Article

Computational Studies on the Kinetics and Mechanisms for NH 3 Reactions with ClO x ( x = 0−4) Radicals

Department of Chemistry, Emory University, Atlanta, Georgia, United States
The Journal of Physical Chemistry A (Impact Factor: 2.69). 03/2007; 111(4):584-90. DOI: 10.1021/jp065397t
Source: PubMed

ABSTRACT

Kinetics and mechanisms for NH3 reactions with ClOx (x = 0-4) radicals have been investigated at the G2M level of theory in conjunction with statistical theory calculations. The geometric parameters of the species and stationary points involved in the reactions have been optimized at the B3LYP/6-311+G(3df,2p) level of theory. Their energetics have been further refined with the G2M method. The results show that the H-abstraction process is the most favorable channel in each reaction and the barriers predicted in decreasing order are OClO > ClO > Cl > ClO3 > ClO4. All reactions were found to occur by hydrogen-bonding complexes; the rate constants for these complex metathetical processes have been calculated in the temperature range 200-2000 K by the microcanonical VTST and/or RRKM theory (for ClO4 + NH3) with Eckart tunneling and multiple reflection corrections. The predicted rate constants are in good agreement with the available experimental data.

0 Followers
 · 
3 Reads
  • [Show abstract] [Hide abstract]
    ABSTRACT: The insertion reaction mechanism of CBr2 with CH3CHO has been studied by using the B3LYP/6-31G(d) method. The geometries of reactions, transition state and products were completely optimized. All the energy of the species was obtained at the CCSD(T)/6-31G(d) level. All the transition state is verified by the vibrational analysis and the internal reaction coordinate (IRC) calculations. The results show that the propionaldehyde (HP1) is the main product of CH2 insertion with CH3CHO. The calculated results indicated that all the major pathways of the reaction were obtained on the singlet potential energy surface. The singlet CBr2 not only can insert the Cα-H [reaction I(1)]) but also can react with Cβ-H [reaction II(1)]. The statistical thermodynamics and Eyring transition state theory with Wigner correction are used to study the thermodynamic and kinetic characters of I(1) and II(1) in temperature range from 100 to 2200 K. The results show that the appropriate reaction temperature rang is 250 to 1750 K and 250 to 1600 K at 1.0 atm for I(1) and II(1) respectively. The rate constant and equilibrium constant are distinct in the range from 250 to 1000 K so that I(1) more easily occurs, while the reactions are not selected in the temperature range of 1000–1600 K.
    No preview · Article · Jan 2008 · Chinese Science Bulletin
  • [Show abstract] [Hide abstract]
    ABSTRACT: In agreement with previous studies, the ground state of ClO4 has been confirmed to be X2B1. Vertical excitation energies and oscillator strengths were calculated by MRCI methods for doublet and quartet states of ClO4. The highest oscillator strength was found for 1 2A1 at 2.95 eV. This state has been identified as the upper state seen by Kopitzky and co-workers in the absorption spectrum of ClO4. Two higher states, 22A1 and 32A1, at 4.19 and 8.12 eV, respectively, also have relatively high oscillator strengths. Rydberg states start at about 9.5 eV. Geometry optimizations were performed by DFT and CCSD(T) methods. After extensive testing, the B3LYP density functional, together with the 6-311 + G(3df) basis set were chosen for calculations. Optimized geometries of seven excited states were obtained. The adiabatic excitation energy of 12A1 (2.40 eV) agrees closely with the observed band origin at 2.46 eV. Three excited states have one or two imaginary vibrational modes. Electron affinity and heat of formation of ClO 4 agree with literature values. None of the quartet states was found to be stable.
    No preview · Article · Oct 2009 · Molecular Physics
  • [Show abstract] [Hide abstract]
    ABSTRACT: The kinetics and mechanism for the sublimation/decomposition of ammonium perchlorate (AP) with and without water present on the decomposing surface have been investigated by first-principles calculations, using a generalized gradient approximation with the plane-wave density functional theory. Calculated results show that H 2O can enhance the sublimation of AP; the sublimation energies with (H 2O) n (n = 0, 1, 2, 3) from the surface were predicted to be 28.1, 21.4, 18.6, and 14.2 kcal/mol, respectively. Notably, H 2O was found not to affect the proton transfer between the NH 4+/ClO 4- ion pair, but was found to significantly enhance the sublimation of AP by co-desorbing with the ion pair. The rate constants for the dominant sublimation processes, the desorption of the molecular complex, H 3N· · ·HOClO 3, and the codesorption of H 2O with AP, (H 2O) n· · ·NH 4ClO 4 (n = 1, 2), predicted by canonical variational transition state theory can be presented by k des = 6.53 × 10 12 exp (-28.8 kcal/mol/RT) (n = 0) s -1, 1.69 × 10 10 exp (-20.3 kcal/mol/RT) (n = 1) s -1, and 1.08 × 10 11 exp (-17.7 kcal/mol/RT) (n = 2) s -1, respectively, with a significant enhancement by the H 2O molecules. Interestingly, the structures of AP in the water complexes are more ionic than those without H 2O in the gas phase. In addition, the energy changes for proton transfer on the surface have been compared with those in solution. The calculated proton transfer energies on the crystalline AP surface with (H 2O) n (n = 0, 1, 2), 31.1, 29.7, and 32.5 kcal/mol, were found to be close to the corresponding values of 30.8, 30.5, and 30.4 kcal/mol calculated by the (H 2O) n-NH 4ClO 4 complexes in solution using the polarizable continuum model. These calculated proton transfer energies on the surface and in solution are close to the experimental values inside AP solid, 26-31 kcal/mol.
    No preview · Article · Jan 2010 · International Journal of Energetic Materials and Chemical Propulsion
Show more