Benton, David James (1972) Kinetic studies by the low temperature stopped-flow method. PhD thesis, University of Warwick.

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Abstract

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.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Flow injection analysis -- Methodology, Flow injection analysis -- Equipment and supplies
Official Date:	January 1972
Dates:	Date Event January 1972 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Moore, Peter
Sponsors:	Science Research Council (Great Britain)
Extent:	90 p.
Language:	eng
URI:	http://wrap.warwick.ac.uk/id/eprint/40005

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