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• W1569563378 abstract "The design and construction of a low temperature stopped-flowapparatus is described. This equipment was developed to study reactionswhose rates are too fast for other rapid flow methods and are not suitablefor investigation by the rapid-reaction techniques which do not involvethe mixing of reagents. Spectrophotometric observation is used andreactions may be monitored in both the ultraviolet and visible regions.The equipment is constructed from chemically inert materials and reactionsmay be studied down to ca. 220K.The low temperature stopped-flow apparatus wan used to study theformations of the 2,2'-bipyridyl, 1,10-phenanthroline and 2,2',2-terpyridylcomplexes of the manganese(II) ion in anhydrous methanol. The reactionsbetween the manganese(II) ion and the first two ligands are too fast forflow methods at ambient temperatures but the kinetics and activationparameters were easily obtained at low temperatures.Whereas in aqueous solution the formation reactions of the divalentmetal ions have activation enthalpies very similar to that of waterexchange at the metal ion, the activation enthalpies of the reactionsstudied in this work are considerably greater than that of methanol exchangeat the manganese(II) ion. It is suggested that a steric effect causesthe high activation enthalpies. Possibly, either the first metal-ligandbond is hindered, or ring closure is rate determining. Evidence that the2,2'-bipyridyl complex is formed with a rate determining ring closure ispresented from consideration of the relative magnitude of the acid andmercuric ion induced dissociation rate constants.The kinetics of dissociation of the mono-(2,2'-bipyridyl)mangenese(II)and mono-(1,10-phenanthroline)manganese(II) ions in anhydrous methanol arealso reported. The rate constants are smaller than the correspondingreactions in water, as is expected from the relative labilising effects ofco-ordinated water and methanol molecules.In Chapter 3, the kinetics of formation of peroxynitrous acid (HOONO) and its decay to nitrate ion (reactions (1) and (2)) are described.HNO2 + H2O2 -H+> HOONO + H2O (1)HOONO -H+> NO3- + H+ (2)The reaction rates are measured at much higher acidities than previousstudies. The existence of an acid catalysed pathway for the isomerisationof the peroxynitrous acid, which previous workers had postulated, isconfirmed.The rate law for the formation process was found to be of the form(a, b and c are constants),d [HOONO]/dt = a[H+][HNO2][H2O2]/(b + c[H2O2])which is consistent with a mechanism in which NO+ is formed as anintermediate.The spectrum of peroxynitrous acid was recorded for the first timeusing a continuous-flow mixing cell.Under the conditions used, reactions (1) and (2) are an example of aseries first-order reaction. It we not possible to measure meaningfulrate constants for the formation process directly because of the overlapwith reaction (2). A least-squares method of analysing spectrophotometricdata for a series first-order reactionA -k1> (B) -k2> Cis described where the unknown parameters are k1, k2 and the extinctioncoefficient of the intermediate B. Care is needed, since two solutionsfit the data equally well. These solutions correspond to a fasterformation of a weakly absorbing intermediate, and a slower formation of amore strongly absorbing species. Methods of resolving the ambiguity are discussed." @default.
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• W1569563378 date "1972-01-01" @default.
• W1569563378 modified "2023-09-27" @default.
• W1569563378 title "Kinetic studies by the low temperature stopped-flow method" @default.
• W1569563378 hasPublicationYear "1972" @default.
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