Yu.M. Gershenzon

The temperature dependence of the reactions H2(v = 1) + D, H

Abstract The temperature dependence of the rate constant for the reactions H2(v = 1)+D and H2(v = 1)+H was studied using a flow-tube method combined with EPR and thermometric techniques. The rate constants (in cm3/molecule s) were found to be kuD = (2.3 ± 1.3)×10−10exp[−(3200 ± 600)/RT]. knD = (1 ± 0.5)×10−10exp[−(3100 ± 300)/RT]. krD ⩽ 2.7×10−13 over the range 254–367 K for H2(v = 1)+D and knH+krH = (1.8 ± 0.7)×10−13 at 254 K and (3.9 ± 1.0)×10−13 at 298 K for H2(v = 1)+H.

Reactions of HO2 with NO, OH and HO2 studied by EPR/LMR spectroscopy

Abstract A combined EPR/LMR spectrometer and fast-flow system has been used to investigate the reactions HO 2 + NO( k 1 ), HO 2 + OH( k 2 ), HO 2 + HO 2 ( k 3 ) at room temperature. The rate constants have been measured: k 1 = (7.0 ± 0.6) × 10 −12 cm 3 s −1 ( P = 7–10 Torr); k 2 = (5.2 ± 1.2) × 10 −11 cm 3 s −1 ( P = 8–10 Torr); k 3 = (1.65 ± 0.3) × 10 −12 cm 3 s −1 ( P = 2.1–24.9 Torr). The conclusion is drawn from analysis of the literature and the present work that k 2 and k 3 do not depend on pressure up to 1 atm.

On the mechanism of heterogeneous relaxation of vibrationally excited nitrogen molecules

Abstract The N 2 vibrational deactivation probability ∈ in the case of a collision with a glass surface has been measured over the temperature range 282–603°K for different values of pressure. The observed temperature and pressure dependence of ∈ is interpreted in terms of two different additive mechanisms of the heterogeneous vibrational relaxation. One of these is connected with the physical and the other with the chemical adsorption of the vibrationally excited N 2 molecule.

Dynamics of vibrational energy exchange in collisions of OH and OD radicals with N2. Application to the kinetics of OH-vibrational deactivation in the upper atmosphere

The dynamics of collisions of OH and OD radicals with the N2 molecule is considered. Vibrational energy exchange rate constants are estimated theoretically. The vibration-to-rotation energy exchange mechanism is shown to be the main contribution for these systems. The results are compared with experimental data and the rate constants obtained from the base ground observations of night sky Meinel emission. Rate constants of vibrational-to-vibrational exchange are too low for the collisions of OH with N2 to be important in hydroxil vibrational energy deactivation in the atmosphere.

Non-Arrhenius behavior of the F+H2 reaction rate at 293–700 K

Abstract Non-Arrhenius behavior was observed in the temperature range 293–700 K for the absolute rate constant for the reaction F + H 2 using an EPR spectrometer combined with a fast-flow system. The measured rate constant (in cm 3 /s) fits the expression k = 2.9 × 10 −13 T exp[−(1390±440)/ RT ].

Study of the reaction of oxygen atoms with vibrationally excited ozone molecules

Abstract The exothermal reaction O + O 3 - 2O 2 is found to be accelerated when ozone molecule vibrations are excited by cw CO 2 laser emission when P Ar = 3 Torr, T = 300 K in a 0.8 cm i.d. vessel. With a laser irradiation intensity of ≈15 W/cm 2 this acceleration made up ≈40% as compared with the equilibrium reaction. Vibrational relaxation of ozone molecules is studied by the method of saturated absorption. The relaxation time of population difference for (000) and (001) levels, T 1 = (3.8 ± 1.7) × 10 −4 s, and the transversal relaxation time, T 2 ≈4 × 10 −8 s Torr/ P , are measured. T 1 is found to be practically independent of pressure at P = 1–15 Torr.

The Kinetic Mechanisms of the Trapping of Atmospheric Gases by Marine Salt Surfaces: IX. The Heat of Adsorption of Cl Atoms on the Surface of NaCl

The heterogeneous trapping of chlorine atoms on the surface of NaCl was studied using two coaxial stream reactors connected to an EPR cavity or a mass spectrometer. The kinetics of trapping was measured by the EPR method over a wide range of chlorine atom concentrations (1010–1013 cm−3) at temperatures of 250–330 K. At [Cl] ≥ 1012 cm−3, chlorine atoms were recorded by the EPR method in the gas phase. At lower concentrations (~1010−3 × 1011 cm−3), Cl atoms were replaced with RO2 radicals by adding hexane RH and O2 at the entrance of the EPR cavity. This was followed by the matrix isolation of RO2 in the cavity at liquid nitrogen temperature. The probability of the trapping of chlorine atoms on the chemically inactive surface of NaCl was found to increase as the concentration of Cl grew. The temperature dependence of the trapping coefficient γ was pronounced at a concentration of chlorine atoms of ~3 × 1010 cm−3, whereas no such dependence was observed at a chlorine concentration of ~ 1013 cm−3. The recombination of Cl atoms was well described by the Rideal-Eley mechanism, and the heat of adsorption of chlorine atoms on the inactive surface of NaCl was estimated at Q = 17 ± 0.6 kcal/mol. It was shown mass spectrometrically that the trapping coefficient γ of Cl atoms decreased with the time of measurements, like the partial coefficient of the formation of the HCl product, whereas the partial coefficient of the formation of the Cl2 product, conversely, increased with the time. The characteristic time of the attainment of stationary values by all the γ coefficients weakly depended on the initial concentration of Cl and equaled several dozen seconds. Reactions of adsorbed Cl atoms formed in the trapping of NO3 radicals by the surface of marine salt NaCl in coastal troposphere are discussed.

An Ab initio study of the structure and energetics of HONF2 and .ONF2

High level ab initio calculations have been performed to determine the ground state energy and geometry of HONF2 (which has not yet been synthesized). Gradient optimization has been done at the self-consistent field (SCF) level with the 6-31G∗∗ and 6-31 + G∗∗ polarized split-valence basis sets. Subsequent perturbative extensions up to fourth-order Moller-Plesset (MP4) have been performed. Geometry and energy of the transition states of HONF2 fragmentation have been determined. HONF2 is found to be stable, the fragmentation barrier being equal to 35.8 kcal mol−1. The barrier of HONF2 fragmentation through the chain mechanism is considerably lower, about 12 kcal mol−1. The heat of formation values for HONF2 and the intermediate .ONF2 radical appear to be equal to −32 ± 2.5 and 8.8 ± 0.4 kcal mol−1 respectively.

Heterogeneous reactions of H and O2 with solid alkanes

The paper describes a new source of surface organic radicals whose special feature is a low rate of the initiation of alkyl radicals in reactions of H atoms with the surface of alkanes. A special reactor design was used to accumulate radicals at room temperature and observe alkoxyl radicals RO by the EPR method. For this purpose, alkanes were deposited on aerosil and placed into an EPR cavity. Thanks to the large area of aerosil loads (∼103 cm2), we were able to obtain a stable signal corresponding to a ∼1017 total amount of alkoxyl radicals (the degree of surface coverage ∼0.1%). When O2 was introduced into the reactor in concentrations of 1015–1016 cm−3, a sharp decrease in the signal from surface organic radicals was observed. The process was described as the first-order (RO)s + O2 → HO2 + ketone reaction with a 1.7 × 10−17 cm3 s−1 rate constant.