Progress in Reaction Kinetics and Mechanism

A series of group 1 and group 2 element cation-exchanged samples of zeolite X were prepared and exposed to a beam of spin-polarised, positive muons (of energy 28MeV/c) from a particle accelerator. Longitudinal field repolarisation measurements were made which identified, in each case, both diamagnetic and paramagnetic muon fractions. One of the paramagnetic components revealed a hyperfine field compatible with values reported previously for free muonium atoms (1585 G), while a secondary component was detected showing roughly half the hyperfine coupling measured for atomic muonium, leading to the conclusion that the muonium had become chemically-bound within the zeolite structure, probably via a single-electron bond to a coordinatively unsaturated aluminium atom. A significant “lost fraction” was also indicated along with superhyperfine and anisotropic components that repolarised in weaker magnetic fields (0-100G). As a general trend, the muonium atom yield was found to increase in order of the increasing radius/charge (riz) ratio, though it is constant within the experimental errors, while the chemically bound muonium was formed in similar proportion in all samples (14.1 + 1.9% of the total muonium polarisation), with the exception of HX and the parent NaX (from which the other zeolite samples were prepared by cation-exchange), in which it was absent. Along with data from zero-field experiments which were also undertaken, this is interpreted in terms of an entrapment or effective “scavenging” of radiolytic electrons by the cations (in the order Na⁺>K⁺; Mg²⁺>Ca²⁺>Sr²⁺>Ba²⁺) whose combination with positive muons is therefore impeded, thus reducing the yield of muonium atoms. Results for LiX are anomalous since the muonium yield is greater than expected, possibly due to a greater degree of covalency in the Li-0 bonding than pertains for the other cations. These results may have some implications for zeolites that suffer longterm radiation exposure/damage when they are used to clean radioactive cations (e.g. Cs ⁺, Sr²⁺) from the waters of nuclear power stations and as materials whose cation-exchange capacity has been deliberately enhanced by prior exposure to ionising radiation.

The reaction of electrochemically generated o-benzoquinones from the oxidation of catechol derivatives as Michael acceptors with dimethyl phosphite as nucleophile has been studied using cyclic voltammetry. The reaction mechanism is believed to be EC comprising oxidation of catechol moiety of these antioxidants followed by Michael addition of dimethyl phosphite. The observed homogeneous rate constants (k_obs) for the reactions were estimated by comparing the experimental voltammetric responses with the digitally simulated results based on the proposed mechanism. In addition, the effects of nucleophile concentration and of substitutent group of the catechols on the voltammetric behaviour and the rate constants of the chemical reactions are described.

The kinetics of the palladium(II)-catalysed ligand exchange of the hexacyanoferrate(II) complex ion with 4-Cyanopyridine (4-CNpy) in aqueous acidic medium have been investigated at 25.0 ∓ 0.1 °C, pH 3.2 ∓ 0.02 and ionic strength, I=0.1 M (KNO₃). The reaction was followed spectrophotometrically by measuring the increase in absorbance of the intense cherry red complex [Fe(CN)₅4-CNpy] ³-, corresponding to metal-to-ligand charge transfer (MLCT) transitions, atλ._max 477 nm. The effects of pH, ionic strength, the concentrations of [Fe(CN)₅^4-], Pd²⁺ and 4-CNpy on the reaction rate have been studied and analysed. The kinetic data suggest that substitution follows an interchange dissociative (I_d) mechanism and occurs via formation of a solvent-bound intermediate. The effects of the dielectric constant of the medium on the reaction rate have been used to visualize the formation of a polar activated complex and an interchange dissociative mechanism for the reaction. A most probable mechanism has been proposed in order to interpret the kinetic data. Kinetic evidence is reported for the displacement of CN^- by 4-CNpy in [Fe(CN)6⁴-]. Activation parameters for the proposed reaction have been evaluated using an Eyring plot, and provided further support to the proposed mechanistic scheme.

Horseradish peroxidase (HRP) was successfully immobilized on amine functionalized TiO₂-coated multiwalled carbon nanotubes (NH₂ TiO₂ CNTs) by a convenient and efficient method. Electrochemical impedance spectroscopy, cyclic voltammetry and amperometry were applied to characterize the HRP/NH₂- TiO₂ - CNT nano-composite. These techniques showed that the NH₂ TiO₂CNTs greatly enhance the electron transfer between HRP and the modified electrode. Owing to the redox reaction of the electroactive centre of HRP, the HRP/NH2-TiO₂-CNTs modified electrode exhibited a pair of quasi-reversible peaks with a peak-to-peak separation (Δ E_p) of 70.6 m V and a formal potential (E°’) of - 367.65 m V (versus Ag/AgCl) in phosphate buffer solution. The charge transfer coefficient (a) and the apparent charge transfer rate constant (k_s) were found to be 0.34 and 2.08 s-1 respectively. The prepared biosensor responded to H₂O₂ with a linear range, detection limit, sensitivity and response time of 1.0 × 10⁻⁹ to 1.0 × 10 ⁻⁷ M, 0.786nM, 28.4 μA A nM⁻¹ and 3 s, respectively.

A theoretical study of the kinetics and mechanism of the cyclization of citronellal in the gas phase was performed using density functional theory methods at the B3LYP level of theory with 6-311G, 6-311G*, 6-31G**, 6-311G**, 6-311+G and 6-311 + +G basis sets at 298.15K, 433.15K, and 473.15K. Equilibrium molecular geometries and harmonic vibrational frequencies of the reactant, transition state and products were calculated. Rate constants and activation thermodynamic parameters were calculated and showed a fairly good agreement with experimental results. The effect of solvent polarity on the reaction was studied. These calculations indicated that the reaction proceeds through an asynchronous concerted mechanism.

The alkaline hydrolysis of landrin (2,3,5-trimethylphenyl-N-methylcarbamate) into 2,3,5-trimethylphenol was examined for various buffers and sodium hydroxide solutions ranging from pH 11.8 to 13.6. This work was carried out by reversed phase liquid chromatography and UV spectrophotometry. The positive activation entropy A Δ S#= +38.40 J mol ⁻¹ K ⁻¹ and the absence of general basic catalysis indicated an ElcB hydrolytic mechanism involving the formation of methyl isocyanate. This result was confirmed by the finding that landrin fits well into Brönsted and Hammett plots, obtained for a series of substituted N- methylcarbamates whose decomposition in aqueous media was established to follow an El cB mechanism.

A computational study has been carried out on the gas-phase retro-ene reaction of diallyl amine. The energies of the stationary points have also been evaluated with the B3LYP and MP2 methods on the optimized structure using the different basis sets. The kinetic activation parameters were calculated at 648 K to enable comparison with experimental data. Two probable mechanisms are suggested for the reaction. Since the radical mechanism produced a high activation energy conflicting with the experimental results, the concerted cyclic pathway of the retro-ene reaction was considered completely and comprehensively. Natural bond orbital analysis showed that the acidic character of the HI atom plays an important role in the reaction process. Wiberg bond indices were calculated throughout the reaction, and an asynchronousity value of 70% was obtained, confirming a distorted chair-like geometry for the transition state with the early and advanced natures of bond formation and bond cleavage. To provide a reasonable analysis of the critical points and the Laplacian of the electronic charge density in the TS, the atoms-in-molecule method was applied. Topological analysis showed the expected ring point for the best transition state corresponding to the cyclic six- membered structure. Comparison between the computational data and experiment showed a good agreement.