Ab initio study of the thermodynamic and kinetic stability of the ammonium radical

An increasing interest in the possible existence of the NH₄ radical has emerged in recent years. In this paper we report an ab initio UHF CI study of the ammonium radical, an investigation of parts of the energy surface around NH₄ and a theoretical prediction of the kinetic parameters of the radical formation and dissociation reactions within the framework of the TST theory. The ground state of the ammonium radical appears to be of the Rydberg type. Its ionization potential is found to be 4.29 eV. The NH₄ formation reaction from NH₂ + H₂ is very slightly exothermic whereas the reaction from NH₃ + H is slightly endothermic. We find a transition state of C_3v symmetry for the dissociation of NH₄ into NH₃ + H. The insertion of H₂ into NH₂ occurs according to a two-step mechanism whose determining step corresponds to the crossing of a saddle point with C_s symmetry previously obtained in the study of the reaction NH₂ + H₂ → NH₃ + H. Finally, we predict for NH₄ and ND₄ lifetimes of 0.1 and 1.4 μs respectively.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

Perturbation theory at large order in reggeon field theory

We estimate the behaviour of the perturbation expansion and the ? expansion at large orders in reggeon field theory. The perturbation expansion is divergent but Borel summable for α₀ < 1. At fixed total rapidity, the Borel summability extends into α₀ > 1.     

The content of unitarity constraints on a pomeron pole of intercept one

We present a consistent picture of a pomeron pole with intercept one, together with its cuts, which evades the decoupling arguments. We use the reggeon cut discontinuity formulae to introduce Gribov's reggeon calculus as an exact solution of multiparticle t-channel unitarity. We show how, within the calculus, two-pomeron iterations of a singular kernel can be responsible for the zero in the triple-pomeron vertex. Using the concept of a bare pomeron pole as a multiperipheral production process which is subsequently renormalised by other effects, we apply the reggeon calculus analysis to inclusive cross sections. We find that the inclusive sum rule decoupling arguments are avoided because of the addition of enhanced absorptive corrections to the conventional Regge pole contributions. However, we show that in this picture the combined pole and two-pomeron cut contribution to the total cross section factories to order (ln s)^?2.We also show that, when the correct helicity structure of the pomeron is taken into account, the s-channel unitarity condition for pomeron scattering amplitudes does not lead to any serious decouplings.     

Renormalization group study of theA+B→⊘ diffusion-limited reaction

TheA+B diffusion-limited reaction, with equal initial densitiesa(0)=b(0)=n ₀, is studied by means of a field-theoretic renormalization group formulation of the problem. For dimensiond>2 an effective theory is derived, from which the density and correlation functions call be calculated. We find the density decays in time as ford<4, with =n ₀ –Cn ₀ ^d/2 +..., whereC is a universal constant andC is nonuniversal. The calculation is extended to the case of unequal diffusion constantsD _A D _B , resulting in a new amplitude but the same exponent. Ford2 a controlled calculation is not possible, but a heuristic argument is presented that the results above give at least the leading term in an =2–d expansion. Finally, we address reaction zones formed in the steady state by opposing currents ofA andB particles, and derive scaling properties.     

Low-temperature reaction of trialkoxysilanes on silica gel: a mild and controlled method for modifying silica surfaces

The room temperature reaction of 4-(triethoxysilyl)butyronitrile, 4-TBN ((C2H5O)3Si(CH2)3CN), on weakly hydrated silica samples pretreated at 393 K has been studied by desorption experiments and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy at different aging times under various water partial pressures. The reaction is demonstrated by the decrease of desorption of 4-TBN with time and the simultaneous disappearance of the 2980 and 1394 cm(-1) signals in the DRIFT spectra, assigned to the CH3 moiety of the ethoxy functions. Water partial pressure is shown to have a crucial effect on the rate and efficiency of the process as, after 6 days, for samples kept at room temperature under vacuum, ca. 50% of the silane has reacted, while for those kept in a water-saturated atmosphere the silane reaction reaches 96%. Although the silane appears to be irreversibly bonded to the surface, no definite conclusion may be drawn from these preliminary results as to the nature of the bonding (grafting or coating). These samples are compared to modified silicas prepared according to conventional methods. The same extent of silane reaction (50%) is achieved for preadsorbed samples kept under vacuum and either cured at 473 K for 30 h or kept at room temperature for 6 days. A mild and controlled modification of silica by triethoxysilanes can thus be achieved by first physisorbing known amounts of the modifying silanes from an organic solvent on pretreated silica and then letting the samples mature for a few days at room temperature in a water-saturated atmosphere.