Pulse-electrolysis Stopped-flow Method Research Articles

Kinetics and mechanisms of the reactions of the electrogenerated tetramethoxythianthrene cation radical and dication with methanol and pyridine derivatives in acetonitrile

In order to clarify the differences in reactivity between cation radicals and dications, the kinetics and mechanisms were analysed in detail for the reactions of the electrogenerated 2,3,7,8-tetramethoxythianthrene cation radical (TMT˙+) and of the dication (TMT2+) with methanol and pyridine derivatives by using a pulse-electrolysis stopped-flow method. From the rate law determined for each reaction, the reaction mechanisms were elucidated, though they are different depending on the couples in the reactions. The reactions of TMT˙+ proceeded by multi-step reactions including the electron transfer of another TMT˙+ to generate TMT before the rate-determining step (rds). In contrast, in the reactions of TMT2+, it was found that the rds was the first or the second step of the reaction; i.e. the unstable cationic intermediate seems to be formed directly, not by the electron transfer. In addition to the excessive charge present in the dicationic state, a mechanism in which TMT2+ is easily accessible to the rds also seems to explain the high reactivity of TMT2+. The differences in the reaction mechanism depending on the nucleophiles were also supported by observing the substituent effects of pyridine derivatives on the reactions.

Substituent Effect on Ion Pair Formation Reactions Between 1,4-Benzoquinone Derivative Anion Radicals and Metal Cations

Substituent effects on the ion pairing reactions between 1,4-benzoquinone derivatives anion radicals and metal cations were studied systematically in homogeneous acetonitrile solution using a pulse electrolysis stopped-flow method. Although a correlation with the formal potentials was found in the reactivity of the 2,3-dichloro-5,6-dicyano-, 2,3,5,6-tetrachloro- and 2,5-dichloro-derivatives anion radicals, the 1,4-benzoquinone anion radical (BQ·–) was more reactive than the 2,3,5,6-tetramethyl-1,4-benzoquinone anion radical (TMQ·–) in spite of the 0.41V negative formal potential of TMQ·–. Regarding this difference, the PM3 calculation indicated that the negative charge density of the O atoms of BQ·– is greater than that of TMQ·–.

Reaction kinetics of electrogenerated 9,10-bis(4-substituted-phenyl)anthracene cation radicals with methanol: A change in mechanism brought about by remote substituents

The reactions of 9,10-bis(4-nitrophenyl)- and 9,10-bis(4-methoxyphenyl)anthracene cation radicals (DNA˙+ and DAA˙+) with methanol (MeOH) in acetonitrile were analyzed using a pulse electrolysis stopped-flow method. The reaction of DNA˙+ was found to proceed via the same rate law as the 9,10-diphenylanthracene cation radical (DPA˙+), i.e. −d[DNA˙+]/dt = k[DNA˙+][MeOH]2, with a reaction rate 6 times faster. In contrast, the rate law governing the reaction of DAA˙+ with MeOH was different from that of DPA˙+, and was found to be −d[DAA˙+]/dt = k[DAA˙+]2[MeOH]. Reflecting this difference in the rate laws, an acceleration of the reaction was observed in the decay of DAA˙+ relative to DPA˙+, though methoxy groups usually stabilize aromatic cation radicals. This unusual remote substituent effect, which involves a change in the reaction mechanism, is discussed in terms of stabilization of intermediates in the proposed reaction mechanisms.

Complex aspects of reaction kinetics between the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone dianion and alkali metal cations in acetonitrile solution

The reactions between the electrogenerated 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) dianion (DDQ2−) and the alkali metal cations (M+: Li+, Na+, K+) in homogeneous acetonitrile solutions were studied using a pulse electrolysis stopped-flow method. Changes in the absorption spectra were observed after the ion pair formation reaction of DDQ2− with Na+ to form Na2DDQ. From the decay profile of free DDQ2−, it became clear that the reaction of DDQ2− with Na+ reached equilibrium through two steps, one fast (several tens of milliseconds) and one slow (several seconds). This complex behavior could be understood by the following two-step mechanism. In addition, a difference of the reactivity toward DDQ2− depending on the size of M+ was evident, though the reaction of DDQ2− and Li+ was too fast to be followed.

Kinetic studies on ion pair formation between 2,3,5,6-tetrachloro-1,4 benzoquinone anion radical and alkaline earth metal cations (RPS 6708)

Abstract Kinetics and mechanisms of the ion pair formation reactions between 2,3,5,6-tetrachloro-1,4-benzoquinone anion radical (TCQ − ) and alkaline earth metal cations (M 2+ ; Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ ) were studied in acetonitrile (AN) by using a pulse electrolysis stopped-flow method. It was clarified that the reactions proceed according to the following reaction mechanism with a rate law of −d[TCQ − ]/d t =2 K 1 k 2 [TCQ − ] 2 [M 2+ ] for all M 2+ . M 2+ + TCQ − ⇆ K 1 M 2+ ·TCQ − M 2+ ·TCQ − + TCQ − → rds k 2 M 2+ · ( TCQ − ) 2 The reaction rate constants (2 K 1 k 2 ) for Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ were determined to be 4.0 (±0.3)×10 9 , 2.0 (±0.3)×10 9 , 7.0 (±0.3)×10 8 , and 1.6 (±0.3)×10 8 M −2 s −1 , respectively. This result indicates that the kinetic interaction between TCQ − and M 2+ becomes stronger as the size of the metal cation becomes smaller. In addition, the solvent effects on the reaction kinetics were analyzed by adding a small amount of N , N -dimethylformamide (DMF) or dimethylsulfoxide (DMSO) to the AN. From the retarded decay curves, which reflected a strong solvation of M 2+ by DMF or DMSO, the equilibrium constants for the interaction between Mg 2+ and a solvent molecule could be estimated as 130 M −1 for DMF and 220 M −1 for DMSO. These results suggest that the reaction of TCQ − with Mg 2+ was hindered by the strong solvation of electron-donating DMF or DMSO, which is competing with the ion pair formation.

パルス電解ストップトフロー法による電解生成活性種の反応解析

By connecting a column electrolysis method with a stopped-flow technique, a pulse-electrolysis stopped-flow method has been developed. This method has many advantages that it can be applied to the detection and characterization of unstable intermediates as well as the analysis of the reaction kinetics of heterogeneous electrode reactions as homogeneous reactions. In addition, this method can be utilized combined with various spectroscopic methods including UV-visible, resonance Raman (RR) and electrogenerated chemiluminescence (ECL) measurements. In this review, the advantages of this method are demonstrated with many successful results in the analyses of the reaction kinetics and mechanisms involving electrogenerated species obtained by using the pulse-electrolysis stopped-flow method.

Detailed analysis of reaction kinetics and mechanisms of electrogenerated tetramethoxythianthrene cation radical and dication

To make clear the differences in kinetic behavior between cation radical and dication, detailed analyses were carried out on the reactions of electrogenerated 2,3,7,8-tetramethoxythianthrene cation radical (TMT +·) and dication (TMT 2+) with nucleophiles (methanol and pyridine) by using a pulse-electrolysis stopped-flow method. Compared with the reactions of TMT +·, which proceed via multi-step reactions including the electron transfer of another TMT +· to generate TMT before the rate determining step (rds), in the reactions of TMT 2+ it was found that the rds is the first or the second step of the reactions; i.e. the unstable cationic intermediate seems to be formed directly and not via electron transfer. Besides the additional charge in the dication state, the mechanisms in which TMT 2+ is easily accessible to the rds also suggest the high reactivity of TMT 2+ in comparison with TMT +·. The differences in the reaction mechanisms arising from the type of nucleophile were also discussed.

Solvent effect on the ion pair formation between 2,3,5,6-tetrachloro-1,4-benzoquinone anion radical and Mg 2+ measured using a pulse electrolysis stopped flow method

Solvent effect on the ion pair formation between 2,3,5,6-tetrachloro-1,4-benzoquinone anion radical and Mg 2+ measured using a pulse electrolysis stopped flow method

Direct observation of ion pair formation between 2,3-dichloro-5,6-dicyanobenzoquinone dianion and sodium ion

The dynamic process of ion pair formation was observed through the changes in absorption spectra measured on mixing solutions of 2,3-dichloro-5,6-dicyanobenzoquinone dianion and sodium ion using a pulse-electrolysis stopped-flow method.

Resonance Raman measurement of electrochemically generated short-lived intermediates by means of a pulse electrolysis stopped flow method

Resonance Raman (RR) measurement with a pulse electrolysis stopped flow method was developed for the purpose of the measurement of electrochemically generated short-lived intermediates. In this method the solution, electrolysed by imposing a current pulse on a column electrode, is delivered to an optical flow cell by means of a stopped flow technique and then the RR spectrum is observed at the optical cell. The RR spectrum of the triphenylamine cation radical (TPA .+), which dimerizes rapidly to produce the dimeric compound tetraphenylbenzidine (TPB), was successfully measured selectively without any interference by the oxidation products of TPB generated in subsequent electron transfer reactions. The RR spectrum of TPA .+ obtained had a different profile from that of the dimer cation radical (TPB .+) but was quite similar to that of the dimer dication (TPB 2+). This result shows the similarity between the vibrational structures of TPA .+ and TPB 2+, which can be explained by their canonical structures. By using the proposed method, it should become possible to observe the RR spectra of electrogenerated intermediates whose half-lives are several tens of milliseconds.

Pulse-electrolysis stopped-flow method for the electrospectroscopic analysis of short-lived intermediates generated in the electrooxidation of triphenylamine

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPulse-electrolysis stopped-flow method for the electrospectroscopic analysis of short-lived intermediates generated in the electrooxidation of triphenylamineMunetaka. Oyama, Koichi. Nozaki, and Satoshi. OkazakiCite this: Anal. Chem. 1991, 63, 14, 1387–1392Publication Date (Print):July 15, 1991Publication History Published online1 May 2002Published inissue 15 July 1991https://doi.org/10.1021/ac00014a010RIGHTS & PERMISSIONSArticle Views386Altmetric-Citations60LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Substituent effects on the reaction kinetics of electrogenerated 9-substituted 10-phenylanthracene cation radicals with water and methanol

Substituent effects on the reactions of electrogenerated anthracene derivative cation radicals with water and methanol were studied systematically by using a pulse-electrolysis stopped-flow method. Five 9-substituted 10-phenylanthracene cation radicals (XPA .+) were produced electrochemically, and their reaction kinetics were investigated. In all the reactions, the rate law was found to be first-order in XPA .+ and second-order in water and methanol. The reaction rate varied greatly depending on the nature of the substituents. While the electrochemical and spectroscopic properties of XPA .+ were correlated to the results of simple Hückel molecular orbital calculation, the order of the reaction rates was explained by the coulombic interaction induced by the substituents on the basis of the reaction mechanism and taking into account the particular conditions. The coulombic interaction caused by the substituents seems to be a significant factor governing the reactivities of aromatic cation radicals.

Measurement of time-resolved absorption spectra of species generated by fast electrochemical processes

For the acquisition of time-resolved absorption spectra in the millisecond range for reaction processes of electrogenerated species, a measurement method involving the use of a pulse-electrolysis stopped-flow method and a xenon flash lamp is described. The method has the advantages that it can be applied for analyses of very rapid reactions of short-lived electrogenerated species and that it has a longer light path and better reproducibility than other spectroelectrochemical techniques. Time-resolved absorption spectra for the reaction of the short-lived 9,10-dichloroanthracene cation radical with methanol, with a half-life of a few milliseconds, were successfully acquired. The results of analyses of the kinetics and mechanism of this reaction are presented.