Properties of Transition Species in the Reactions of Hydroxyl with Ammonia and with Itself

Abstract

Quantum transition state theory was used to model two hydrogen atom abstraction reactions of hydroxyl radicals. Moments of inertia for the transition species were calculated from geometries obtained by ab initio or bond energy bond order methods. The transition state equation was fit by nonlinear least squares to experimental rate constants to determine parameters of the transition species. For the reaction of hydroxyl with ammonia, the geometric mean, ω b , of four transitional vibrational frequencies was found to be 660 ( 20 cm -1 , the activation barrier height, V b , including zero point energy, to be 8.5 ( 0.5 kJ mol -1 , and the barrier thickness, at half its height, to be 65 ( 30 pm. For the disproportionation reaction of hydroxyl radicals, the corresponding values of the first two parameters, based on the most recent experimental data, were 890 ( 20 cm -1 and 2.7 ( 0.8 kJ mol -1 . Ab initio calculations on the latter reaction gave ω b ) 746 cm -1 at the HF/6-31G(d,p) level and V b ) 13.6 kJ mol -1 at the MP4/6-311G(d,p) level. The curvature of the Arrhenius plots can be explained by the contributions of the low-frequency vibrations and, for the disproportionation reaction, of the low-lying electronic states.

Multichannel RRKM-TST and Direct-Dynamics CVT Study of the Reaction of Hydrogen Sulfide with Ozone

“…In order to calculate the tunneling factor, it is assumed that a particle with an energy of E approaches an unsymmetrical Eckart barrier. In this method, the tunneling factor can be calculated as normalΓ = exp ( V k B T ) ∫ 0 ∞ κ false( E false) ⁡ exp true( prefix− E k normalB T true) ⁡ d E k B T where V is the effective barrier height corrected for the ZPE and κ( E ) is the transmission probability for a particle with the energy E approaching an Eckart barrier. To calculate the tunneling factor, a numerical integration program from Brown was used.…”

Gas-Phase Reactivity of OH Radicals With Ammonia (NH3) and Methylamine (CH3NH2) at Around 22 K

“…In Scheme 1, a simplified illustration of the relative energies to the reactants of the stationary points along the minimum energy pathway (MEP) for the OH + NH 3 system is shown. Some studies proposed that product formation in reaction R1 occurs from the transition state and reported energy barriers ranged from 2.03 kcal/mol to 8.94 kcal/mol (Giménez et al, 1992;Bowdridge et al, 1996;Lynch et al, 2000). However, other investigations proposed that reaction R1 occurs through a H-bonded PRC at the entrance channel (Corchado et al, 1995;Bowdridge et al, FIGURE 4 | (A) Temperature dependence of k 1 (log-log plot) in the 22-4,000 K range.…”

“…The observed dependence of k 1 with temperature is positive, i.e., the rate coefficient increases when temperature increases, and the reported activation energies range from 0.5 to 9 kcal/mol in the 230-2,360 K range (Zellner and Smith, 1974;Hack et al, 1974;Perry et al, 1976;Silver and Kolb, 1980;Fujii et al, 1981Fujii et al, , 1986Salimian et al, 1984;Stephens, 1984;Zabielski and Seery, 1985;Jeffries and Smith, 1986;Diau et al, 1990). The computed energy barriers in the 200-4,000 K range were found to be between 2.0 and 9.05 kcal/mol (Giménez et al, 1992;Corchado et al, 1995;Bowdridge et al, 1996;Nyman, 1996;Lynch et al, 2000;Monge-Palacios et al, 2013b;Nguyen and Stanton, 2017). The reaction mechanism of R1 was also theoretically investigated (Giménez et al, 1992;Espinosa-García and Corchado, 1994;Corchado et al, 1995;Bowdridge et al, 1996;Nyman, 1996;Lynch et al, 2000;Monge-Palacios et al, 2013b;Nguyen and Stanton, 2017).…”

“…The computed energy barriers in the 200-4,000 K range were found to be between 2.0 and 9.05 kcal/mol (Giménez et al, 1992;Corchado et al, 1995;Bowdridge et al, 1996;Nyman, 1996;Lynch et al, 2000;Monge-Palacios et al, 2013b;Nguyen and Stanton, 2017). The reaction mechanism of R1 was also theoretically investigated (Giménez et al, 1992;Espinosa-García and Corchado, 1994;Corchado et al, 1995;Bowdridge et al, 1996;Nyman, 1996;Lynch et al, 2000;Monge-Palacios et al, 2013b;Nguyen and Stanton, 2017). The formation of a pre-reactive complex (PRC) at the entrance channel is accepted, as illustrated in Scheme 1, and quantum mechanical tunneling has been reported to be an important contribution to k 1 , leading to the observed non-Arrhenius behavior (Espinosa-García and Corchado, 1994;Corchado et al, 1995;Bowdridge et al, 1996;Nyman, 1996;Monge-Palacios et al, 2013b;Nguyen and Stanton, 2017).…”

Chronically anticoagulated patients who need surgery: Can low‐molecular‐weight heparins really be used to “bridge” patients instead of intravenous unfractionated heparin?

“…One of the most extensively studied reaction species is the OH radical, its important role as an oxidizing agent in both atmospheric and combustion processes has led to the assimilation of a large body of kinetic data for its reactions with both radical and stable species. For instance, the reaction of NH 3 with OH radical had been studied experimentally and theoretically by chemists 6–9. The rate constants of the oxidation reaction of the methylamine by the OH radical have also been measured by many research groups 10–14.…”