I= HETEROGENEOUS ELECTRON TRANSFER RATE CONSTANTS OF SOME SUBSTITUTED BIPYRIDINIUM HALIDES
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Title of Thesis
HETEROGENEOUS ELECTRON TRANSFER RATE CONSTANTS OF SOME SUBSTITUTED BIPYRIDINIUM HALIDES

Author(s)
Naheed Kaukab Bhatti
Institute/University/Department Details
Department of Chemistry/ Quaid-i-Azam University Islamabad
Session
2002
Subject
Physical Chemistry
Number of Pages
235
Keywords (Extracted from title, table of contents and abstract of thesis)
bipyridinium halides, electrode reaction, linear sweep, heterogeneous electron

Abstract
In this study heterogeneous electron transfer rate constants (kº) were determined both experimentally and theoretically. For experimental determination of rate constants cyclic voltammetric technique was used. In this respect peak separation methods were used including equations formulated by Gileadi, Nicholson and Kochi. Values of kºexp for reduction processes of dications and monocations of methyl viologen, diquat, benzyl viologen and n-butyl viologen were determined at hanging mercury drop electrode (HMD) and platinum disk electrode(pt.disk). The studies were carried out in three solvent mixtures including dimethyl formamide(DMF), dimethyl sulfoxide(DMSO) and formamide(FM) with water at different temperatures ranging 293K-313K.

It was observed from cyclic voltammograms (obtained experimentally) and from values of kºexp (in the range of 10-5 cm.s-l) that all systems behave reversibly. Their activation parameters such as change in free energy of activation(”G*exp), change in enthalpy of activation ”H*exp) and change in entropy of activation(”S*exp) have been evaluated. It was found that the values of change in free energy of activation obtained by employing different equations were comparable. It was also found that ”S*exp have small values for the reactions studied, so ”H*exp values can be considered as almost as ”G*exp Values of kºexp seemed to be the function of temperature and solvent polarity.

Higher values of rate constants were obtained with HMD as working due to its lower work function as compared to pt.disk electrode. In formamide reactants exhibited anomalous behaviour at pt.disk electrode. In case of smaller viologens second reductions were not observed at pt.disk electrode in FM mixture.

Theoretical rate constants were also evaluated for the same process under the experimental conditions using different equations such as Marcus equation with the inclusion of work term, its modified version obtained by replacing kel with barrier crossing activation energy parameter(K) and the equation which also take into account the effect of pre-exponential term, Vn, solvent dependent nuclear frequency factor.

Theoretical values of kº were obtained at different thickness of reaction site from the surface of the electrode. And an attempt has been made to locate the position of reaction plane from the electrode surface by comparing the theoretical values of kº with the experimental ones. The comparison between the experimental and theoretical results gave good agreement.

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10117.95 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
1200.44 KB
2 1 Introduction 1
603.83 KB
3 2 Mechanism Of Electrode Reaction 8
1614.19 KB
  2.1 Linear Sweep And Cyclic Voltammetry 8
  2.2 Single Electron Transfer In A Reversible Process 9
  2.3 Multi Electron Transfer 22
4 3 Electroanalytical Techniques For The Determination Of Heterogeneous Electron Transfer Rate Constant 27
623.99 KB
  3.1 Cyclic Voltammetric Method 27
  3.2 Digital Simulation For Evaluation Of K º 32
  3.3 Other Methods 34
5 4 Theories Of Electron Transfer 35
1971.71 KB
  4.1 Potential Energy Surfaces 35
  4.2 Marcus Theory 38
  4.3 Theories Of Hush, Dogonadze-Levich And Others 53
  4.4 Effect Of Solvent And Temperature On Rate Of Electron Transfer 54
6 5 Experimental 58
2822.29 KB
  5.1 Materials And Chemicals 58
  5.2 Measuring Assembly 60
  5.3 Purification Of N 2 Gas 63
  5.4 Measurements Of Peak Separation And Current 63
7 6 Results 67
953.63 KB
  6.1 Experimental Results 67
  6.2 Theoretical Results 69
  6.3 Comparisons Of Values Of Experimental And Theoretical Rate Constants 70
8 7 Discussion 216
613.07 KB
  7.1 Effect Of Method Selected 216
  7.2 Effect Of Solvent 217
  7.3 Effect Of Working Electrode 220
  7.4 Anomalous Behaviour Of Monocations In Fm Mixture At Pt. Disk Electrode 220
  7.5 Effect Of Temperature 221
  7.6 Effect Of Side Chain 222
  7.7 Comparison Of K º 1 And K º 1 223
  7.8 Activation Parameters 223
  7.9 Theoretical Results 225
  7.10 Concluding Remarks 228
  7.11 References 229