I DISSOCIATION OF CYANOGEN HALIDES IN SHOCK WAVES A Thesis presented by PETER JOHN KAYES in partial fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry Cellge of Science .:.rd Techn.ology j,..21y 1972 2 ABSTRACT In this thesis, an investigation of the decomposition of cyanogen bromide and cyanogen chloride is reported. The light emitted from the CN radical behind incident shocks is used to monitor the kinetics and mathematical modelling techniques are used to simulate the observed intensity profiles, so that the reaction mechanism can be elucidated and rate constants for the important elementary steps determined. In Chapter I, the background theory to reaction rates and shock tube operation is described. The experimental methods employed throughout this work are presented in Chapter II. 4n introduction to the theories of luminous systems and the use of intensity measurements in kinetic studies is given in Chapter III. The absolute calibration, using the light emitted from shock heated sulphur dioxide as an intermediate. standard, is also reported in this chapter.- The numerical methods, used in the simulation of the experimental intensity profiles, are discussed in Chapter IV. In Chapter V, a review of the kinetics and emission from CN containing systems is presented. The experimental results for the decomposition of cyanogen bromide and cyanogen chloride are reported in terms of an apparent rate of reaction in Chanters VI and M. The simulation of the apnarent rate and the detailed intensity profiles, using the mathematical modelling techniques described in Chanter IV, are also discussed. In all four separate mechanisms for the decompositions are described: classical chain kinetics and more unconventional energy chain mechanisms involving vibrationally excited species are discussed. In Chapter VIII a separate study of the ecuilibrium emission from shock heated sulphur dioxide/argon mixtures (some dosed with oxygen) 3 is presented and discussed in terms of the 0 + SO chemiluminescence. 4 ACKNOWLEDGEMMTS It is my pleasure to thank Dr. Bryan P. Levitt for his guidance, supervision and unfailing encouragement during the course of this work. I am indebted to Professor R.M. Barrer, F.R.S. for providing the research facilities in the Department of Chemistry at Imperial College. I thank the Science Research Council for my Research Studentship. I particularly thank Mr. J. Bilton for conducting the mass spectroscopic analysis of the gas samples; Mr. H. Cobley (and the Workshop staff), Mr. A. Madell (and the staff of the Glass-blowing section) and Mr. D. Alger (and the staff of the Instrument section) for their help in maintaining and improving the shock tube and associated equipment. I thank the University of London Computing Centre for the use of the CDC 6600 computer and Imperial College Computing Centre for the use of their IBM 7090/4 and CDC 6400 computers. I also wish to thank the staff of the Program Advisory Service at Imperial College for their help and suggestions in developing the program PROFIL. Finally, it is my privilege to thank my parents for their continued interest and support in the completion of this work. 5 PREFACE The contents of this thesis are divided into three parts. In part I, the general theory underlying the determination of reaction kinetics and the application of shock tube methods are outlined. The experimental methods, which are common to the kinetic investigations reported later in the thesis, are also given. In Part II, the experimental results and discussion of the kinetics of the thermal decomposition of the cyanogen halides, BrCN and C1CN , are presented. A separate kinetic investigation of the equilibrium emission obtained behind incident shocks into sulphur dioxide/argon mixtures, is reported in Part III. It is worthwhile commenting that the SO 2 work was conducted first and arose out of the absolute calibration procedure reported in Part I. The study of the pyrolysis of cyanogen bromide followed this and represents the bulk of the work undertaken. The final brief investigation of the pyrolysis of cyanogen chloride is closely related to the cyanogen bromide work, both in experimental technique and analysis. 6 CONTENTS PART I THE INTRODUCTION ArD THE DESCRIPTION OF THE SHOCK TUBE TECHNIQUES PAGE CHAPTER I GAS PHASE KINETICS 1:1 Introduction 14 1:2 The Reaction Rate Constant 14 1:3 Theoretical Considerations 14 1:3:1 Dissociation and detailed rate theory 16 1:3:2 Concluding remarks 22 1:4 The Shock Tube Applied to the Investigation of Reaction Kinetics 23 1:5 Shock Wave Formation and Propagation 24 1:6 Shock Tube Coordinates and Nomenclature 15 1:7 Time Scales 26 1:8 Processes Occurring Behind Incident Shock Waves 28 1:9 Boundary Layer Formation 29 CHAPTER II EXPERIMENTAL =HODS 2:1 Introduction 34 2:2 The Shock Tube 34 2:2:1 The driver section 34 2:2:2 The test section 36 2:2:3 The dump tank 37 2:2:4 The windows 38 7 PAGE 2:3 The Glass Vacuum Line 38 2:3:1 The gas handling limb 40 2:3:2 The pressure measuring limb 40 2:4. The Pumping System 41 2:5 Leak Detection 44 2:6 Diaphragms 45 2:7 Gas Preparation 46 2:8 Gas Purification 47 2:9 The Instrumentation 48 2:9:1 The optical system 50 2:9:2 The monochromators 50 2:9:3 The electronics 52 2:9:4. The timer photomultipliers 52 2:9:5 The pulse shaper 53 2:9:6 Signal matching 53 2:9:7 The quantitative photomultipliers 54 2:9:8 The oscilloscopes 55 2:9:9 The overall rise—time 55 CHAPTER III EMISSION FROM SHOCK HEATED GASES 3:1 Introduction 56 3:2 Thermal ?mission 57 3:2:1 Quenching probabilities 58 3:3 Chemiluminescence 62 3:4. The Absolute Calibration Using the Thermal Emission from Shock 64 Heated Sulphur Dioxide as an Intermediate Standard 3:4:1 Procedure 67 8 PART II - THE THERMAL DECOMPOSITION OF THE CYANOGEN HALIDES: BrCN AND C1CN PAGE CHAPTF12 IV COMPUTATIONAL TECHNIQUES 4:1 Introduction 72 4:1:1 Machine methods 74 4:2 The Program (FROFIL) Description 76 4:2:1 Input 77 4:2:2 Preliminary calculations 78 a) Shock front b) Equilibrium 9) The rate constants d) Experimental data 202:3 Integration 81 4:2:4 Integration control 83 4:2:5 Diagnostic information 84 4:3 Flow Corrections 86 4:4 The Analysis 87 4:4:1 The preliminary analysis - The apparent rate 87 constant (Ke) )+:4:2 Detailed fitting - The simulated profiles 91 CHAPTER V CN Ca1TAINT7G SYSTEMS 5:1 The Kinetics of CKX Systems 94 9 PAGE 5:1:1 Previous investigations 94 5:1:2 Theoretical calculations 100 5:1:3 The thermochemistry of the CN radical 102 5:2 Emission from Systems Containing Carbon and Nitrogen 107 5:2:1 Previous investigations 107 5:2:2 Equilibrium populations of the electronic states of CN 110 5:2:3 The AV = Al band of the CN violet (B2E 7(22:) 112 emission 5:2:k Impurities 112 5:3 The Emissivity of the CN Molecule 114 5:3:1 Experimental 116 5:3:2 Results 116 CHAPTER VI THE PYROLYSIS OF CYANOGEN BROMIDE 6:1 Introduction 120 6:2 Results 120 6:2:1 The emission profiles at A = 11 21.5nm 120 a) Lean mixtures b) Rich mixtures 6:2:2 The rate of rise of emission at 421.5 nm 125 a) Lean mixtures b) Rich mixtures c) Intermediate mixtures d.) Time resolution e) Optical thickness 6:2:3 The alnarent seconi order rate comtant (Ka) 137 6:2:4 Errors 142 6:3 Schere - A 'Simple' Dissociation lechanism 143 10 PAGE 6:3:1 Discussion — The computer predictions for scheme I 143 a) Rich mixtures b) Lean mixtures 6:4 Scheme II — A Simple Chain Mechanism 147 a) Initial estimates for the rate constants 6:4:1 Discussion — The computer simulations for scheme II 156 a) Rich mixtures b) Lean mixtures 6:1+:2 Conclusions from scheme II simulations — Comparison of 170 computer and experimental profiles 6:4:3 Possible modifications to scheme II 178 a) Possible effects due to contamination 6:5 Scheme III — An Energy Chain Branching Mechanism 180 a) General considerations 6:5:1 A + BC ---*— ABv° + C reactions 182 a) Energy chains — Previous investigations b) Reactions involving vibrationally excited cyanogen c) The degree cf vibrational excitation (a 0) for the vo reaction: BrCN + CN --4— C N + Br 2 2 6:5:2 The thermodynamics of C2'N 2 v, 193 6:5:3 Estimates for the rate constants 196 6:5:4. Discussion — The computer simulations for Scheme III 198 a) Lean mixtures b) Rich mixtures 6:5:5 Conclusions from scheme III simulations 206 a) Lean mixtures — 3000 to 4300K b) Lean mixtures — less than 3000K c) Rich mixtures /, a) The significance of the best fit values of ' '12 and 1;13 11 PAGE 6:6 The Comparison of Results from Schemes II and III 218 6:6:1 The comparison with the predictions of Keck and Kalelkar 222 6:6:2 Uncertainties in the rate constants as determined from 223 the computer modelling 6:7 Scheme IV - The Solution by Iteration of the Average Energy of 223 Excitation (Ev) at the Moment of Dissociation a) Assumptions b) The rate constants c) The computation 6:7:1 Discussion - The computer predictions of from scheme IV 226 6:8 Concluding Remarks 230 6:9 Recommendations for Future Work 233. CHAPTER VII THE PYROLYSIS OF CYANMEN CHLORIDE 7:1 IntrodUction 232 7:2 Results 232 7:2:1 The emission profiles at X = 421 5 nm 232 7:2:2 The apparent rate constant (Ka) 232 7:3 Discussion and Comparison with the Computer Predictions 236 7:4 Conclusion 244 7:4:1 An energy chain mechanism 244 7:4.:2 Lean mixtures: 'Shuffle' mechanism 246 7:4:3 Rich mixtures and Lean mixtures at high temperatures 247 7:5 Recommendations for Future Work 248 PART.