Progress in Reaction Kinetics and Mechanism

The interaction of glycyl-L-leucine (L-I/H) with [(H₂O) (tap)2RuORu(tap)2(H₂O)]²⁺ (tap=2-(m-tolylazo) pyridine) has been studied spectrophotometrically in aqueous medium as a function of [(H₂O) (tap)₂RuORu(tap)2(H₂O)²⁺], [glycyl-L-leucine], pH and temperature. The reaction was monitored at 600 nm where the spectral difference between the reactant and product is a maximum. At pH 7.4, the interaction with glycyl-L-leucine shows two parallel steps, i.e. it shows a non-linear dependence on the concentration of glycyl-L-leucine; both processes are ligand dependent. The rate constants for the processes are: k_l 10⁻³ s ¹ and K₂∼10⁻⁵ s⁻¹. The activation parameters calculated from Eyring plots are: Δ H^#1 = 15.8∓0.6kJ mol⁻¹, ASf = −232 + 2J K ¹ mol⁻¹, AH₂^# =31.7∓3.5kJmol⁻¹, AS* = −216∓ 10J K⁻¹ mol⁻¹. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. from the temperature dependence of the outer sphere association equilibrium constant, the thermodynamic parameters were also calculated, which gave a negative AG° value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The product of the reaction has been characterized from IR and ESI-mass spectroscopic analysis.

The interaction of diethyldithiocarbamate (Et₂DTC) with diaquaethylenediamineplatinum(II) has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], temperature and pH at constant ionic strength. The reaction proceeds via rapid outer-sphere association complex formation followed by two consecutive steps: the first is ligand dependent and second is assigned to intramolecular ring closure. The low enthalpy and negative entropy of activation indicate an associative mode of activation for both steps.

Comparison is made of the kinetics of oxidations of phenol with H₂O₂ catalysed by two Schiff base manganese(III) complexes in micellar media using cationic surfactant cetyl trimethylammonium bromide (CTAB) and nonionic surfactant polyoxyethylene lauryl ether (Brij35). The characteristic spectra of the two Schiff base complexes in H₂O₂-buffered solution were recorded and analysed. Both Schiff base manganese(III) complexes exhibited remarkable catalytic activity for phenol oxidation by H₂O₂ under different conditions.

Ranitidine hydrochloride (RNH), chemically known as A-dimethyl-5-[2-(l- methylamino-2-nitro-vinyl)-ethylthiomethyl]furfuryl amine hydrochloride, is an antiulcer drug and its oxidation-kinetic study is of much significant in understanding its mechanistic chemistry in redox reactions. Consequently, the kinetics of oxidation of RNH by sodium N-chloro-p-toluenesulfonamide (chloramine-T or CAT) have been examined in HC10₄ and NaOH media at 298 K. The stoichiometry and oxidation products of RNH - CAT reaction are the same in both media, however, their kinetic patterns were found to be different. The reaction rate exhibits first-order kinetics with respect to [CAT]_O and [RNH]_O in both media. The rate of reaction increases with increase in [H⁺], but it decreases with respect to [OH ^-] with a slope of unity in both the cases. Decrease of the dielectric constant of the medium decreases the reaction rate in both media. Variation of the ionic strength of the medium, and addition of /7-tolucncsul foil amide or NaCl, have no significant effect on the rate. The reaction was studied at different temperatures and activation parameters have been deduced. Ranitidine-S'-oxide was identified as the oxidation product of RNH by GC-MS in both media. The conjugate free acid CH3C₆H₄S02NHC1 of CAT is postulated as the reactive oxidizing species in both cases. The observed results have been explained by plausible mechanisms and related rate laws have been deduced.

Kinetic studies for the non-isothermal decomposition of un-irradiated and y-irradiated zirconium acetylacetonate, Zr(acac)₄, with 10³ KGy total y-ray dose were carried out in static air. The results show that decomposition proceeds through multiple steps and ends with the formation of Zr O_z as a final solid residue without any significant changes in the thermal behaviour between un-irradiated and y-irradiated samples of Zr(acac)₄. The non-isothermal data for both un- irradiated and y-irradiated Zr(acac)₄ were analysed using both model-free and model-fitting approaches. Irregular variation of E_a with α indicates a complex decomposition mechanism involving multi-step processes including melting, sublimation and degradation and more than one model could be suggested to explain the results. Calcination of Zr(acac)₄ at 600° C for five hours led to the formation of Zr0₂ monodispersed nanoparticles, characterised by X-ray diffraction, FT-IR, SEM and TEM. This is the first example of preparing Zr0₂nanoparticles by solid state thermal decomposition of Zr(acac)₄.

The gas phase elimination kinetics of 2-chloroethyltrichlorosilane (1), 2-chloro- ethylethyldichlorosilane (2), 2-chloroethyldiethylchlorosilane (3), and 2-chloro- ethyltriethylsilane (4), have been studied at the ab initio B3LYP/6-311 + G**, B3PW91 /6-311 + G** and MPW1PW91/6-311 + G** levels of theory. The B3LYP/6-311 +G** method is in good agreement with the experimental data for the activation parameters. The calculated data demonstrate that, in the elimination mechanism of 2-chloroethylsilane and derivatives, the polarization of the C₁-C1₃ bond in the sense C₁ ^δ+ C1^-δ ₃ is a determining factor. Based on the optimized ground state geometries using the B3LYP/6-311 + G** method, the natural bond orbital analysis (NBO) of donor - acceptor (bonding-antibonding) interactions revealed that in accordance with the increase of activation energy, the HOMO-LUMO energy-gaps in the ground state structures increase. Moreover, the order of energy barriers obtained could be explained by the number of electron-releasing chlorine atoms substituted at the silicon atom. NBO analysis shows that the occupancies of σc1-c13 bonds decrease for compounds 1-4as 4<3<2<1, while those of ^σ*C1-C13 bonds increase in the opposite order (4>3>2>1). This trend explains the comparatively easier thermal decomposition of the <σ c1-c13 bond in compound 4 compared to compounds 1-3.

A theoretical study on the regioselectivity of 1,3-dipolar cycloaddition reactions between methyleneamine N-oxide (nitrone) and cyclopent-3-Ene-1,2-Dione (DPhl), pyrrole-2,3-dione (DPh2), furan-2,3-dione (DPh3) and thiophene-2,3- dione (DPh4) has been carried out by means of several theoretical approaches, namely, activation energies, Houk's rule based on FMO theory and DFT reactivity indices. The calculations were performed at the DFT-B3LYP/6-31G(d) level of theory using Gaussian 03. The present analysis shows that these reactions have an NED character. Moreover, the results obtained from energetic and electronic approaches, with the exception of Gazquez - Mendez rule based on NPA analysis, confirm that 1,5-regioisomer is the major product.