3 s-1 are as follows: CO2 - 10 ± 2; N2 - 3.2 ± 0.6; SF6 - 7 ± 1; He - 1.1 ± 0.3; Ne - 1.3 ± 0.3; Ar - 2.8 ± 0.6; Kr - 6 ± 1. The measured values of kMa-X are close to the values calculated from absorption measurements. Considering the known rate constants kMb-a for the b1Σg+-a1Δg transition in the gas phase we found that the ratio kMa-X/kMb-a was constant and independent of a collisional partner according to the "spin-orbit based" mechanism of intensity borrowing proposed by Minaev (THEOCHEM, 1989, 183, 207). However, this ratio amounted to (1.3 ± 0.2) × 10-4, which is considerably lower than the theoretically predicted value of (3-6) × 10-4.