Mass spectrometric studies of gas phase reactions of atoms with small molecules

Chr·istchurch, New Zealand

by

M., R .. Dunn

In Memoriam

This thesis describes work which was carried out in the Department o:f Chemistry in the University of Canterbury between February 1 968 and Februai'Y 1 97·1 •

The worl\: is entirely original except where otherwise stated in the text. It was conducted under the supePvision of Professor- L.F.Philli:ps and I should like to express my most sincere thanks to him for his advice and encouragement. I am also grateful to Dr M.J.McEwan fop acting in a similar capacity duping Pr>ofessor Phillips' absences overseas, and to Dr C.G.Freeman for many helpful discussions.

My thanks are particularly due to my parents and sister for their wholehearted support of m,y eff'orts over the past six years, and to Mi.ss Eileen Cruickshank, who typed the manuscript f'or this thesis, and assisted in its pPeparation in many other 'Nays.

I am indebted to the Department and to the University for> the provision of' a Teaching

Fellow-ship during the greater part of' my puPsuance of' this course of study.

ii

PUBLICNriOll]

The ~ollowi11g papers relating to the research described in this thests have been accepted ~or publication:

1 ., "Mass Spectrometric Study of the Reaction

o~ Hydrogen Atoms with Nitrosyl Chlor>ide", M.R.Dunn, M.r .. r.sutton, C.G.Fr>eeman,

M.J.McEwan and L.F.Phillips,

Joux•nal of Physical Chemistry, ]2, 722 ( 1 971) ..

2. 11_{M ass Spectr'ometr•ic Study o~ the Reaction}

of Nitrogen Atoms with Nitrosyl Chloride11 ,

M.R.Dunnt C.G.Freeman, M.J.McEwan and L .. F.Phillips,

Journal of Physical Chemistry, in press.

3.

"Mass S:pectrometric study o~ the Reaction of Oxygen Atoms with Nitrosyl Chloride", M.R.Dunn, C.G.Freeman, M.J.McEwan and L.F.Philli:ps,

In acldi tion, the :following paper has been submi ttecl f'or publication;.

4.

11_{Photomett•ic and Mass 8pectromet1•ic Observations}

on the Reaction of' Hydrogen Atoms with Cyanogen", M.R.Dunn, C.G.Freeman, M.J.McEwan and

L.F.Phillips, submitted to

TITLE

DEDICATION PREFACE

PUBJ.J ICAT IONS CONTENTS

IT.~LUSTRATIONS

ABSTRACT

CHAPTER 1

CONTENTS _ , . , . ? 1 ;

-1 .. -1 HISTORY OF ATOMS l\liD THEIR REACTIONS IN THE GAS PHASE

1 .2 EXPERIMENTAIJ 11ETHODS IN ATOM REACTION

STUDIES

. 1 .21 Procedures for the Production and

Reaction of Atoms

1 .22 Discharge-Flow Systems

iv

( i)

(ii)

( iv) (viii)

( ix)

1 •

1 •

1 .23 Determination of Atom Concentrations

1 .3

in Flow Systems

1 .24 Use of Mass Spectrometers in

Studying Atom Reactions

IJ\1'J.'RODUCTION TO THE PRESENT WORK

10.

1 6.

1 8.

1 .31 Aim and Scope of This Project 18 ..

1

.32

Previous Studies Related to this

CHAPrER 2

APPARATUS

2.11 The Reaction System

2.1 2 The Mass Spectrometer•

24.

24.

"

28 ..

2.,2 MATERIALS 33.

2.3

CALIBRATION AND MEASUREMENT PROCEDURES

37.

2.31

Relative Sensitivity

Determinations

_{37 ..}

2.

Procedure f'or l~'low Experiments

40.

2 ..

33

Titrations of Atom Concentrations

43 ..

2.34

Primary Rate Constant Measurements

45.

2.35

Reactant Stoichiometry

Determinations

2.36

Photometric Determination of Stoichiometry

2.37

Product Analyses CHAP.PER 3

3.1

TH:m MASS SPECTRUM

3.2

REACTION PRODUCTS

STOICHIOMETRY THE REAO'J:IION

3 .,4 'l1HE RATE CONSTANT FOR rl'HE PRIMARY

REACTION

3&5

DISrnJSSION

4-9.

51 •

53.

58.

CrTAPTER

4

W

HE!J&.!lQlL.QJl:_QXYG~J.'f ATOMS WITH Bl11lQ.SY.k...Q_~

vi

75.

4.1

GENERAL FEATURES

75.

4.2

REACTION Pl10DUCTS AND IJ\lTERMEDIATl!;S

75.

L~.3 REACTION S~POICHIOMETRY

77.

4.4

THE RA'l'E CONS'l'ANT POH THE PRIMARY REACTION

4.5

DISCUSSION

CHAPTER 5 .T,HE REACTIOJi 01" NITROGEN ATOMS WI'rH NITROSYL CHLORIJ:?E

5.1 GENERAL FEATURES 89.

5. 2 REAC'l' I ON PRODUCTS Ai'I,T]) INTERMEDIATES 90.

5.3

STOICHIOME'rRY OF THE REACTION

93.

5.4

THE RATE OF THE PRIMARY RBJACTION 98.

5.5

DISCUSSION

103.

CHAPTJ.i~R 6 THE REACTION OF HYDROGEN ATOMS YVITH

CYANOGEJ1 111 •

·"

..

6.1 INTRODUCTION 111 •

6.2 Ml~SUREMENr S 11 2 ..

6.21 Determination of the Primary

Process 11 2.

6.22 The Rate of the Primary Reaction 11 3.

CHAPTER 7 SUMMARY @P OONOLUSIOli§

7 .. 1 SUMMARY OF RESULTS

7.11

Rate Constants

7.12 Reaction Stoichiometry Measurements

7 . .,13 Reaction Products

7.14

Other Features

7 .,2 GE!NERAL OBSERVATIONS ON THE REACTIONS

1 21 "

1 21 .. 1 21 ..

122.,

1 2!j..,

125 ..

OF NITROSYL OHI,ORIDE 1 26.

7.3 SUGGESTIONS FOR FURTHER WORK 1 30.

APPENDIX I ~HEI\M9QIDllHICATJ J2:8T.~ 1 33.

APPENDIX I I 1 3L~ •

A2 .. 1 Ji1_{low city in the Reaction Tube,}

and the Reaction Time ..

A2.2 Partial Pressu.res of' Reagents. A2.3 Rate Constants.

. APPENDIX III .QOMPUTJ!!R pROGRAMS

· REF,ERENOES

134 .. 136. 137.

143 •

· 2.,1 Schematic D:l.agr·am of' the Reaction

System

The Mass Spectrometer Ion

2.3 Circ!ui t :f'or F1_{ilament Power Supply}

Viii

Following

Page

28.

a""lcl Electl"on J:l~mi ssion Regnlation 32.

Compttter Pl"ogl"~Sm DIFCORAT

and sample output 146.

Computer Program SIMUL8

A mass spectrometer was used in conjunction

with a discharge--flow system to study the reactions

of hydrogen, oxygen and nitrogen atoms with nitrosyl

chloride, and of hydroge~ atoms with cyanogen.

For each of the nitrosyl chloride reaction

systems, a rate constant was obtained for the

primary reaction, the pr•oducts and intermediates

were·identified, and the overall stoichiometry

was determined. From this information a detailed

mechanism for the reaction was deduced.

In the cyanogen study, the order and rate

constant for· the primaPY reaction were determined.

The measu1")ed rate was used to estj.mate the heat of

1 •

REVIEW AND INTRODUCTION

P' tnt:"

me-m-oo-..s·~~---1.1 HISTORY OF ATOMS AND THEIR REACTIONS IN THE GAS PHASE

Wi.th the discovery by Lewis 1 in ·1900 of' a golden-yellow a.fter-glow which persisted after the passage of an electrical discharge through nitrogen at low

pressure, a new branch of gas kinetics chanced to be f'ounded. From his examinations2 of the properties and react3.vity of this glowing , which he nam(:'ld "act nitrogen", StruttJCe later showed that its behaviour was in accordance with its containing free

nitrogen atoms. Hydrogen atoms were first produced

in 1912 by Langmuir3 , who used a tungsten f':l.lament to pyrolyze molecular hydrogen. A decade later Woqd4 and Bonhoeffer5 .developed the electrical discharge technique for production of atoms by dissociation of molecular hydrogen. This discharge method was also applied by Wood6 to the generation of oxygen atoms. There were a number of qualitative studies during the next fifteen years of the reactions of atoms produced by this means7-12 , but the elucidation of many of the

kinetic and mechanistic details of such reactions proved beyond the available experimental resources. Little further development of technique he.d occut•red by the middle of the century, when Steacie's review1

3

of atom and free radical reactions was published. In the 1950's the prospect and advent o~ extra-terrestrial exploration engendered a renewal

ot

interest in atom-molecule reactions, which were known to play a key role in the upper atmosphere. Under-standing the behaviour of the latter entailed, inter alia, a systematic study of the photochemistry and kinetics of inter-reaction ot: species known to be present there, including hydrogen, nitrogen and

oxygen atoms, hydroxyl radicals, ozone and oxides of nitrogen, as well as molecular oxygen and nitroge.n. Similarily, the development ot: new propellants, often

involving unusual materials such as hydrazine, required the characterization, in isolation, of reactions

involved in combustion processes. During normal combustion t.he interrelations among chemical, hydro-dynamic and heat and mass transfer phenomena :frequently preclude :full appreciation of the events occurring.

3.

ldnetics, which could conveniently be tested in their applj.cation to these relatively simple reactions.

As new end diverse experimental methods were developed, studies atom-molecule reactions became increasingly more prevalent throughout the f'j.f'ties and into the sixt Latterly, with further encouragement from ita relevance to air pollution, the field has become one of' prolific endeavour, so that even reviewers are hard pressed to cope with its manifold expansion.

ance of' comprehensive reviews testifies to an unabat-ing interest in atom and radical reactions among gas kineticists, despite the vigorous growth of such

shoots as ion-molecule reactions .. It is thi'OUgh a

surv~y ·of such reviews that the maturing of the subject in recent years can be gauged.

'l1_{he Chemical Society's "Annual Reports" hs.s had}

its section on kinetics developed from Sugden's review

(1959)

1

4,

largely descriptive of experimental methods, into a series of compendia of rate data

with textual commentary on more significant experimental results and technique

(1967,

68, 69) 16 , 1

7,

18 • An

measure-ment o£ individual processes and an awareness of the importance of energy distribution in Pl"Oducts and reactants is evident. Within a similar series in Annual Reviews or Physical Chemistry the

prolifer-ation of quantitative in:forma.tion led from Boudart' s

(1962) 19

consideration of a number of peripheral topics in reaction dynamics to the narrower and more tightly disciplined reviews by Benson and De More

( 1965) 20

and Mahan (

1966)

21 ; however 11 Kaufman's

22

.

. .

(1969)

zeal in this reapect•allowed Spicer and Rabinovitch

(1970)

2

3

to return to more expansive discussion within selected topics.

The series 11_{Progress in Reaction Kinetics",}

edited by G.Porter, contains detailed reviews on

th _e d ti d ti .p 24 h 1 . . 25 .

Pl'O U<'} _on an reac ons o.~.. oxyge11 , a ogen ;

.

26

27

hydrogen and nitrogen atoms, with tabulations of rate data. An extensive monograph on active nitrogen has appeared28 , while a series

of

critical reviews on groups of releted Peaations, under the title uHigh

Temperature Reaction Rate Data", is providing long awaited discerr~ent among conflicting kinetic

deter-;>"q

minations. Finally, there exists an annotated bibliograph?0 covePing the period

1900-1966,

which provides convenient access to the published

5.

1 • 2 EXPElUMENTAL METHODS IN ATOM 11EACTION STUDIES 1 .21

The energy required to produce atoms t'rom stable molecules may be supplied in various ways, e.g. by heat from a filament, light from a photolysis lamp~

or by collision with fast moving electrons

in

a.n electrical discharge. While the ideal of producing a single reactive species in et completely known state

is rarely achieved by any of' the methods currently used, the generally higher rea~tivity of atoms and the specificity of' methods for their detection has permitted the thering of' accurate kinetic data for their reC~ctions.

The systems used to study atom reactions may conveniently be classified according to the means of production of atoms. viz.: Thermal, photolysis, combustion, beam and discharge methods, though some overlap does occur among these

ca

tegol"ies.

(a) Thermal methods.

(ca. 2000 K) requirea31• Recently, the pyrolysis of nitrous oxide in passage over a 11_Nernst11 _{glower ha.s}

been used to pl"od.uce oxygen atoms .f'ree from other reactive species32•

(ii) Shock tubes: The passage of' a shock wave through a gas causes a rapid and uniform temperature rise, often ot: several thousand degrees. The

dissociation of' a molecular and subsequent re-combination or other reaction of the atoms and radicels thereby produced ma.y be observed using absorption spectroscopy or time-o.f'-flight mass

spectrometry. The subject is regularly reviewed33-36 • (b) Photochem:l.cal methods.

(i) Flash photolysis: Energy may be imparted to components of' a gaseous mixture by its subjection to a brief, high. intensity f'lash37; this may result in homogenous heating as in a shoclc tube, or an inert gas may be admixed to maintain isothermal conditions. The technique of kinetic absorption spectroscopy, as

developed by Porter, Norrish and associates38, in which spectra or transient species are recorded at

Vt:lrying times af'ter the initiating flash, hCls yielded

a tremendous wealth of' kinetic deta1139,40.

in the ultra-violet region of the spectrum can be Used to selectively dissociate species present, either directly41 or thr•ough excitation of an inter-mediate sensitizer42, to yield atoms in their ground state. Electronically excited atoms ma.y be produced directly by using radiation of sufficiently high

energy, or by resonance excitation of' ground state atomii3

,44.

(c) Combustion methods.

(i) Explosion limits: Explosive mixtures of gases will not detonate beyond certain regions of composition, temperature and p1•essure, the limits of which are controlled by rates of chain branching propagation reactions. Foreign gases introduced into the mixture may react competitively with th~

active species involved, and from the consequent variation in explosion limits the rates of such reactions may be deduced45,4

6. ·

(ii) Flame studies: In the laminar, post combustive region of suitable stabilized premixed fuel-oxidant flames, the concentrations of atoms

and radicals normally present or produced from

T.he flame is used as a high temperature flow system ( typica.lly 1500-2000 K) f'or the obtaining of kinetic and thermodynamic information on the species under study4G,47. Diffusion flames, in which the two

reactants are initially separate and the ct:>mbustion zone is the primary region of interest, are also used. in elementary reaction studies.

(d) Collisional methods.

Crossed molecular beems, both thermal and ..

velocity selected, have been used to yield inform-ation on reactive and non-reactive ("elastic") scatteri-ng the species involved. Detection may be by some active surface, movable throughout the range of scattering angles to be studied, or

. •' . .·

by mass spectrometer. Though in principle capable of' giving ini'orina.t ion on the behaviour of reactants in selected energy states, the technique is beset with experimental and interpretative difficulties. Xt has been used chiefly to study alkali metals and their halidesLt.B because of' the ease with which beams of these materials can be detected.

(e) Discharge methods.

9.

low pressure1' 2, and the application

o~

this method by subsequent workers4-12 to the production o:f atoms

in a flowing gas stream :formed the bas .of' what is currently the most extensively used procedure for the study o:f atom-molecule reactions~ discussed in the next sub-section.

1.22

The :formation of a reactive species during the passage of' a gas at low pressure through an electrical discharge is the essence of operation of a discharge f'low system; such a system can be used to study the reactions o:f excited molecules and of' ions besides those of' atoms. Early workers4-i 2 used a discharge between metal electrodes in the f'low o~ gas, but modern systems generally employ electrodeless micro-wave or radiof'requency discharges49, which avoid contamination o:f the reactants and give stable operation throughout the commonly used pressure range of 0.1 to 10 Torr. Atom precursors are

normally diatomic gases, and yields of' atoms may usually be enhanced by the addition of' a rare gas ca.1 .. rier in considerable excess.

con-centrstions, and of means f'or following the consumption

of

reactants and appearance of' products within flow systems, that accounts f'or their preponderance among

techniques for studying atom-molecule re<:lctions.

Atoms may be produced within them by methods other

than those involving discharges, e.g. by pyrglysis3 ,31 or photolysis, and the analytical procedures as

out-lined next may be used in conjunction with any of those

methods.

1.23 ~~u.of A1o~once~i£~~n ~low S~st~Q

The generation of f'ree atoms is often accompanied

by the formation of other active species, and this

introduces a problem of specificity into detection

methods. Moreover, while many properties in which

atoms differ from other species (e.g. spectra, mass,

chemical reactivity) can be used as a basis for

detection, some methods are of' little quantitative

use, while the majority of others are useful in

· determining only relative atom concentrations, and

need calibration by some other technique to establish

absolute concentrations.

The various methods for estimation of atoms will

be considered according to the principle of detection,

'I 'I •

chemical methods.

(a) Calorimetric methods.

As the dissociation energies of' hydrogen, oxygen and nitrogen molecules are 103, 117 and 225 kcal/mole, respectively, considerable heat is liberated upon

recombination of' atoms of' these gases. Two main techniques employ this principle to detect atoms:

( i) 'rhe Catalytic Probe: 'l'his consists of' a small piece of metal ("'-"1 mm2 area) attached to a thermocouple, the leads of' which are encased in an inert material. As comparatively f'ew atoms are removed f'rom the system, the temperature rise by which their concentration is estimated is small,

minimizing cooling ef'f'ects and variation of' catalytic f:lctivity. The temperature rise is ideally e linear function of' the atom concentration, relative values

of' which may be reli(;lbly estimated5°,

~nput ·on activation of the dj.scharge the heat supplied from atom recombination can be determined, provided no other active species are removed by the calorimeter. '!'hough the method gives a s1:1tisf'actory measure of'

absolute concentrations of' hydrogen atoms using a platinum wire detector, the presence of excited molecules leads to overestimation of' oxygen atom concentrations unless a silver-oxide coated probe is used.

(b) Physical methods.

(i) The Wrede-Harteck Gauge. Such gauges50 , first described by Wrede52 and Harteck53 , were the

first capable of' yielding absolute atom concentrations. They depend f'or operation on the pressure difference which established when atoms effusing thr·ough a small hole in the reaction tube wall are recombined upon a catalyst enclosed in a small volume, the

resultant molecules effusing more slowly in the other direction. The gauge requires time to attain

13.

(ii) An Isolation method. Recently Elias

54

described a method in which the pressure decrease

due to recombination o~ atoms within a sample isolated f'rom a flowing stream of' gas was used to determine their concentr·atlon. Comparison with the results of estimating concentrations by chemical methods con.f'ir>med stoichiometry of' nitrogen oxide

"titraticms" :t'or• hydrogen, nitrogen and oxygen atoms.

(c) Spectroscopic methods .

. (i) Optical spectroscopy. While absorption

spectroscopy has been used to determine concentrations of' ground state hydrogen, nitrogen and oxygen atoms

55,

such absorption is in the experimentally awkward

vacuum ultra-violet region of' the spectrum, and the method is more widely used for measurement of'

concentrations of other species, particularly radicals56. Emission from excited molecules produced in recombination

o~

atoms has been used to estimate chlorine atoms56 , while the intensities of' the N

2 f'irst positive

(B3rr _{. g} A3z +) _u system and the NOR _~ (B2n

~

x2rr ) bands have been used in determining the. concentrations of' nitrogen and oxygen atoms present in the same system

57,58,59. _{Chemiluminescence from the reactions of}

considered under chemical methods.

( iq Electron Spin Resonance. Qne of the more sensitive and specific techniques for measuring

concentrations survived early

60 of atoms and free radicals t

calibration problems61 in its

esl"'

a.pplication to this field, and was soon developed to yield accurete atom concentrations62• A number of reections have been recently studied in flow tube -esr systems 63-69.

(d) Chemical methods.

The chemical reactivity of gas phase atoms affords a variety of means for their detection. IIeterogeneous reactions of hydrogen atoms with metals7° and metal oxides71,7 2 have been used in their detectionv but for quantitative work wholly gas phase reactions are

"end-point" corresponding to complete consumption of'

atomic species is barely reached, the·atom f'low then being determined f'rom that of' the titrant if' the

stoichiometry is known. In this way nitric oxide may be used for titrating

nitrogen7L~

atoms, and ni tr•ogen dioxide for hyd.Pbgen ?5 and oxygen 76 ... atoms. Variation of atom concentrations within a f'low

system may sometimes be f'ollowed by observation of afterglow intensities consequent upon the addition of trace additives.

(e) The Mass Spectrometer.

Being able to distinguish not only between

particles of different mass, but also by suitable choice of ionizing energy between ground and excited state

species, mass spectrometers have been frequently employed in atom and radical reaction studies77.

Extremely rapid scanning instruments78 can be used to follow the kinetics of reactions occurring under

transient conditions, such as in flash photolysis and shoclc tubes. More commonly, a conventional mass spectrometer is arranged to sample continuously from a fast flow reaction system, measurements being made under steady state conditions and kinetics determined

to their reaching the sampling leak. In each case a pressure drop of several orders of magnitude occurs between the reection system and the ion source of' the instrument, effectively "freezing" the reactions under study during analysis.

1. 24 :gse of lt,ass SJ2eCtPqr.uet.~!l..,Situd~!Qg .Atom Reactions. The flow sampling technique fir·st used by Eltenton79 and Ingold and Lossing80 to study f'ree radicals by mass spectrometry was extended to the study of' atom I'eactions by Schi:f':f' and assoctates, who characterized the

. 81 82

products of oxygen and nitrogen discharges, and by Kistiakowsky and cowoX>kers83 , 84• Phillips and

Schif'f'3~)~,

used a flow system of the type employed in the

present study to measure rates :f'or some o:f' the reactions of' pydrogen, oxygen and nitrogen atoms with nitrogen oxides and with ozone, including the most rapid known reaction of' ground state atoms, that of' hydrogen

atoms with nitrogen dioxide88• The method has been widely employed to study addition reactions of e.tomic hydrogen9° and oxygen91 , reactions between atoms and

17.

employed in combination with the mox•e recently developed technique

67.

Another application of the discharge flow technique has been to study of' ion-molecule reactions,

particularly tho::.:;e of interest in terrestrial and other planetr:Jroy ionospheres95 • 96 • Such reactions

~are

generally some orders of' magnitude faster than those of' neutral species, and extremely fast flows are required. The re£lcting mixture of ions and neutral species is

1 .3 INTRODUC'l'ION TO TiiE PRESENT WORK

1 .31 Aim and Sco e o_f' This_Er.Q;:Le9~~·

In this investigation a mass spectrometer was used in conjunction with a discha.rge-:f'low system to study the re.:1ctions o:f' nitrosyl chloride with hydrogen, nitrogen and oxygen atoms, and also the kinetics of' the initial reaction between cyanogen and atomic hydrogen. The study of nitrosyl chloride ·reactions wes undertaken to determine their suitability as atomic titrant reactions, and to compare them with

similar reactions of nitrogen dioxide. Rate constants were obtained f'or the pr•imary reactions, the products and overall stoichiometries were determined, and the relative :tmportances of' dif'f'erent reaction pathways were assessed. The cyanogen study was cont'ined to an examination ot: its relative reactivity toward atomic

19.

of' the ON radical was deduced. 1

.32

(a) Reactions of' Nitrosyl Chloride with Atoms.

(i) H + ONCl Clyne and Stedman97 studied the reaction of hydx'ogen atoms with nitrosyl chlox•j.de in

a discharge flow system, finding it suitable~as a

gas-phase titration f'or the measurement of' hydrogen atom concentrations in a syst·em in which the walls were coated with phosphoric acid. They consj.dered

that the reaction occurring under such conditions could be represented by a single step

H + ONCl -) HCl + NO,

and found its ra to be greater than 1.5 x 10-12 cm3 molecule-1

sec~

1

.

Cashion and Polanyi98 observed

ini'rf!-red emission from levels up to v

=

9

of

vibrationally excited hydrogen chloride produced in this resction, showing thet all of' the 65 kcal of'

.·

available energy may appear as vibration of' the

newly-formed bond. Using the combination of a mass spectrometer and discharge flow system, Niki, Stedman and Steffenson99, concurrently with the present study,

reaction to be unaffected by the presence of vibrationally excited hydrogen.

( ii) N + ONCL A paper by Biordi59 describes the effects of addition of nitrosyl chloride to the flowing afterglow of discharged nitro'gen. The

observed emission indicated the simultaneous.presence of oxygen and nitrogen atoms until a dark endpoint was reached {with the nitrosyl chloride flow half' that of nitrogen atoms initially) and of' oxygen atoms and nitric oxide thereafter, as the relative nitrosyl flow was increased. A mechanism involving several fast reactions consuming the nitrosyl chloride, followed by a number slower ones governing the decay of active species, wae. proposed to expla:l.n the behaviour of the system.

(iii) 0 + ONCl. This reacti9n does not appear

to hEtve been studied previously.

(iv) Cl + ONCl. From the efficiency with which small amounts of nitrosyl chloride inhibited the chain production of phosgene from carbon monoxide and chlorine, Burns and

Dainto~

1'01 evaluated the rate constant f'or the reaction

Cl + ONCl .-4 C~ + NO

which .red.uces to 3.3 x 10-12 cm3 rnolecule-1 sec-1 at

300 K. Ashmore and Chanmugam102 f'ound that nitrosyl chloride had a similar i.nhibitin.g ef'f'ect on the reaction of' hydrogen w:i.th chlorine, attributed to its reaction with both hydrogen and chlorine atoms·, and deduced a

rate f'or the reverse of' the ubove reaction. .. The

usefulness of' nitx•osyl chloride as a titrent f'o1~ chlorine atoms produced :i.n a discharge-flow system was later

demonstrated by Ogryzlo10

3.

(b) Reactions of' Cyanogen and Cyanide Radicals Cyanogen, the cyanide radical CNt and various other derivatives are important intermediates in

reaction systems involving hydrocarbons or halogenated hydrocarbons and active nitrogen104-i07. In a f'low system study, Haggart and Winkler108 showed that.

hydrogen cyanide was the only molecular product of' the· reaction between hydrogen atoms and cyanogen in the presence of' excess molecular hydrogen, and confirmed the previously suggested109,110 chain mechanism:

H + C

2N2..., HCN + CN,.

CN + H

₂

~ HCN + H.

Most mechanisms suggested for the relat:tvely complex reaction o~ nitrogen atoms with cyanogen involve as intermediates CN

2 and CN radi.cals 111

Safrany & Jaster have proposed a bl"~mching mechanism involving C atoms and CN and

c

₂N radicala115. For the 0 +

0~

2

reaction, Boden and Thrush116 established an ini tia.l rate o:f' ( 2. 5±0 .3) x 1 0 i3 exp( -11 OOO!i12000/Rrr)

cn3 mole-1 sec-1, and con:t'irmed the mechanism established earlier by Setser

&

Thrush11

7:

/NCO+ ON

0 +

C

2

N

2

~ ~

~

NO + CO + ON NON + CO

The CN radicals produced in these systems react rapidly with excess cyanogen, with the atomic and molecular gases present, and with various additives

which may be introduced. Flash photolytic studies118-121

have yielded rate constants for thermal reactions of' CN with o_{2 • NO,} c~

₂

and CN itself', and estimates of'

values f'or several other reactions. Measuring

concentrations ot: CN 1~adica.1s _{by absorption spectroscopy,} Iwai et a1106 obtained a rate constant of' 3 x 10-14 cm3 molecule-1 sec-1 f'or the reaction of' CN with H₂, a

value in accord with an estimate of' 7 kcal/mole t:or its activation energy122• This technique was also used by Boden and Thrush116 to determine rates f'or CN reactions with atomic and molecular oxygen, and with

23.

vartly heter•ogeneous, since the l"emoval of' ON from the system was more J:apid than could be accounted for vary Paul and Dalby's118 value of 3.6 x 10-15

cm3 molecule-1 sec-1 for the rate of homogeneous reaction, Reactions of ON and other j,ntermediates

L in the N +

o

2N2 system were further studied by

Safran;y· and Jaster, who investigated the inhibitol"Y

122

effect of various additives ~.

Emission from electronically excited CN radicals 111 112 117 124-7 ''•

has been widely observed ' ' ' in these and other reaction systems. Iwai et a1128 give

T

B

G

·o

X

f

PUM

24.

2. 1 APP ARA'l'US

2.11

The fest f'low reacti,on system is depicted in

figure 2.1. Atoms were f'ormed by partial dissociation of ,fi\ diatomic gas as it passed either alone Ol"' in an inert car·rier through an electrodeless discharge sustained by a microwave antenna (.M) in the quartz section

(Q)

of the 12 mm diameter inlet tube (A). The 2450 MHZ BUlTPlY for the discharge was provided by a "Raytheon" 125 watt diathermy unit, operated at between 5 and 50)b output power~ as required to

produce varying atom concentrt:1tions under the dlf"ferent flow and ssure conditions used. The discharge

region we.s separ~:.lted by two right angled bends from the .17 mm i. d. pyrex reaction tube (R) in order to a.void photolysis of' reactants by radiation f'rom the discharge. The other reactant was introduced at

(B),

passing through a length of' flexible PVC tubing (T) to enter the reaction vessel via an axial 3 mm o.d. tube which terminated in a 4.5 mm diameter, 6 hole spherical inlet jet (J). The axial tube was

centt~al positioning o:t' the jet within. the :t'low tube. This jet assembly could be moved over a 30 em distance along the :flow tube using a pair o:t' externally mounted magnets (F) which acted upon a soft iron slug (E)

lj.nlced to its upper end. In this way the time between the mixing o:t' reactants and their reaching

the mass spectrometer sampJ.j.ng leak (L) was continuously variable over a considerable range under constant :t'low conditions. A :fixed inlet jet (C), located 35 em :from the sampling leak, served to introduce NO and N0₂ :for 11_titration11 _reactions.

Pressures in the reaction tube could be measured by means of a Consolidated Electro~vnamics Corporation

"Micromanometer" type Lt1550 connected at D. A

Cl.iff'erential pressure device with a range o:t' 0 to 0.500

-L~

Torr, and a precision of 1.0 x 10 Torr, this instrument was supplied with a continuous backing vacuum o:t'

-6

ca. 1 0 Torr by a mercut~y diff'us ion pump and an Edwards 11_Speedivac11 _{1SC150B mechanical :rorepump.}

Af'ter leaving the reaction tube the gases passed through a large bore tap (X), by which their f'low could be throttled down in necessary, to a cold trap at

-78

0, and thence to a Welch model 1397 2-stage

26.

With the tap (X) fully open a linear flow velocity of about 10 m.sec-1 in the reaction tube was attainable at a total pressure of ca. 0.1 Torr. For photometric studies a housing (P) containing a 1P21 photomultiplier tube and suitable filters was mounted just upstream of the large-bore tap.

The flow rate of the carrier gas was measured with a calibreted capillary flowmeter; the flows of

all reactants were controlled using Edwards "Speedivac" type LB1B needle valves.

A small, central portion of the gas stream was continuously sampled into the mass spectrometer ion soucce through the leak (L). Sampling leaks were prepared from

7

mm o.d. pyrex tubing by the following

d due t J . m H 1 29 A 1 th .p t b i

proce ure o .~. erron • eng . o~ u ng

was drawn down to form solid rod at a point where the walls had been thickened, cut off in this region, and

then ground back using fine carborundum grit until a hole sufficient to give the required sampling

characteristics was formed. With an orifice of 30 micrometre diameter, a leak made in this fa.shion gave rise to an ion source pressure of 1.0 x 10-5 Torr

orthophosphox•ic acid in order to minimize heterogeneous recombination of' atomo, using Ogryzlo's103 procedure. The ion source was msintained at slightly a.bQve

28.

2.12 ·~h~LJ.1.P..~.§._§p~c_tromet~r.:

The mass spectrometer used to analyse the gaseous reaction mixtures in this work was constructed by

Proressor L.F.Phillips o~ this department during 1963, and is similar to that used by Phillips and Schir:r

85

in Montreal. It was a single order, direction-focussing instrument incorporating a 90° de~lection, 6-inch

radius magnetic anedyser. The ion-accelerating voltage was variable over the range 550 to 3000 volts, and was

set a constant value, usually 2000V, during measure-ments. The analyzer field was produced by an electro-magnet, the current through which could be varied

either manually or by a motor•-driven system scanning the mass spectrum.

The mass r~nge or the instrument was f'rom 1·to 160 with an ion accelerating potential of 1000V, and its

resolving pow~r about 1 in 100. This low resolution was of advantage since it was often necessary to keep a chosen mass peak in focus over a long period and any higher resolution would have made greater demands upon the stability of the power supplies used in the

instrument. The low resolving power was adequate to separate mass tral perucs produced from the

TOP VIEW

c

L-~

,...-~-

---0

~

R

SIDE VIEW

R

B

.._...,

...

,

___ _

/

I

L \'

\ I

\.

..__,...-

I ...

-... 01'

8

s

M

F

DETAIL OF ION ACCELERATOR:

B

c:======:::t

g~~~~~

I

c

e:

F

H

H

[image:41.596.93.471.81.797.2]

29.

'rhe ion source of the mass spectrometer was of' the electron bombardment type, and is represented in

figure 2-2. The electron gun was of simple construction, designed to achieve maximum sensitivity of' the instrrnnent. It consisted of' a

5

mm long filament (F) supported by

mounts (M) about 3 mm f'rom a 1 .5 mm ·x 6 mm slit ( S), and backed by a shield (H). The f'ilament was main-tained at a negative potential with respect to the ion box (B), of which the plate bearing the slit (S) f'ormed one wall, and the shield (H) was connected to

the negative end of' the filament. Having traversed the ion box, electrons wer•e collected in a trap (C), which was positively biased with respect to the ion box

(B), as was the ion repellel"' (R). A drawout potential f'or the ions was applied between the box (B). and. pla.tes (D), while a beam focussing voltage. could be applied between plates (D) ·and (G). Plates (E) were at earth potential, and the accelerated ion beam (I) then

passed through collimating slits into the analyser. In order to facilitate the detection of free

transparent tungsten mesh ( '1') and then flowed directly into a cold t1~ap and mercury dif'f'usion pump. The

ionizing electron beam was collimated by a weak mB-gnetic f'ield gener·ated by two small magnets (X, Y) externsl to

the ion source, and in line with electron gun and trap. By altering the positions of' these magnets

slightly the electron beEiffi could be aligned to optimize sensitivity and stability of' the instrument.

Thoriated

iridiu~

f'ilaments 130 , which gave steady emission of' electrons at moderf:ite temperature, and proved extremely :t:>esistant to a.ttacl'>: by the highly corrosive gases present in the system, were used throughout this work. With careful use they would survive several hundred hours of' operation, but

runn1.ng them at too high a temperature hastened their demise, These filaments were prepared by coating 0. 005 inch ( 0. 251-i- mm) diameter iridi urn wire with a

layer about 0.001 inch thick of' f'inely powdered thoria, using a cataphoretic process in an alcohol bath131 .

The ion be~m, after passing through the analyser tube, was collected on a polished~ stainless steel plate, maintained by a shielded battery at a positive potential or 45 volt with respect to its surroundings

31 ..

Ion currents were detected by a vibrating reed electro-meter: originally a Cary model 31 , later a Cary model

401 • rrhe input Voltage to the electrometer Was

developed across a resistor of either 1

o

1 0 , 1

o

11 or

1012 ohm value. Its output was supplied to a Leeds and Northrup "Speedomax G11

1 0 mV Chart reco·rger with 0 .. 25 second response for full scale deflection and, during photometric experiments, to a Solartron type LM 902.2 Digital Voltmeter, which could be read easily within the darkened room.

The electrometers were normally operated ln critically damped mode, except when detecting the lowest signals. The limit of' ion current detection was e.bout 3 x 1 o-1 7 ampere (L.e. 200 particle sec-1),

thou.gh .electr·ical noise in the building of'ten precluded measurement of' such low currents. The sensitivity limit of the mass spectrometer towards argon correspond-ed to a pressure of 3 x 1

o-

7 To.rr in the reaction

system under the best conditions attained, but was typically an order of magnitude higher than this.

Detecti.on limits for other gases ranged from the argon value to about two orde1.,s of magnitude higher. The background pr·e ssure in the ion source was usually

was normall;;r less than 8 x 1 o-5 Torr, as measur·ed by a

Veeco RG75 ionization gauge he and RG-2A controller. The ion source and analyzer tube were each

evacuated thx>ough liquid air temperature traps by an

l~dwards 11_Speedivac11 _{2M3B mercury diffusion pump ..}

These pumps were backed by a Welch model .1402B single stage rotary pump.

The power supplies for the electromagnet, magnet scanning control and ion accelerating vol were based on those of McKinney al132• In quest of better stability the circuit for filament power supply, emission regulation and supply of sundry voltages to the ion source was repeatedly modified. Its pyrolytic self-destruction on one occasion nece tated :tts ovm

~eplacement and the obtaining of a new vibrating reed electrometer .. A circuit which gave satisfactory performance is shovrn in figure _• All these power supply circuits were run from a Claude Lyons TS-lR AC voltage regulator, and generally proved stable and reliable in operation. Breakdown of insulation on parts of the system subjected to high voltage,

~

~V heatet'

T

ll.V:

I

T

5c:>O Hz.

OSCILlATOR

I

100 A

~o .... F

w""!L

+ ~lz

FIGUR

2.3

2.2 MATERIALS

Welding grade argon, 99~99% pure with less than 1 0 p. p.m. oxygen content, was dried by sage through a cold trap at Dry Ice temperature bef'ore being

adm:l.tted to the system. Industrial grade oxygen, from a cylinder selected for low nitrogen content, was similarily treated, while industrial grade dry hydx•ogen was purj.f'ied by passage over copper heated to 400°C and passed through a Dry Ice cooled trap.

Matheson 11_prepurif'ied11 _{nitrogen, with an oxygen content}

•

of' about 8 p.p.m., was passed through a liquid air cooled trap containing type 5A molecular s ve before use, Matheson "ultra.-high purity" oxygen containing 5 _{p.p.m. N2 and less than} 3 p.p.m, H₂

o,

was used

either directlyt or af'ter passage through an ice f'illed. trap at Dry Ice temperature.

Hydrogen atoms were formed by discharging hydrogen-argon mixtures, containing less than

3%

hydrogen :for nitrosyl chloride kinetic studies, but up to

30%

hydrogen f'or reaction with cyanogen.

Nitrogen atoms were produced either from pure nitrogen with no argon present, or from mixtures of argon and

34.

of' nitrogen atoms with nitric oxide added through the titra.nt inlet.

Nitrogen oxides :for the titration of' atoms were prepared from Matheson 98.5% pure nitric oxide. This was purif'ied by trap to trap distillation and repeated f'rsctionation. Nitrogen dioxide was derive~ from this pure nitric oxide by reacting it with an excess of' pure oxygen, the remainder of which was pumped away after the product had been frozen on a large

surface at liquid air temperature. These preparation procedures are described in detail by Nightingale et a1133 .

Nitrosyl chloride was obtained from the Matheson company. The 97°/o pure cylinder gas was trapped and pumped on at liquid air• temperature, then condensed

on to P

2

o

₅, and allowed. to melt and wet the solid before being fractionally evaporated. Af'ter this

the material was repeatedly fractionated using a

LeRoy still134 at temperatures.between 170 and 230°K, · until its mass spectrum indicated no further increase

in purity. Mass spectral analysis indicated the

N0

2• '!'he above method of' purification was based on

17.5

that used by BuPns and Dainton :; • The preparativ·e, storage and handling sys for ONCl was entirely coated in black paint to prevent photochemical decomposi·t ion.

Isotopically labeled nitrosyl chloride was prepared by reactj.on of' enriched nitric oxide (Bio-Rad Laboratories,

99

atom

%

15N) with an excess of'

chlorine. The chlorine was previously purif'ied

by trap-to=trap distillation. The nitric oxide was condensed on to solid chlorine at 80°K, the

materials then being allowed to react as they warmed, Af'ter several repetitions of' the f'reeze-thaw cycle

the

o

15NC1 was separated by fractionation using a Le Roy still. In. o~der to obtain best conversion of'

the

15

No this method dif'f'ers f'rom that of' Timmons and Darwent136 f'or the preparation of' normal ONCl in the

use of' an excess of' chlorine, which is more difficult to separate f'rom the f'inal product.

Matheson 98.57~ pure cyanogen was trapped at liquid air temperatuJ•e and pumped on to remove non-conclensibles, then f'ractionated using a Roy still between 160

36.

All stopcocks in the nitrosyl chloride handling system were lubricated with 11

2.3 CALIBRATION M~D MEASUREMB~T PROCEDURES

In order to detel'mine the partial pressures of' different gases present in the flow system during reaction, it was necessary to know the sensitivity of

~

the mass spectrometer towards them. Sensitivities for argon, nitrogen, hydrogen and oxygen were measuPed directly. The tap between the flow system and the pump was closed, the was admitted to the reaction tube and micromanometer, and the gas pressure and corresponding mass spectral peak height were recorded simultaneously, In the range 0 to 0.5 Torr, it was generally found that the peale height increased linearly w.ith pressure, to within 356 for all these gases. In practice a number of measurements of the sensi ti vit:les of argon and whichever the other gases was· to be used in the flow experiments were per•f'ormed al ter·nately in a preliminary experiment, the sensitivity for the diatomic gas relative to argon then being used in a

of main experiments.

38.

determined by an indirect method. Mixtures of lcnown composition of each of' these gases with tn•gon were prepared, and introduced into the reaction vessel under static conditions. The sensitivit for the gases, rela.ti ve to argon, wer:e determined from the

...

ratio of peak heights in the spectrmn. It was found necessary to "conditionn the reaction ves and mass spectl:>ometer by f'illing it with several samples of the gas mixture over a period of ten minutes before measure-ments were made, in order to obtain consistent relative sensitivity values. Several samples for measurement were also normally used.

All mixtures of were prepared us a Texas Instruments Model

144

quartz Bourdon gauge (range

0 - 300 Torr) to determ.ine the partial pf'essures the components. For N0

2, allowance was made for the fact that an appreciable proportion was present as N₂

o

4 1_{38 .}

Under conditions of measurement in the flow system, the equilibrium proportion of N2

o

₄

was negligible. Moreover, i,t can be shown that the dissociation of N2

o

₄

to N02 is rapid enou~h to be complete before

mass spectrometric sampling during flow experiments1

39.

For most gases the peak due to the parent ion

measurements o:f gas concentrations, but the exceedingly low parent peak of' nitrosyl chloride* was not detectable under such conditions, and fragment-ion pealts were

used instead. Un . .f'ortunately, the largest f'ra.gment :l.on peak, due to NO+, could not normally be used to · monitor the concentration of ONCl, e.s the sensitj:vity of the instrument at mass 30 towards this gas and

towards the NO produced in its reactions was identical, within limits experimental error. For most

experiments the ONCl concentration was measured using the NCl+ peak at mass

49,

which is only 1.3% as

intense as ND+, and the low sensitivity for ONCl using this peak was a major source of difficulty throughout the stuCI.y of its reactions. It was assumed that any . NCl l?roduced in reactions of ONCl would not contribute

to the peak at ~ass

49,

since no such peak was observed :from the products of reaction of' N with 01₂0, where NCl radicals were certainly present1

4°.

Relative sens ivity values were found to remain constant to within about 5)'b over considel"able periods of' time~ provided constant ion source conditions

prevailed. They were normally checked weekly when a

40.

series of experiments w~s in progress, and small variations time-interpolated. Typical values for the relative sensitivit of' the mass spectrometer . towards the g6lses used, referring the heights of their

peaks to the hej.ght of'. the argon peak a.t mass 40, were:

2.32

H2, 0.20; N2, 0.54;· 02, 0.37; 012 at "mass 70, 1.18; NO~ 0.47; N02, 0.093;

CZN2'

0.90;

· ONCl at mass 30, 0.47; ONCl at mass 49, 0.0063.

Pr:i.or to any experiment in which the mass spectro-meter was to be used, sufficient time was allowed for the various power supplies to warm-up, so that stable output sign1;1ls could be obtained. After the filament had been operating at a steady emission level for some time, the system was flushed with argon and the magnets external to the ion source adjusted to obtain optimum sensitivity of the instrument towards this gas. If' substantial changes in ion source conditions had been made, relative sensitivity values for the various gases to be used were determined, as described in the previous section, before proceeding with flow experiments.

gases ·could be obtained .from thus using current relative sensitivity values. When successive determj.nations of' the carrier gBs sensitivity were constant to within

5%

over the full pressure range of' the micromanometer

(0 to 0.500 Torr), the flows of' the carrier and admixed gases were adjusted to the desired values, the height of' the carrier gas peal{ recorded, the temperature noted, and the carrier flow rate determined from the capillary flow-meter. The carrier gas peak height together with the mean sensitivity value permitted calculation of' the carrier pressure within the f'lowing gas near the mass spectrometer sampling leak. Under f'<.1st .flow condi tiona this could not be detex•mined direct}y, owing to the considerable pressure drop between the micromanometer inlet and the samplini

42.

then performed as previously, and the initial and f'inal values obtained compared. Agreement was usually to withln 10;:6, and mean values were used in such cases, interpolated values being used otherwise.

Experiments were normally three to f'ive hours duration and some variation in the overall sensitivity of' the mass spectrometer occurred between initial and :final calibrations. Sensi ti vi ty f'luctuatj.ons were followed by monitoring the height of' the carrier gas peak at f'requent intervals during an experiment. Measured peak heights f'or all other reactants could

then be scaled using this peak height and the peak heights obtained in calibration, to give values

independent of' variation in instrumental perf'ormance. Between initial and f'inal calibrations, titrations were perf'ormed to establish atom concentr•ations, and measurements were made of' the primary rate constant of'

2 . .33

The titrents and reactions used in determining atom concentrationB were as f'ollows:

(i) For hydrogen atoms, NO., was the titrant,

_...

the re;;>ction scheme being: 88

H + N0

2 ...,.. NO + OH

OH + OH ~ H₂

o

+ 0

0 + OH · 4 0

2 + H 0 + N0₂ ~ NO +

o

₂,

( 2-1)

(2-2)

( 2-.3)

(2-4)

giving the overall reaction stoichiometry

(ii) For oxygen atoms, N0

2 was also used, in the

ree>.ction

24 ,81

(2-4)

(iii) Nitrogen atoms were determined using nitric oxide: 84 , 85

N + NO ( 2-5)

The stoichiometry of the oxygen and nitrogen atom imat:ton revctions, (2·-4) and (2-5), is thus 1:1; for hydrogen atoms, two moles of atoms react with three moles of N0₂. The llYdrogen atom reaction is subject to error in the presence of sufficient excess of molecular hydrogen 141

'

11

~·

2

,

since then the reaction

competes with (2-2), and leads to the overall s toi.chiometry j

H2 + N0₂ -il H

20 + NO,

44.

so that some N0₂ is consumed without removing hydrogen atoms. The stoichiometry of these reactions has been

62 ~

confirmed by ear measurements , and by the isolation method of Elias54.

The procedure adopted for these titrations was to add an excess of titrant to the flowing gas stream, and to observe the decrease in"titrant parent peak when atoms were produced by activating the microwave discharge. About a twofold excess of' titrent gas was normally used, and the discharge was turned. on and off a number of times, the mean change in peak height being recorded. 'l'he atom concentrli'ltion was calcule.ted f'rom·the change in peak height and known

titrant gas s:e(!;J.Sitivity, scaled accor'dj.ng to the

mean carrier gas peak height before and after titration. The titr·ant inlet jet W.;tS suf'f'iciently i'ar f'rom the

sampling leak f'or the titration reactions to be complete before mass spectre.l analysis.

prior to final calibra.tj.on. It was found that atom concentr•ations were repx•oducible to with

5%

between successive ti tre1tj.ons, and intex•polated values were used in determining the results from intervening measurements. Blarut titrations were performed

when argon Wf:IS used as a carrier•, with it f'lowing alone

through the discharge tube, to ensure that no titrant was consumed by active species produced from the

argon, or by photolysis by light from the discharge region.

2.34

primar~ Re1te Cons~ant Measurem~

Despite the great variation in the rates of the primary steps and the differing relative rates of

primary and secondary stages of the l"eactions studied, there was a general procedure common to all primary. rate constant measurements. The differing conditions under \mich these measurements were made in each

reaction system will be discussed in the chapter devoted to that reaction.

Rate mea:::mrements were made under· condi tiona of incomplete consumption of the r•eactants. An excess of atoms was norme1lly used, but where secondary

reactions did not inter:rere some measu:~:>ements were

46 ..

The contact time between reectants, and hence the extent of' the reflCtion,. could be varied by altering the distBnce between the movable inlet jet and the mass spectrometer sampling leak. Reduction of' the linear gas flow velocity within the reaction tube, by almost closing the tap be.tween it and the pump, enabled the time scale to be extended i'or study of' the slower ret1ctions.

In perf'orming the measuz•ements, which wex•e

preceded and f'ollowed by determinations, of' the atom concentration, the carrier gf:ls peak height wa.s first checked and the position of the movable jet set.

A ~oderate amount of molecular reactant was introduced to enable the mass spectrometer to be focussed on to .the correct peak for its measuremen·t, then a background measurement made of this peak with the reactant absent. Next the flow of' the molecular reactant was restored and adjusted to give the desired partial pressure, and the peak height was measured a number of times with and without the discharge operating. Finally, the background signal was deterrnlned once moPe, and the carrier peak. height again measured. Averages of'

For n:i.tl'osyl chloride reaction studies, flow conditions and reagent pressures we1~e chosen such that the reactant gas could be added within 10 em of' the sampling leak, so as to minimize erl"ors due to viscous pressure drop under fast flow conditions, and atom recombination wi.th slower flows. In

determining the rate of the reaction between hydrogen atoms and cyanogen, it was found necessa.ry to set the movable inlet jet at its greatest distance from the sampling leak, in order to give time for the reaction to proceed to a sufficient extent. In this case the effect of atom recombination was allowed for in

calculations of the rate.

2.35 Heactan~,--Stoichiometry Determinations

In determinations of' the stoichiometries of.the reactions of nitrosyl chloride, the method of' perform-· ing the measurements was similar to that previously described f'or the titrations used to estimate atom concentrations. An excess of nitrosyl chloride was added via the movable jet, which was set at its

48.

The procedure for stoichiometric runs was

straightforward. After initial calibration of' the system, alterrwte measurements were made of the consumption of reactElnt Elnd titrant, using the ssme concentration of' atoms. These observations were interspersed between checks of' the csrrier gEls peak height. Blank determinations, with only argon passing through the discharge, were performed f'or both reactant and titrant, except when nitrogen was the carrier gas. Varying excesses

the stoichiometr

nitrosyl chloride were used~ and of' its reactions with the atoms deduced f'rom the relative consumptions of ONCl and the titrants.

'l'he pressure drop which would occur bet.ween addition of reactC~nt and mass spectrometric sampling

is greater in these measurements than in those of the reaction rates. Although wj.th the fastest flow the drop was about 10/b of' the total pressure, the similar locations of the reactant and titrant inlet jets

invest te the stoichiometry. Observations were made downstream or the mfiss spectrometer sampl

leak using a 1P21 photomult ier tube in copjunction with either a Spectrolab P type interference filter·

(~L5 nm half width at 625 nm) to detect the nitrogen

afterglow, or a Corning

7-39

lter to detect

em iss ion from the NO

ft

-bands. Em iss ion f'rom both these systems occurs concurrently with the reaction, provided that some nitrogen atoms remain; thus the

nendpoint" at which the photomultipliex• signal drops to zero corresponds to the stoichiometric constunpt.ion or atoms by the molecular reactant59. As similar

behaviour exhibited by the N + NO titration reaction,

a direct compar, son technique could be used to establish the relative concentrations of atoms and nitrosyl chloride57,58 . Concentrations o:r reactant and titr~nt were each.determined in the absence o:r

50.

Experiments of' this type were normally carried out with the room darkened. After initial and before

'

final calibr•&tions, the .nitr·ogen atom concentrat:ton was determined by titration in the usual way.

Tllroughout the remainder of the experiment the mass spectrometer was kept focussed on the ma.ss 3G peak, which had a considerable ba.ckground intensity in the absence of intl~oduced reactants.

To esteblish the reletive concentrations o.f N and ONCl at the point o.f stoichiom~tric balence, the

.following procedure was followed alternately with NO and ONCl. The di8charge was set in operation, and

an excess or the molecular reactant added, so that an "air ef'terglow11 _{emission was yisible in the}

. react: ion tube. The reactant flow was then reduced and set as nearly: lW possible to give the "dark

endpoint11 _{of' the reaction, as indicated. by a minimum}

in the photomultiplier current. To determine the

reactant concentration the discharge was then extinguish~d,

the mass 30 peak height measuredt then the background peak me!:lsured with the react8nt flow shut of'f'. The discharge was operated (;It a constant power level, so that atom concentrations varied as little as possible.

ot: the nett mass 30 sj.gnals of NO and ONCl, since the former gives an estima of' N atom concentrations whiich ca.n be directly Peh'~ted to those of' ONCl yielded by

the latter.

2.37

Examinations were made of' all reaction 10ystems in order to determine the nature of' their pr•oducts and intermediates. In the reactions of' oxygen and nitrogen atoms with nitrosyl chloride, such studies were also carried out using the isotopically labelled material,

o

15NC1. These measurements were generally qualitative or only semi-quantitative, since mass spectr•al sensitivities f'or radical intermediutes were not known, and products were sometimes unobservable against large background peaks. Some of' the mo:t;>e usef'ul observations were negative ones, showing the absence of' certain pPoducts. For example, the f'ailure to detect an unreactive species such as N₂

o

among the products of' a react:i1on is conclusive evidence against its formation in more than trace amounts. Even

52 ..

Product searches were carried out at both long

and short reaction times, using as great a variety

of' l'eaction conditions as possible. 11

Blan1t11

measure-ments vdthout one or other of' the reactants were