Chemical kinetics and reaction mechanisms

The study of the reaction kinetics can yield tremendous insight into the mechanisms of a chemical reaction. APL stopped-flow and laser-flash photolysis instruments are excellent tools for measurements of reaction kinetics to provide insight into chemical reaction mechanisms.

Chemical kinetics involves the the study of rates of chemical processes. Measurements of the rates of reactions under different experimental conditions (for instance pH, solvent, concentration and temperature) allow the construction of mathematical models that satisfy and describe the characteristics of a chemical reaction. This model provides insights into the reaction mechanism and transition states involved in the chemical process.

Stopped-flow spectrophotometers like the SX20 allow the following of reaction kinetics, initiated by the mixing of two (or more) reactants, from the millisecond time range onwards. This allows a very diverse range of mechanism of many different types of fast chemical process to be studied. For a more in depth explanation of the stopped-flow method please read the tutorial.

Laser flash photolysis using the LKS.60 allows the reactions occurring in the nanosecond time range to be studied. Reactions are initiated by a very brief pulse of laser light. Then data is collected using a number of spectrometric techniques. This allows ultrafast reactions processes to be studied, provided that the process can be initiated with a flash of laser light. For a more in depth explanation of the laser flash photolysis method please read the tutorial.

Relevant Stopped-flow References

Below are listed 2 selected recent references of studies of chemical reaction mechanisms using our SX range of instruments are listed below. A complete searchable database with all references can be accessed by logging into the APL members area.

Authors	Title	Year	Keywords	Journal/Proceedings
Brendan Garrett and Richard A. Henderson	Direct studies on 5-coordinate intermediates formed during substitution at tetrahedral Fe sites: role of bound nucleophile in labilisation of leaving group [Abstract]   [URL]	2007	Fe–Cl bond dissociation, Inorganic kinetics, stopped-flow	J CHEM SOC DALTON TRANS, 2007, Iss 31, pp 3435-3439
Abstract: The substitution reactions of the tetrahedral Fe sites in [FeCl4]–, [Fe2S2Cl4]2–, [Fe4S4Cl4]2– and [{MoFe3S4Cl3}2(µ-SEt)3]3– with 4-RC6H4S– (R = MeO, Me, H, Cl or NO2) all involve rapid binding of the thiolate to a Fe site and formation of a kinetically and spectroscopically detectable intermediate. Kinetic studies allow calculation of the rate of Fe–Cl dissociation from the 5-coordinate site of the intermediate (k2R). The rate of Fe–Cl dissociation from the intermediate exhibits a marked dependence on the nature of the bound thiolate with log10(k2R) increasing in a linear manner with the calculated NBO charge on the sulfur atom of the coordinated thiolate. This behaviour indicates that Fe–Cl bond dissociation at the 5-coordinate intermediate involves a process in which Fe–thiolate bond shortening occurs prior to movement of the Fe–Cl bond.
Ria Yngarda, Seelawut Damrongsirib, Khemarath Osathaphanb and Virender K. Sharmaa, Corresponding Author Contact Information, E-mail The Corresponding Author	Ferrate(VI) oxidation of zinc–cyanide complex [Abstract]   [URL]	2007	Ferrate(VI), Cyanide, Zinc–cyanide complex, Kinetics, Oxidation, Rinse water	CHEMOSPHERE, 2007, Vol 69, Iss 5, pp 729-735
Abstract: Zinc–cyanide complexes are found in gold mining effluents and in metal finishing rinse water. The effect of Zn(II) on the oxidation of cyanide by ferrate(VI) (FeVIO42-, Fe(VI)) was thus investigated by studying the kinetics of the reaction of Fe(VI) with cyanide present in a potassium salt of a zinc cyanide complex (K2Zn(CN)4) and in a mixture of Zn(II) and cyanide solutions as a function of pH (9.0–11.0). The rate-law for the oxidation of Zn(CN)42- by Fe(VI) was found to be -d[Fe(VI)]/dt= k[Fe(VI)][Zn(CN)42-]0.5. The rate constant, k, decreased with an increase in pH. The effect of temperature (15–45 °C) on the oxidation was studied at pH 9.0, which gave an activation energy of 45.7 ± 1.5 kJ mol-1. The cyanide oxidation rate decreased in the presence of the Zn(II) ions. However, Zn(II) ions had no effect on the cyanide removal efficiency by Fe(VI) and the stoichiometry of Fe(VI) to cyanide was approximately 1:1; similar to the stoichiometry in absence of Zn(II) ions. The destruction of cyanide by Fe(VI) resulted in cyanate. The experiments on removal of cyanide from rinse water using Fe(VI) demonstrated complete conversion of cyanide to cyanate.

Relevant Laser Flash Photolysis References

Below are listed 5 selected recent reference of studies of chemical reaction mechanisms using our LKS range of instruments are listed below. A complete searchable database with all references can be accessed by logging into the APL members area.

Authors	Title	Year	Keywords	Journal/Proceedings
Ayman A. Abdel-Shafi, Jose L. Bourdelande and Sameh S. Ali	Photosensitized generation of singlet oxygen from rhenium(I) and iridium(III) complexes [Abstract]   [URL]	2007	singlet oxygen, rhenium(i), Iridium(III), laser flash photolysis	J CHEM SOC DALTON TRANS, 2007, Iss 24, pp 2510-2516
Abstract: Photophysical properties in dilute acetonitrile solution are reported for a number of iridium(III) and rhenium(I) complexes. The nature of the lowest excited state of the complexes under investigation is either metal-to-ligand charge transfer (3MLCT) or a ligand centred (3LC) state. Rate constants, kq, for quenching of the lowest excited states by molecular oxygen are in the range 1.5 × 108 to 1.4 × 1010 M–1 s–1. Efficiency of singlet oxygen production, fΔT, following oxygen quenching of the lowest excited states of these complexes, are in the range of 0.27–1.00. The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition between a non-charge transfer (nCT) and a CT deactivation channel. The balance between CT and nCT deactivation channels, which is described by the relative contribution pCT of CT induced deactivation, is discussed. The kinetic model is found to be successfully applied in the case of quenching of the excited triplet states of coordination compounds by oxygen in acetonitrile, as was proposed for the quenching of π–π* triplet states by oxygen.
C. Busset, P. Mazellier, M. Sarakha and J. De Laat	Photochemical generation of carbonate radicals and their reactivity with phenol [Abstract]   [URL]	2007	phenol, carbonate radical, laser flash excitation,	J PHOTOCHEM PHOTOBIOL A-CHEM, 2004, Vol 185, Iss 2-3, pp 127-132
Abstract: The reaction of carbonate radical with phenol in aqueous solution has been investigated in systems in which carbonate radicals were generated by UV irradiation of an aqueous solution of [Co(NH3)5CO3]+ (pH 8.0 phosphate buffer). Both steady state and time resolved photolysis experiments were performed. Upon continuous irradiation of complex phenol mixtures, phenol was converted into benzoquinone and dihydroxybenzenes. Benzoquinone was the major by-product in the early stages of the reaction. Laser flash excitation (266 and 355 nm) of the cobalt complex clearly showed the formation of the carbonate radical. When phenol was added to the solution of the complex, a second species was observed which was assigned to the phenoxyl radical. The second-order rate constant of reaction between phenol and carbonate radical was found to be equal to 1.6 × 107 M-1 s-1, in agreement with literature data of 2.2 × 107 M-1 s-1.
Pascal Wong-Wah-Chung, Salah Rafqah, Guillaume Voyard and Mohamed Sarakha	Photochemical behaviour of triclosan in aqueous solutions: Kinetic and analytical studies [Abstract]   [URL]	2007	Triclosan; Irgasan DP300; Photolysis; Dioxin; Photocyclization	J PHOTOCHEM PHOTOBIOL A-CHEM, 2007, Vol 191, Iss 2-3, pp 201-208
Abstract: The mechanism of the direct photolysis of the anti-microbial triclosan in aqueous solutions was investigated by using steady state and laser flash photolysis. Quantum yields were determined for the disappearance of triclosan and formation of chloride anions in steady state irradiations in the absence and in the presence of oxygen as well as a function of pH. The photoreactivity was found to be efficient with the anionic form and in the absence of oxygen. Following laser flash photolysis (226 nm), three transients were found (triclosan triplet state, solvated electron and phenoxyl radical). Several primary and secondary stable photoproducts were elucidated by means of LC/MS/MS data. They were found to arise from four main photochemical processes: isomerisation, cyclization (leading to the formation of dioxin derivatives), dimerisation of the phenolic moiety and hydrolysis. The ionic chromatography showed that the loss of chloride anion in triclosan phototransformation represents an important degradation pathway. The formation of oligomeric products was also observed for prolonged irradiation time. A detailed mechanism for the formation of the primary products is proposed and discussed. The very important photocyclization reaction is more likely involving the triplet state pathway and the homolytic dissociation of the ether bridge occurs from the singlet excited state pathway.
Malgorzata Insinska-Rak, Ewa Sikorska, Jose L. Bourdelande, Igor V. Khmelinskii, Wieslaw Prukala, Krzysztof Dobek, Jerzy Karolczak, f, Isabel F. Machado, Luis F.V. Ferreira, Ewa Dulewicz, Anna Komasa, David R. Worrall, Maciej Kubicki and Marek Sikors	New photochemically stable riboflavin analogue—3-Methyl-riboflavin tetraacetate [Abstract]   [URL]	2007	Flavin analogue; 3-Methyl-riboflavin tetraacetate; Riboflavin; TD-DFT method; Triplet states; Photosensitizer; Singlet oxygen; Photodegradation; Crystallographic characteristics; Time-resolved fluorescence spectra	J PHOTOCHEM PHOTOBIOL A-CHEM, 2007, Vol 186, Iss 1, pp 14-23
Abstract: Spectroscopic and photophysical properties of a flavin analogue – 3-methyl-riboflavin tetraacetate – were studied in methanol, ethanol, water and acetonitrile. We compared experimental spectroscopic data with the results of theoretical predictions, obtained using the TD-DFT method. Based on these calculations, we assigned (π,π*) symmetry to both the lowest excited singlet and triplet states. We found the title compound to be a very efficient photosensitizer of singlet oxygen production (φΔ = 0.61). The triplet state quantum yield of 3-methyl-riboflavin tetraacetate was determined as 0.54 in methanolic solutions. Photodegradation quantum yield measurements demonstrate that the title compound may be used as a much more stable substitute of riboflavin, being two orders of magnitude more photostable (φR = 2 × 10-5). We also present exhaustive crystallographic characteristics of 3-methyl-riboflavin tetraacetate, along with time-resolved fluorescence spectra of its polycrystals.
Anthony Harriman, Laura J. Mallon, Gilles Ulrich, Raymond Ziessel	Rapid Intersystem Crossing in Closely-Spaced but Orthogonal Molecular Dyads [Abstract]   [URL]	2007	charge transfer , chemical sensors, dyes/pigments, fluorescence, triplet state	CHEMPHYSCHEM, 2007, Vol 8, Iss 8, pp 1207-1214
Abstract: A borondipyrromethene (bodipy) dye is equipped with a 4-pyridine residue attached via the meso position. The strong fluorescence inherent to this class of dye is extinguished on protonation of the pyridine N atom. For the corresponding N-methylpyridinium derivative, fluorescence from the dye fragment is also extensively quenched due to the onset of a light-induced charge-shift reaction. The resultant charge-transfer state (CTS) is weakly fluorescent and decays primarily by way of population of the triplet excited state localized on the bodipy dye. Time-resolved spectral studies provide rate constants for all the steps involved in the forward and reverse charge-shift reactions. An interesting feature is that the lifetime of the CTS, around 1 ns, correlates with the viscosity of the solvent as might be expected if the rate-limiting step involves a substantial change in geometry. There is an unexpectedly small activation energy for the reverse charge-shift reaction, even allowing for the fact that this involves triplet formation. Local fluorescence is restored on cooling to 77 K.