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Title: Kinetic studies by the low temperature stopped-flow method
Author: Benton, David James
ISNI:       0000 0001 3460 7191
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1972
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The design and construction of a low temperature stopped-flow apparatus is described. This equipment was developed to study reactions whose rates are too fast for other rapid flow methods and are not suitable for investigation by the rapid-reaction techniques which do not involve the mixing of reagents. Spectrophotometric observation is used and reactions may be monitored in both the ultraviolet and visible regions. The equipment is constructed from chemically inert materials and reactions may be studied down to ca. 220K. The low temperature stopped-flow apparatus wan used to study the formations of the 2,2'-bipyridyl, 1,10-phenanthroline and 2,2',2"-terpyridyl complexes of the manganese(II) ion in anhydrous methanol. The reactions between the manganese(II) ion and the first two ligands are too fast for flow methods at ambient temperatures but the kinetics and activation parameters were easily obtained at low temperatures. Whereas in aqueous solution the formation reactions of the divalent metal ions have activation enthalpies very similar to that of water exchange at the metal ion, the activation enthalpies of the reactions studied in this work are considerably greater than that of methanol exchange at the manganese(II) ion. It is suggested that a steric effect causes the high activation enthalpies. Possibly, either the first metal-ligand bond is hindered, or ring closure is rate determining. Evidence that the 2,2'-bipyridyl complex is formed with a rate determining ring closure is presented from consideration of the relative magnitude of the acid and mercuric 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 are also reported. The rate constants are smaller than the corresponding reactions in water, as is expected from the relative labilising effects of co-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 previous studies. The existence of an acid catalysed pathway for the isomerisation of the peroxynitrous acid, which previous workers had postulated, is confirmed. 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 an intermediate. The spectrum of peroxynitrous acid was recorded for the first time using a continuous-flow mixing cell. Under the conditions used, reactions (1) and (2) are an example of a series first-order reaction. It we not possible to measure meaningful rate constants for the formation process directly because of the overlap with reaction (2). A least-squares method of analysing spectrophotometric data for a series first-order reaction A -k1> (B) -k2> C is described where the unknown parameters are k1, k2 and the extinction coefficient of the intermediate B. Care is needed, since two solutions fit the data equally well. These solutions correspond to a faster formation of a weakly absorbing intermediate, and a slower formation of a more strongly absorbing species. Methods of resolving the ambiguity are discussed.
Supervisor: Not available Sponsor: Science Research Council (Great Britain)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry