Progress in Reaction Kinetics and Mechanism

Benzene, para-xylene and mesitylene were adsorbed into silicalite, Na - morde- nite, Na - ZSM5 and Li - X zeolites, and studied using the longitudinal field muon spin relaxation (LF-μSRx) technique. The zeolites ZSM5/silicalite and mordenite contain microporous channels while zeolite X has only supercages. For cyclohex- adienyl radicals/benzene adsorbed in Na-ZSM5, silicalite and Na - mordenite, a fraction was detected with a common reorientational activation energy in the region of ca 5kJ mol⁻¹; however, in all cases there appeared a second fraction with an activation energy of ca 12kJmol’. [In Li-X only a single fraction was observed with E_a = 8.1kJ mol⁻¹, from molecules adsorbed in supercages]. Given that high loadings of benzene, beyond the saturation capacities of the zeolites were employed, we believe the two distinct motional distributions represent the channel intersection and channel/external-surface niche locations in ZSM5/silicalite, the latter having the higher activation energy. An extraneous fraction may also be present (possibly as a thin film coating the zeolite grains), which probably also contributes to the detected ca 5 kJ mol⁻¹ component in which, as at the channel-intersections, the motion tends toward bulk behaviour. Mordenite has only a single accessible channel structure and is hence devoid of the relatively unrestricting channel intersecting pores that are present in ZSM5/silicalite. However, the channels are wider (ca 7 A diameter) and we may ascribe therefore, the ca 5 kJ mol⁻¹ fraction to the formation of benzene clusters within these channels that exhibit bulk-type behaviour, while we assign, in analogy with the results for ZSM5/silicalite, the ca 12kJ mol⁻¹ fraction to molecules adsorbed in niches on the external surface. It is thought that the essential difference between the two activation energies is that the ca 5 kJ mol -¹ processes involve molecular motion within clusters of benzene molecules with properties similar to the bulk phase (6.6 kJ mol -¹ was measured in pure benzene), while those characterised at ca 12kJ mol⁻¹ reflect single benzene molecules or small, ordered molecular clusters interacting with a zeolite surface. The results for dimethylcyclohexadienyl radicals (derived from p-xylene) are rather similar, but indicate slightly reduced activation energies. The larger mesitylene molecules can penetrate zeolite X and (more slowly) mordenite, but are excluded from the internal pores of ZSM5/silicalite.

The reaction of silylene with acetylene was observed by the density function theory (DFT) method at the 6 - 311 g (d,p) level. A new reaction pathway has been identified involving the tetra-conjugated: SiH₂CH==CHH₂Si:. Thermodynamic and kinetic analysis confirms the reaction to be spontaneous at room temperature. The new intermediate proposed in this work offers a resolution of the conundrum of the silylene/acetylene reaction.

The kinetic parameters for the chemical reaction of nano-NiO of different particle sizes with aqueous sodium bisulfate solution were determined; additionally, the influence of particle size on the kinetic parameters were studied and were discussed. The results show that: there are clear effects of the particle size of nano-NiO on the rate constant, the reaction order, the apparent activation energy and the pre-exponential factor; thus the rate constant and the reaction order increase, and the apparent activation energy and the pre-exponential factor decrease, with decrease of the particle diameter; the logarithm of the preexponential factor, and the apparent activation energy, exhibit a linear relationship with the reciprocal of the particle diameter respectively. The bases of the role of the particle size are that the reaction order is influenced by the molar surface area of nano-NiO, the apparent activation energy by the molar surface energy, the preexponential factor by the molar surface entropy, and the rate constant by both the molar surface energy and the molar surface entropy.

A new macrocyclic (N-donor) cobalt(II) complex, denoted CoR, was synthesized and characterized, and used as a mimetic peroxidase in the catalytic oxidation of phenol by H₂O₂. The catalytic system displayed high catalytic activity and the catalytic character of a metalloenzyme, although it did not attain the catalytic efficiency of enzymes. The final products were maleic and oxalic acids. Intermediate products, such as catechol and hydroquinone and p-benzoquinone, were detected by high-pressure liquid chromatography (HPLC) and UV-Vis spectrophotometry. The effects of temperature and the pH of the system on the rate of the catalytic oxidation were investigated. A reaction mechanism and kinetic model for the catalytic oxidation are proposed, which are supported by the results of the experimental and calculation data.

Oxidation of some lower oxyacids of phosphorus by benzimidazolium dichromate results in the formation of the corresponding oxyacids containing phosphorus in a higher oxidation state, via an intermediate complex formation.

The reaction between benzensulfonyl chloride and aniline was studied in various compositions of methanol with ethyl acetate, propan-2-ol and water at 25° C. Second-order rate coefficients of this reaction were determined; those in pure solvents were in the order of water > methanol >propan-2-ol> ethyl acetate. Rates decreased sharply with the mole fraction of ethyl acetate in methanol- ethyl acetate mixtures and showed an increasing trend with water content in methanol-water mixtures. Linear free energy relationship regressions showed that the polarity, hydrogen-bond donor ability and hydrophobicity of the media were the main factors affecting the reaction rate coefficient.

The kinetics of interaction between glycyl-glycine (L-L'H) and [Rh(H 2 O) 5 OH] 2+ [Rh(H₂O)₅OH] ²⁺ have been studied spectrophotometrically in aqueous medium as a function of [Rh(H₂O)₅OH] ²⁺], [glycyl - glycine], pH and temperature at constant ionic strength. At pH 4.3, the interaction with glycyl-glycine shows two parallel paths: both processes are ligand-dependent. The rate constant for the processes are: k₁ ∼ 10 ⁻³ s ⁻¹ and k₂∼10⁻⁵ s⁻³. The activation parameters calculated from Eyring plots are: ΔH₁^≠ - 26.7 ± 0.3 kj mol⁻¹, ΔS₁^≠ =-207+1 JK⁻¹ mol⁻¹, ΔH₂^≠ = 92.4 ± 1.3 kJ mol⁻¹ and ΔS₂^≠ = −42±4 JK⁻¹ mol⁻¹. The ligand glycyl-glycine, with different donor atoms (N and O), attacks two rhodium(III) ions (Rh(III) being a borderline acid), with different reactivity. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for both interaction processes. From the temperature dependence of the outer-sphere association equilibrium constant, the thermodynamic parameters calculated are: ΔH₁^∘ = 21.8 ± 1.9 kJ mol⁻¹, ΔS₁^∘ = 124± 6 JK⁻¹ mol⁻¹, ΔH₂^∘ = 12.7 ±0.5 kj mol⁻¹ and ΔS₂^∘ 89±2 JK⁻¹ mol″⁻¹ which give a negative AG° value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer- sphere association complex for both the parallel paths. The product of the reaction has been characterized from IR and ESI-mass spectroscopic analysis.