NBSIR 74-516

Chemical Kinetics Data Survey Ylll.

Rate Constants of ClOx of

Atmospheric Interest

R. T. Watson

Department of Chemistry

University of California

Berkeley, California 94720

June 1974

Interim

Distributed by

Chemical Kinetics Information Center

National Bureau of Standards

Prepared for Climatic Impact Assessment Program

Department of Transportation

Washington, D. C. 20590

NBSIR 74-516

CHEMICAL KINETICS DATA SURVEY VIII. RATE CONSTANTS OF CiO^

ATMOSPHERIC INTEREST

R. T. Watson

Department of Chemistry

University of California

Berkeley, California 94720

June 1974

Interim

Distributed by

Chemical Kinetics Information Center _ National Bureau of Standards

Prepared for Climatic Impact Assessment Program

Department of Transportation

Washington, D. C. 20590

•"•r.u o» *

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director
ii

Table of Contents

Introduction and Survey of methods

Calorimetric Probe

Emission Spectroscopy

Ultraviolet SpectrophotometTy

Mass Spectrometry

Resonance Fluorescence

Resonance Absorption

Electron Paramagnetic Resonance Spectrometry

Infrared Spectroscopy

Miscellaneous Techniques

Errors and Uncertainty Limits

(a) Discharge Flow Systems

(b) Flash Photolysis Systems

Units

Acknowledgement

Page

2

3

3

5

78

11

12

12

13

13

14

15

15

16

Table I . Rate Constant Measurements Categorized by

Analytical Technique

17

3. Compilation of Rate and Photochemical Data

2 I. Reactions of Ground State (

1* 2/2 ^ Chlorine Atoms

19

19

(1) Cl( P

(2) CK^P

(3) Cl(-P

(4) Cl( P

3/2

3/2

3/2

3/2

(4A) C^( P3/2)

(5) Cl( P

3/2

(6) Cl( P3/2

(7) Cl( P

(8) Cl( P

3/2

3/2

(9) Cl( P

(10) Cl( P

3/2

3/2

+ O3 ^ CIO + 0^

+ OCIO ^ 2 CIO

+ O^ + M ^ ClOO + M

+ CH^ -y CH3 + HCl

H2 HC^ + H

+ ClOO -> CI2 + O^

+ ClOO -> 2 CIO

+ Cl^O ^ Cl^ + CIO

+ NO + -> CINO +

+ NO + O^ CINO + O^

+ CINO -> Cl^ + NO

19

19

19

20

20

23

24

24

24

24

25
iii

(11) Cli^P^^^) + + M ^ CINO2 + M 25

(12) Cli^P^^^) + CINO2 CI2 + NO^ 25

(13) Cli^P^^^) + C1{^I>^^^) + Ar ^ CI2 + Ar 25

2 II- Reactions Producing Ground State ( P -.^/2 ) Chlorine Atoms,

and /or Involving CU) Radicals. 26

(14) 0(^P) + CIO -> Cl(^P3/2^ + °2* ^-^^'-^^ ) 26

(15) NO(^n) + CIO -> NO2 + Cl(^P3/2^

2

(16) CIO + CO ^ CO2 + Cl( 26

(17) CIO +03^ ClOO + O2 27

-> OCIO + O2

(18) CIO + ^2 ^ Products 27

(19) CIO + CH^ -y Products 27

(20) CIO + C^E^ ^ Products 27

(21) CIO + C2H2 ^ Products 27

(22) CIO + N2O ^ Products 27

(23) CIO + NH3 Products 27

(24) CIO + CIO ^ CI + OCIO 28

(25) CIO + CIO ^ CI + ClOO 28

- ^^2 ^ ^2

(2 6) CIO + CIO + M -> CI2 + O2 + M 29

(27) 0 + CI2 ^ CIO + CI 30

(2 8) OH + HCl -> li O + CI 30
(2 9) 0 + HCl OH + CI

(30) ClOO + M ^ CI + O2 + M

III. Reactions of OCIO and Cl

^ O which Involve CIO Radicals

(31) 0 + OCIO ^ CIO + O2

(32) NO + OCIO NO2 + CIO

(33) N + OCIO ^ NO + CIO

(34) H + OCIO ^ OH + CIO

(35) 0 + Cl^O ^ CIO + CIO

IV. Optical Absorption Cross-Sections

(36) CI2 + hv ^ 2 CI

(37) HCl + hv ^ H + CI

(38) ClOO + hv ^ CIO + 0(-'-D, ^P)

(40) OCIO + hv -> CIO + 0(-'-D, -^P)

CINO + hv ^ CI + NO

(39) CIO + hv ^ CK^P^,^) 3/2 36

r + 0(-^D, ^P)

38

38

Cl^O + hv ^ CIO + CI 3g

CFCI3 + hv (120 < A < 200 nm) 39

CF^Cl^ + hv (120 < A < 200 nm) 39

Figure: Absorption Cross-Sections as a function of wavelength 40

References
1

chemical Kinetics Data Survey

VIII. Rate Constants of CJ^O of Atmospheric Interest

R. T. Watson

Department of Chemistry

University of California

Berkeley, California 94720

Abstract

tative data on reactions of chlorine atoms and the

are compiled. Preferred values for rate constants

possible. Optical absorption cross sections are given

that may photolyze in the atmosphere. The

chniques used to obtain the rate data are discussed.

The quant

i

chlorine oxides

are given where

for the species

experimental te

Key words: absorption cross sections, atmospheric chemistry, chemical

kinetics, chlorine, chlorine dioxide, chlorine monoxide, data evaluation,

gas phase, optical, rate constants, stratospheric chemistry.
2

Introduction

The Chapman mechanisra''' for the production and destruction of ozone

has been shown to be inadequate to explain the observed vertical ozone

2 column concentrations m the stratosphere . Catalytic cycles which

3 4 destrov ozone, based on HO XXand NO and more recently CIO , have been ^ X

proposed to account for the large discrepancy between the observed and

calculated concentrations. Accurate values for the rate constants of

2 reactions involving ground state atomic chlorine ( ^3^2^ and the oxides

of chlorine, (CIO, ClOO and OCIO) are required as a function of

temperature, in order to perform model calculations which would estimate

the perturbation upon the ozone column concentration due to the

injection of chlorine containing species into the stratosphere, i.e. CI2

HCl, and freons (i.e. CFCl^, CF^Cl^).

This compilation of the rate data critically reviews the published

data and forwards with a minimum of explanation a set of preferred values

for either the rate constant at 298K and/or (whenever possible) the

Arrhenius expression. Rate data exists for most of the reactions which

are thought to be of importance in the stratosphere, but unfortunately

most of it was obtained at 298K, and consequently estimates must be made

as to the temperature dependence of the rate constants. Certain

reactions are more important in the chemistry of the stratosphere than

others, i.e. CI + 0^ -> CIO + 0_; 0 + CIO CI + 0„ ; NO + CIO ^ N0„ + CI;
CI + CH^ ^ CH^ + HCl and HO + HCl ^ H^O + CI; and of these, the first

three have only been studied at 298K. However, due to the rapidity of

these reactions at 298K (all ^ 1 . 7 x 10 cm"^ molecule s

"'") it is un- likely that any of them have an activation energy greater than 2.1 kJ

mol (0.5 kcal mol "''). Consequently, this means that the rate constants

for all three reactions are known to within a factor of 2 at all stratospheric temperatures. The other two reactions have both received recent

investigation and their rate constants as a function of temperature are now reasonably well established.

i4any of the early published results have recently been shown to be

erroneous due to misinterpretation of experimental data indirectly re—

5 suiting from insufficient sensitivity m the detection apparatus . Thus

the modeller must be discriminating in his choice of rate constants and

absorption cross-sections, and consequently the writer feels that a brief review of the experimental techniques used to obtain the results

reported in this compilation is justified.

Calorimetric Probe

The calorimetric probe method of monitoring chlorine atoms has been

used in one study reported^, and was calibrated to determine absolute

atom concentrations by use of the CINO titration reaction. This

14 15 -3 technique is both insensitive ( [CI] ~ 3 x 10 - 3 x 10 atom cm ) and

non specific and thus of limited use.

Emission Spectroscopy

Several studies of third order reactions have been performed by

2 monitoring the concentration of ground state chlorine atoms via the

3/2 *3 1+ 7—11 2 Cl^ ( ^ Ig ) afterglow . The recombination of P3/2 chlorine
3 atoms, and the subsequent fate of the ^0^+ state of molecular chlorine

can be written"*"^:

:'P3/2) ^ ^^("^3/2^ ^ ^ - ^l2('nouH

Cl^i^n +) ^ Cl„(-'-y ^) + hv(> 500nm)

z ou z g

CI (^P^^2) +<^l2 ^^^ou+^ ' CI2 (non-radiative) +C1 (^P2/2^

It has been shown that I, = I°[C1] , where I, represents the emission

A A A 12 intensity at wavelength A . n was shown to vary with A, (i.e.

A

- n^-on = 1.7(M=Ar) and n^^„ = 1.0(M=Ar) ) and not have a constant

52 0 nm 9 20 nm

9 value of 2 as had been earlier reported . Therefore, when using this

technique to monitor relative Cl atom concentrations, care must be taken

to use the correct value for n, . Results for some early work which used

A 7 this method must be slightly modified (the reported rate constants are probably low by ~ 2/n - 15%) as n was taken to be 2, rather than 1.7

A A as later determined. This method of monitoring Cl atom concentrations

is moderately insensitive, but useful v/hen [Cl] is lO"*"^ - lO"*"^ atom cm

^

1 3 Due to the rapidity of the Cl + CINO ^ NO + CI2 reaction , ClNO can be

* used to titrate Cl atoms. The critical extinction of the red Cl^ after

^. glow is one method for determining the titration end-point, and this was

14 12 recently used in a series of Cl + RH reactions , where [CIJ^ >_ 5 x 10

-3

atom cm . Under low pressure discharge flow conditions, the extent to

which Cl atoms are removed between the ClNO inlet and the observation

point at which CI2 is monitored due to the Cl + Cl + M ^ CI2 + M and

the Cl + NO + M ^ ClNO + M reactions can be calculated, but is normally

negligible. The heterogeneous wall removal of atomic chlorine can be

inhibited by coating the reactor surfaces with H^PO^.
5

Ultraviolet Spectrophotometry

The technique of monitoring both stable and labile species by UV

absorption spectroscopy has been used in conjunction with discharge flow

(CIO, OCIO, 0^)"^^""^^; flash photolysis (CIO, OCIO, CI2O, ClNO)"^^"^^ and

2 6 2 7 molecular modulation ' systems. The absolute concentrations of these

species can be monitored as a function of reaction time by follov/ing the

optical absorption of the species and using the experimentally

determined values for the absorption cross sections. Although the

— 1 8 2 absorption cross sections are fairly high {ClO^-j-j

2 nm

~ '''•2 x 10 cm molecule . OCIO^^,-, r- = 1.14 x 10

"'"'^ cm^ molecule . .= 351.5 nm 3260.4

— 17 2 — 1(28) 1.10 X 10 cm molecule ; all to base e) , this method for following these species is still relatively insensitive compared to molecular

beam mass spectrometry. The discharge flow systems used multiref lection

absorption cells where a typical optical pathlength was 20 cms, resulting

m 13 - 3 a limit of sensitivity for CIO, OCIO and 0^ of ~ 10 molecule cm

The molecular modulation technique was two orders of magnitude more

sensitive, using an optical pathlength of 400 cm and measuring modulation

- 3 - 5 11 amplitudes of 10 -10 . Thus, concentration modulations of -10

- 3 molecules cm could be observed for CIO and ClOO.

15 The first study of reactions (1) (CI + 0^ ^ CIO + 0^) and (2)

(CI + OCIO ^ 2 ClO)"'"^ could only report lower limits for k.^^ and at

300K due to limited detection sensitivity. When studies of the 0 +

"I o ] q ic

OCIO ^ CIO + O2 and NO + OCIO ^ + CIO reactions were performed, several elem^entary processes were occurring simultaneously, and

due to the fact that the rapidity of reaction (2) was not fully realised.
6

its role in the overall mechanism of these reactions was not appreciated.

This resulted in the value for k^^ (0 + OCIO) being overestimated by two

orders of magnitude. Rate constants for reactions 14 (0 + CIO CI + O^)

and 15 (NO + CIO ^ NO2 + CI) , which are of utmost importance in the

3 stratosphere, were obtained by indirect methods whereby the 0( P) and NO

(in seperate experiments) competitively reacted with CIO and OCIO. The

autocataly tic nature of the reaction mechanisms was not known and

18 19 16 consequently the published values for k^^ ' and k^^ must be rejected.

The reaction mechanism for the X + OCIO reaction can be written, (X =

0, NO)

X + OCIO XO + CIO; k . 5 X 10~^^ cm^ molecule"^ s""*-

X + CIO XO + CI ; k > lO""'"-'- cm^ molecule"-"- s""""

CI + OCIO CIO + CIO; k = 5.9 x lO"^""" cm^ molecule"-'- s~^

The bimolecular self-disproportionation of CIO radicals has been

studied in a wide variety of chemical systems using UV spectrophotometry

15 17 56 19 23 24 25 in conjunction with discharge flow ' ' , flash photolysis ' ' '

26 and molecular modulation experiments, however, the results are at

variance with each other. The low pressure discharge flow technique has

29 conclusively shown, both by direct (using atomic resonance absorption)

and indirect evidence that, Cl atoms are generated from the decay of CIO

radicals, but not 100% efficiently. This technique has been used to

show that the reaction is overall 2nd order and has an activation of

^ 10 kJ mol 2.4 kcal mol "'). Results using the high pressure flash

photolysis method record that there is no generation of Cl atoms,

and that the overall decay is strictly 2nd-order (with possibly no
7

activation energy) . The third technique^ molecular modulation

spectroscopy, yielded results that indicated that the CIO radical decayed

by both overall 2nd and 3rd order processes, producing atomic chlorine

in the former. Thus, any results used to compute the following important

equilibrium constants from these results must be used with caution. The

discharge flow results indicate the possibility of two primary decay

reactions, but the experimental decay data cannot differentiate between

tne pathways and measures the overall rate of decay. Also, the measured

activation energy refers to the overall activation energy for CIO decay.

CIO + CIO ClOO + CI ; K-^ = ^24^^6

^-i * CI + 0^ + M ^ ^ ClOO + M ; = ^3/^^30

^30

Mass Spectrometry

The discharge flow technique has been used in conjunction with mass

spectrometry for several studies reported in this compilation. Many of

the preferred rate constants were measured using a system which utilized

efficient collision free sampling between the flow tube and the ion source

5 2 9 30 of the mass spectrometer ' ' . The ion currents of both labile and

stable species were shov^n to be linearly proportional to the concentrations

of their flow tube precursors and could be calibrated on an absolute

basis. Typical limits of sensitivity, with a signal to noise ratio of

9 - 3 unity, were, CIO (m/e 51) = 1.5 x 10 molecule cm ; OCIO (m/e 67) =

8 — 3 9 — 3 5.0 X 10 molecule cm 0^ (m/e 48) = 2.0 x 10 molecule cm . Therefore,

it can be seen that this technique is ~ 4 orders of magnitude more

sensitive than UV absorption spectrophotometry for monitoring CIO and
8

OCIO and as such is well suited to study the kinetics of fast reactions.

The preferred values of the rate constants obtained using this technique

used psuedo first order conditions, whereby, complicating 1st and 2nd

order secondary reactions were eliminated. Reactions due to trace con- centrations of active impurities produced in the microwave discharge

(i.e. NO, 0, H) were carefully eliminated utilizing the discharge bypass technique and/or by chemical scavenging.

The mass spectrometric study of the CI + CH^ reaction used a similar but somewhat less efficient, and less sensitive sampling system

14 than the above . However, the study was performed using pseudo 1st

order conditions whereby, if there had been any secondary reaction between the atomic chlorine and the product methyl radicals it would not

have affected the reported rate constant.

Time of flight mass spectrometry has been used to determine the

o 1 overall reaction rate and s toichiometry for the 0 + Cl^ reaction . Both atomic oxygen and molecular chlorine were monitored and this

technique yielded results in good agreement with those obtained using

32 chemi lumines cence to monitor 0 atoms.

Resonance Fluorescence

The technique of atomic resonance fluorescence is both highly

specific and sensitive, and thus, well suited to the study of rapid

5 3 3 3 4 reactions. Both discharge flow ' and flash photolysis experiments

4 5 2 have monitored the fluorescent flux of the CI 4s ^2/2 ~ ^ ^ ^3/2

transition at 138.0 nm. The results of the discharge flow experiments

showed that the fluorescent flux was linearly proportional to the con2 2 12 -3

centration of P3/2 chlorine atoms when [Cl, ^3/2-' — '^^ atom cm
9

This is in slight contrast to the flash photolysis results which show

11 12 that when the chlorine atom concentration is in the range of 10 - 10

_ 3 atom cm , the fluorescent flux is not linearly proportional to the

0 9 atom concentration, but obeys the following: I a [CI] ' . The paper

r 2 did not report the nature of the relationship between 1^ and [CI, ^2/2^

11 -3

when the latter was < 10 atom cm , but it would be expected that I„ — - F

35

was linearly dependant upon the atom concentration m this region

2 The lower limit for detection of '^2/2 '^'^ atoms using the discharge flow

10 -3(13) system was reported to >_ 5 x 10 atom cm , but later after system

9 -3(5) modification this value was reduced to ^ 3 x 10 atom cmi

3 Kinetic studies involving 0 P atoms have been performed using the

5 discharge flow technique . The fluorescent emission used to m.onitor the

3 3 4 3 concentration of 0 P atoms was that produced by the 0 3s - 2p P^

transition at A 130.6 nm. The limit of detection was reported to be

9 - 3 12 - 3

~ 4 X 10 atom cm , and from this value to <_ 10 atom cm the

intensity of fluorescence was found to be linearly proportional to the

atom concentration.

Hydroxyl radicals have been monitored in a discharge flow system

2 using the emission produced at 309 nm, the band head of the A J

-> X

(0-0) transition^^ . Hydroxyl radical concentrations <_ 10''"'^ radical

-3

cm can oe detected, thus, allowing reaction conditions to be chosen

whereby the problem of the bimolecular disproportionation of HO radicals

12 -3

IS eliminated. Under these conditions, where [HO]^ ^ 10 radical cm , the only process removing HO radicals, besides its interaction with the

added reagent, is the heterogeneous 1st order wall removal. Hov/ever, if

the reaction is performed using pseudo first order conditions and the fixed

observation point technique (as v/as the case) then 1st order reactions
10

such as wall recombination do not need to be considered in the analysis

of the decay data^"^'"^^.

It is apparent that the technique of atomic and molecular resonance

fluorescence allows even the most rapid reactions to be performed under

psuedo first order conditions, whereby, the problem of complicating

secondary reactions can be successfully eliminated. The technique

12 becomes insensitive at high absorber concentrations (> 10 particle

cm '^), although the lamp design and operating conditions control the

range in which the intensity of the fluorescent flux is linear (or

nearly) with atom concentration. The technique of atomic resonance

fluorescence is powerful in that it can be used to directly observe in

a quantative manner trace concentrations of both, (a) impurity atoms

produced either photo lytically or in a microwave discharge, and (b) atoms

produced in the course of a reaction. This can be achieved by alteration

of the chemical composition of the discharge lamp.

Calibration of the fluorescent flux intensity is normally achieved

by producing known concentrations of atoms, either photoly tically or by

use of a stoichiometric reaction and observing the fluorescent flux

produced. Even though this procedure may/or may not be to within an accuracy of 10%, it introduces no error into the rate constants reported

in this compilation as all studies were performed under psuedo first

order conditions, whereby, the concentration of the species being

monitored was always significantly (a factor of 5) lower than the un- monitored species, therefore, only relative atom concentrations were

required

.
11

Resonance Absorption

This technique has been infrequently used in the studies to be

reported in this compilation. It is typically about two orders of magnitude less sensitive than resonance fluorescence, and thus of limited use

-12 3 -1

in the study of rapid bimolecular reactions (k > 10 cm molecule

s

"'"), where psuedo 1st order conditions are required to eliminate

complicating secondary reactions. Hydroxy 1 radicals have been monitored

using this technique in a flash photolysis study of the HO + HCl

39 reaction . The only other studies to use this technique were; (a) an investigation of the s toichiometry of the CI + ClNO reaction, in which

40

a lower limit was reported for the rate constant , and (b) an investigation of the reaction products of the bimolecular disproportionation of

29 CIO radicals at low total pressure (~ 1-5 torr) . Both studies were

performed in a discharge flow apparatus and monitored the concentration

2 4 5 2 of chlorine atoms at A 138.0 nm (CI 4s P3/2 ~ ^ P ^3/2^' '^^^ 12 - 3 lower limit of detection was ^ 2 x 10 atom cm . Nitric oxide was

40 also monitored in the former study at A 180 nm (1-0 transition of NO,

2o+ 2

£ - D 2, ~ X ^1/2^ ' """^ which the lower limit of sensitivity was reported

13 - 3 to be >_ 2 x 10 molecule cm

Calibration of the emitting lamp, which is normally achieved in a similar manner to that for fluorescent experiments, could be eliminated

if the f value of the electronic transition were known, and if the lamp

were totally unreversed with pure Doppler broadened lines. However, the

latter criterion is difficult to achieve and often the f values are un- known. VJhen kinetic experiments are performed under second order
12

conditions, or the s toichiometry of a reaction is being measured, then

the absolute concentration of the species being monitored is required,

and any calibration inaccuracies will be reflected in the accuracy of

the final result. Absorption experiments suffer to a greater extent

than fluorescent experiments to drift in the lamp output, but are able

29 to measure higher concentrations, which in some systems is essential

Electron Paramagnetic Resonance Spectrometry

Two kinetic studies have used e.p.r. in conjunction with the

3 discharge flow technique. The decay of ground state ( P) oxygen atoms

in the presence of excess HCl was monitored by following the intensity

45 2 of their e.p.r. spectrum . The production of ^3/2 ^^1°^^^® atoms was

observed in a qualitative manner.

46 In a study of the HO + HCl system, absolute concentrations of

2 2 both CI ( ^3^2^ ^ were required in order to analyse the kinetic

data. Absolute concentrations were obtained by double integration of

47 their spectra, and by using published e.p.r. transition probabilities

Consequently, an error in the calibration would lead to an error in the

value of the calculated rate constant. The lower limit for detection of

11 -3 ... HO radicals was reported to be 2-5 x 10 molecule cm . The sensitivity

of this technique for monitoring atoms is similar to that of atomic

resonance absorption.

Infrared Spectroscopy

Infrared spectroscopy has been used with the molecular modulation

technique to identify the ClOO free radical as an intermediate in the

photolysis of CI2/O2 mixtures, and to obtain a value for a product of

2 6 rate constants . Kinetic data obtained from this system must be
13

analysed in terms of a large complexity of interacting reactions. Thus,

the reported rate constants rely upon the analysis using the correct

matrix of reactions, and simplyfying assumptions. However, as I.R.

absorption cross sections are normally lower than UV absorption crosssections the technique is rarely used for kinetic experiments, except

with the molecular modulation technique which uses long optical path-

-4

lengths and is capable of measuring small modulation amplitudes (10

lO"^) .

Miscellaneous Techniques

Some early studies were performed whereby reaction mechanisms and

rate constants were determined from somewhat limited and indirect ex- perimental data such as: pressure change as a function of extent of

414 2 4 3 reaction ' ; the chemical analysis of unreacted reagents or reaction

44 products from a competitive study with a known reference rate constant.

These studies involved complex kinetic schemes and the results must be

regarded as inferior to the more recent direct determinations.

Errors and Uncertainty Limits

The difficulty in assigning reliability limits to the rate data is

that the preferred value is normally a single measurement. Although

several reactions have been studied more often, many of the determinations

can be eliminated due to their indirectness (subject to an accumulation

of errors) or incorrect data analysis. Besides the error limit placed

on the reported rate constant by the original workers, it is difficult

for the writer to arbitrarily increase this error limit. However, a few

comments on systematic errors are included to show that ~ ±25% is probably

the upper limit to place on single value determinations. Five recent
14

3 investigations of the 0( P) + NO^ reaction have been performed using a wide variety of techniques, and tne reported rate constants v/ere all

within 10% of each other, which strongly suggests that systematic errors

51-54

are not normally significant . Most of the preferred values which

was obtained from single m.easurements were performed using the discharge

flov/ technique.

a. Errors in discharge flow systems

The fundamental measurements in a discharge flow system are: P^,

total flow tube pressure; T, temperature; A, cross-sectional area of the

flow tube; and f^, the flow rates of both carrier and reagent gases.

It is possible that errors in these measurements could accumulate and

cause an error of 10% in the measured rate constants. Systematic errors

may also arise if, (1) the gas flow profile is laminar, rather then plug,

and (2) axial diffusion is significant. As the rate of radial diffusion

increases, the flow velocity profile tends towards that of plug flow,

4 8 but the problem of back diffusion increases. A recent paper has

calculated the magnitude of the possible errors introduced into the

determination of rate constants if the flow velocity profile is laminar,

and plug flow has been assumed. The magnitude of the error is dependant

upon the type of observation technique used; flow tube dimensions, total

pressure, and the first order rate constant. Calculations were performed to show that the errors caused by this effect v/ere always less than

52930 49 50 10% ' ' , in agreement with earlier views ' that the errors caused

in assuming plug flow were small. In flow systems with numerous inlet

jets, the formation of laminar flow is unlikely due to the jets acting

as centers of turbulence. The effect of back diffusion is to under-
15

estimate the true rate constant, and the following expression shows the

relationship between the true (k) and measured (k') rate constants:

k,k' are 1st order rate constants

k = k' (1 + ^1^) U - flow velocity

U

D = diffusion coefficient

The magnitude of the errors introduced by neglecting back diffusion can

be seen to be greatest under slow flow conditions, but are still normally

< 10%. The last two effects both underestimate the true rate constant

and so it is conceivable, but unlikely that they may add to lead to an error of -25%.

b. Errors in flash photolysis systems

The fundamental measurements in a flash photolysis system are: t,

time; T, temperature; and , the reagent and diluent gas pressures.

The largest systematic errors present in early flash photolysis systems

stemmed from (1) the pressure measurements, and (2) complicating secondary

reactions which limited the accuracy of the data analysis. Since the

advent of accurate low pressure capacitance manometers, the accuracy of

pressure measurements has improved considerably and the errors are now

<_ (1-3)%. Previous discussion has shown that the use of modern detection techniques,

.such as atomic resonance fluorescence, has minimized the effect of secondary

reactions in the systems studied. Diffusion of species

out of the reaction zone can be accurately determined and should not

lead to an error in the data analysis of > 5%. Consequently, it is felt

that systematic errors should not exceed 10% in total.

Units

Rate constants for bimolecular reactions are expressed in units of 3-1-1 6 "2

cm molecule s , and those for termolecular reactions in cm molecule
16

s . The expression used for a rate constant as a function of temperature is k = A exp(-C/T) where C = E/R, E being the "activation energy",

and R the gas constant. To obtain E in kJ mol multiply C by 0. 008314

and to obtain E in kcal mol , multiply C by 0.001987. Absorption cross

2 -1

sections are given in units of cm' molecule

isiumbered Reactions

The numbered rate constants referred to in the introduction are keyed to the Table of Contents. Tables I list the reactions

studied under the various techniques used. An asterisk denotes a study

in which the result is used by the writer to forward a preferred value.

Acknowledgements

I am indebted to 14. A. A. Clyne for many discussions on the chemistry

of CIO , and to H. S. Johnston for nis useful advice.

This work was supported by the Climatic Impact Assessment Program

by means of an interagency agreement between the Department of

U.S.

Transportation and the/Atomic Energy Commission.
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19

Compilation of Rate and Photochemical Data

2 Reactions of Ground State ( '^ 2/2 ^ Chlorine Atoms.

2 CI ( ^ 2/2 ^^^^''^^Q^s

(1) ci(^P

3^,^ ) + O3-— cio(^n) + 0

^ *Clyne and Watson, 1974^°. (1.8^±0.3g) x lO"^"'- 300 K (a)

Clyne and Coxon"^^ ,

~ 19 6 8 . > 6.7 x

10~"^^ 300 K

(a) Preferred value. Obtained using psuedo 1st order conditions.

(2) CK^P ^^,^

) + OCIO--^ 2 cio(^n)

*Bemand, Clyne and V^atson^, (5.9±0.9) x 10 x

1973, exp(0±120/T) (298-588)K (b)

Bemand, Clyne and Watson^, (6.1±0.9) x 10 29 8 K (b)

1973.

Benand, Clyne and Watson^, (5.9 + 0.9) x lO""^"^ 298 K (b)

1973.

Basco and Dogra^^^, 1971a. (8.5±1.2) x lO""""^ 300 K

Clyne and Coxon""-^, 1968. > 1.0 x lO""^^ 300 K

(b) The preferred value was obtained by combining the results

of three separate sets of experimental determinations,

using different techniques.

^3

(3) CI + 0 ^ + II —^ ClOO + M

-34 Stedman in "Clyne and 5.6 x 10 (200-300)K

Coxonll, 1968.

Clyne and Coxon^^, 1968. < 5.6 x lO"-^^ 300 K

Nicholas and Norrish^^, 1.7 x lO"^"^ 300 K (c)

1968.

(c) The authors used several kinetic assumptions in their calculations, which if incorrect would lead to their value for

to have been landerestiraated.

*Preferred value.
r 20

(4) CI (^P 3/2-

+ CH. CH -, + HCl

Clyne and VJalker 14 1973. 5. 1 X lO"-'--'- X

exp (-1790±37/T)

.-13

Davis, Braun and Bass

44

34

Knox and Nelson 1959,

(1.3±0.1) X 10

1970. (1.5±0.1) X 10

4 X 10~^^ X

-13

Pritchard, Pyke and

Trotnan-Dickenson^ 3 , 1954

exp (-1940/T)

4.6 X 10~^^

exp (-1920/T)

(300-686)K

300 K

298 K

(193-593)K

(293-488)K

(d) The preferred value is based on the average value of k^ at

29 8K, and the activation energy reported by Clyne and

Walker. Therefore,

k.* - (5.6+0.9) X 10~^^ X exp (-1790±37/T)

k/ (300) = (1.4±0. 1) X 10

(d') These values were not considered as they were determined

relative to k(Cl+H2) and differ by a factor of 2 from the

other results at 298K.

Cl( + HCl + H

Hertel

81

1931.

Potts and Rollefson82

1935

83 Rodebush and Klingelhoefer , 1933

84 Steiner and Rideal ^1939

E,a (24.3 ± 4 .2) kj 1-1 mol -300 K

E;a ~ (18.9 ± 4 .2) kj mol

1 <170 K

(24.3 + 4 2) kJ T -1 mol >200 K

1 .37 X 10-1^ 298 K (e 1)

5 .5 X 10-1^ 273 K (e 1)

1 .08 X 10-1^ 298 K (e 2)

4 .1 X 10-15 273 K (e 2)

1 .17 X 10-11 1071 K (e 3)

8 .67 X 10-12 1013 K (e 3)

7 .67 X 10-12 960 K (e 3)

6,.00 X 10-12 901 K (e 3)

5,.90 X 10-12 1071 K (e 4)

4,,57 X 10-12 1013 K (e 4)

4..07 X 10-12 960 K (e 4)

3 ,,27 X 10-12 901 K (e 4)

(d)

(d)

(d-)

(d )
21

8 5 Ashmore and Chanmugam , 1953 8.0 X 10

-13 523 K (e 5)

Fettis and Knox^^,1964. (1.38 + 0.1) X 10

exp(-2750 + 70/T)

-10

X (273-1071)K (e 6)

(1.4 ± 0.1) X 10

-14

298 K

87

Clyne and Stedman ,1966.

-13

1.47 X 10 373 K (e 7)

-14

1.95 X 10 294 K Ce 7)

1.29 X

10"!'^ 273 K (e 7) 7.0 X 10"^^ 195 K (e 7)

88 Westenberg and de-Haas , 1968. 2.0 X 10"-'"^exp(- 2160/T) (251-456)K (e 8)

-14

1.35 X 10 298 K

"Benson, Cruickshank and Shaw , (8.0 ± 2.0) X 10

-11 X (479-610)K (e 9)

1969.

34

"Davis, Bass and Braun ,1970

90 Galante and Gislason ,1973.

14 Clyne and Walker ,1973,

exp(-2655 ± 200/T)

(1.1 ± 0.2) x 10

14

(1.4 + 0.1) X 10

REVIEW

-14

11-

(3.7 ± 0.6) X 10 "X

exp(-2146 ± 51/T)

2.76 X 10"1^

(5.6 ± 1.2) X lO'^^x

exp(-2254 + 69/T)

,-14 2.95 x 10

298 K

298 K (e 10)

(e 11)

(195-610)K (e 12)

298 K

(195-496)K (e 13)

298 K

(e 1). These results should have low or zero weighting when calculating a best

value for k^^ due to the inaccurate manner used to determine [Cl], and due to the

observed dependence of k^^ upon the history of the reactor surfaces. These values

were calculated by Fettis and Knox-,86 from the reaction probabilities reported in

reference 83.
22

fe 2) . Same data as in (e 1) . These values were calculated by Benson

et al^89. See remarks in (e 1).

(e 3) . These results should have low weighting due to the uncertainty

regarding the effect of the presence of molecular (91). The values of k

(H + HCl) reported by Steiner and Rideal^^ have been recalculated using more recent thermodynamic data to yield values for k^^.

(e 4) . Same data as in (e 3) . These values are those given by Benson et al (89)

.

The values of k.^^ reported in (84) were recalculated using more recent thermodynamic data and lowered by a factor of 2 based on the report (91) that the

presence of molecular oxygen alters the rate of hydrogen exchange.

(e 5) . The chemical system is complex and numerous simply lying assumptions

were made. However, recent kinetic measurements justify most of these assumptions.

Their value of k^^ depends only upon the equilibrium constant for k(Cl2 + NO) /k(Cl + ClNO),

and not upon Burns and Dainton's value of k(Cl + ClNO) which they used, and is

not believed (by the writer) to be incorrect. Consequently their value may be

approximately (± 20%) correct.

(e 6) . A review of references (80, 81, 82). Their interpretation of the

published data is shown in the table.

(e 7). Calculated by Benson et. al. (89) using the reported values of k_^^

in conjunction with the thermodynamic values of Kgq • These values predict that

the activation energy increases with temperature, which was not observed over the same temperature range in reference 88. The published values of k_^^ are in fair

(± 307o) agreement with those published for k_^^ in reference 88. No obvious

systematic error is noticable but these results are at variance with the direct

determinations of k^^ (especially reference 34)

.

(e 8.) The Arrhenius expression is shown for the study which directly measured

• '^-4A also reported but the results are not shown. Kgq (exp) was found

to be a factor of 2-3 lower than that calculated from thermodynamic data. The

value at 298 K for k^^ is in excellent agreement with reference 34, but this is

fortuitous in that their measurement of k^^ should be seriously underestimated due

to the regeneration of Cl via the rapid H + CI2 reaction. Consequently this result

should not be considered. The back reaction should not suffer from such error and

could predict a much higher value (factor of ~2.5) for k^^.

(e 9.) The chemical system was somewhat complex, but the study should not

suffer from any serious error. Benson et. al. also reviewed earlier data and

concluded that their Arrhenius expression was unchanged when including this data.

However, the reliability of the earlier data is questionable.
23

(e 10) . This study performed at 298 K should be the only study free from

any complicating secondary reactions. Psuedo l^t order conditions were used,

and the absence of complications was well verified.

(e 11) . This review judged that the data from the direct studies of k^^ were accurate, whilst those (87,88) measured for were in error by a factor

of 2. The explanation forwarded for the factor of 2 is certainly questionable

with no direct evidence to support it. However the conclusion that it is inaccurate

data which results in K^^ (calculated) ^k^^/k.^y^ is valid.

(e 12). This rate constant expression includes data from references (87,

88 (k_^^) 89 and 34), but rejects the determination of k^^ from reference 88.

(e 13). This rate constant expression is based on data from references

(87, 88 (k_^^) and 34). However, the weighting given to reference 34 now seems

too low.

The writer feels that only the data in reference 34 are free from error. Several

other studies support this room temperature value. However there is disagreement

about the temperature dependence of k^^ and it is not clear how to resolve the

matter. For the present the expression of Benson et. al. (89) should be

used when values are needed over a range of temperatures. Reference (34 and 89) are essentially in agreement at room temperature. For model calculations the

expressions given by Clyne and Walker (14) may be used to test the sensitivity of

the model to the value of k, . .

(5) CK^P, ,^ ) + ClOO -j Cl ^ + 0^

Johnston, Morris and 1.56 x lo""'''^ 298 K

Van den Bogaerde26, 1969.

Johnston, Morris and ^^Af- = 10 8 29 8 K

Van den Bogaerde^S, 1969.

2 2 Nicholas and Norrish , k^/k^ = 15

1968. 5 6

(f) This value was obtained from the analysis of a complex reaction

system,, and is dependent upon the value used for AK^rClOO, which

IS not well established. ^

(g) These two values are markedly different. The value of 15 is in

agreement v/ith a value that can be derived from a study by

Porter and Wright (1953).

(f)

(g)

(g)
24

(6) CK^P -,^,^ ) + CIQO—^ 2 CIO

-12 Johnston, Morris and 1.44x10 298K

Van den Bogaerde^S, 1969.

(h) This value was obtained from the analysis of complex reaction

scheme, and was obtained relative to k5. Therefore, both k5

and the ratio k^/'k.^ have to be accurately known to obtain a value for k^. Although the ratio ks/kg v/as not dependent upon the

value of AHfClOO, the kinetic analysis for ks required AHf ClOO to

be known. ^

(7) CK^P ,^,^ ) + Cl

^ O Cl .^ + CIO

on — 1 ^ *Basco and Dogra , 1971b. 6.8 x 10 300 K

Edgecoralje, Norrish and > 6.7 x 10 "'"^ 300 K

Thrush23^ 1957.

(8) CK^P

^^,^

) + NO + —^ CIHO +

Clark, Clyne and Stedman'^, (9.7±1.4) x lO""^^ 293 K (i)

1966 .

7 Clark, Clyne and StedmanS Ea (M=Ar)=: - (4 . 6± . 4 ) kJ 471 K

1966. ,-1 mol

Ashmore and Spencer'^"'", 1959. (2.1±0.6) x 10~^^ 471 K

(i) Clyne and Stedman (1968), reported that this value may be too

low due to a systematic error in their method of monitoring

[CI] , as it has been shown that the intensity of the chlorine

afterglow is not always proportional to [CI] 2. Therefore, kg

and kg have probably been underestimated by ~ 15%, and this

leads to a preferred value for k-* of (1.1±0.2) x 10~31.

o

(9) CK^P

^^,^ ) + NO + O p CINO + 0

^ Clark, Clyne and Stedman"^, (l.l±.l.) x

10~^"^ 293 K

1966 . * See note (i) above. The preferred value for k is

(1.3±0.3) X 10-31. y
25

2 10

(10) Cl( P ,^^-, ) + CINQ —=^ NO + Cl -^

*Clyne and Cruse"^^, 1972. (3. 0 + 0 . 5) x lO""'--'- 298 K (j)

40 -12

Clyne, Cruse and Watson , >_ 8 x 10 29 8 K

1972.

Burns and Dainton^^, 1952. (1. 9±1.0) x lo""""""" x 298-328 K

exp (-533±160/T)

(j) This direct determination of k]_Q at 298K is preferred to the

study of Burns and Dainton, where several systematic errors

were likely.

2 ^11

(11) CI ( ^ 2/2 ^ ^'^'^ 2 M—^ CINQ ^ + M

*Clyne and White"^°, 1974 . 7. 2 x lO"^""" 298 K (k)

(k) Provisional data, subject to revision

]^ (12) CK^P

^^,^

) + CINO ^ —^ Cl ^ + NQ

^

1 0

' Clyne and VJhite , 1974 . ^12^^11 ^ ^ 1 298 K (1)

(1) k]_2 is greater than the second order comcponent of k^-j^ at low

pressure (1 torr) . Therefore, k^^ > > 3 x 10"-'-^.

(13) CK^P

^^,^

) + CK^P .,^,^ ) + Ar Cl ^ + Ar

8 - 34 Clyne and Stedman , 1968. 5.6 x 10 x (195-500)K (m)

exp (910±350/T)

(1.2±0.1^) x 10~^^ 298 K

Hutton and VJright^ , 1965. (1.2±0.2) x 10~^^ 298 K (m)

Bader and Ogryzlo^, 1964. 1.1 x lo""^^ 313 K (m)

*Lloyd,^''" review. 6.0 x 10 '^'^ x exp (900±250/T) (200-500)K

(m) All three determinations of k]_3 are in agreement (±10%) at

298K. The efficiency of N2 as a third body should not be

too dissimilar from Ar. -d[Cl]/dt = 2kj_^[Cl]2 [Ar] .
26

II.

2 Reactions producing ground state ( P ^^^

) Chlorine atoms, and/

or involving CIO radicals.

(14) 0(^P) + CIO—^- CK^P -,^,^ ) + 0 ^ (^A

*Bemand, Clyne and Watson,^ (5.3 + 0.8) x lO""^"'' 298 K (n)

1973.

Bemand, Clyne and Watson^, (5.7±2.3) x lO"^"*- 298 K (o)

1973.

Basco and Dogra"^^, 1971a. 1.2 x lO""^""" 300 K

TO — 1 1 Clyne and Coxon , 1966a. > 1.0 x 10 300 K

Freeman and Phillips^^, > 1.3 x lo""'""'" 300 K

196 8.

(n) This value is preferred due to the direct nature and the

high specific sensitivity of the study, which utilized atomic

resonance fluorescence to monitor atomic oxygen.

(o) This value was obtained using molecular beam mass spectrometry

and is in good agreement with the preferred value.

]^ (15) NO(^n) + cio(^n)--^ No ^ + ck^p ^^,^)

*Clyne and Watson^°, 1974. (1.7±0.2) x 10~^^ 298 K (p)

Coxon"""^, 1968. > 3 x lO""""-^ 300 K

(p) A direct determination of the rate constant, the value was

reported in "Bemand, Clyne and Watson (1973)" - to be

published by Clyne and VJatson in J.C.S. Faraday I.

k

(16) cio(^n) + CO—

^ co^ + cK^p ,^,^)

*Walker^'^, 1972. 1. 7 x lO"-""^ 587 K

Harker^"^, 1972. 1. 4 x lO""*-^ 300 K (q)

(q) This value is rejected due to its indirect determination

from a complex reaction scheme.
27

^17

(17) CIO + 0 -, ^ ,0C1Q +

jll7 ClOO +

Clyne and Coxon"^^, 1968. £ 5 x lO""*"^ 300 K

29 -15

Clyne, McKenny, Watson , £ 5 x 10 29 8 K (r)

1974.

(r) Provisional data subject to revision. The rate constants

reported do not differentiate between the two reaction paths.

^1 o (18) CIO + —^

Products

Walker^"^, 1972. < 8 x 10~^^ 670 K (s)

Clyne and Coxon"""^, 1968. 1 5 x lO""""^ 300 K

^19

(19) CIO + CH ^ —^ Products

Walker-^^, 1972. ^ 4 x lO"""-^ 670 K (s)

^20

(20) CIO + C ^H^ —^

Products

VValker^"^, 1972. < 5 x lO"^^ 670 K (s)

(21) CIO + C -,H^ —^Products

Walker^^, 1972. < 5 x lO""""^ 670 K (s)

(22) CIO + U

^ O —^

Products

Walker""-"^, 1972. 1 7 x lO""""^ 587 K (s)

(23) CIO + NH ., Products

Walker""-"^, 1972. < 5 x lO"^^ 670 K (s)

(s) These values were directly determined using the discharge

flow technique coupled to UV absorption spectrophotometry

and/or a line of sight mass spectrometer.
28

(24) CIO + CIO

"24 2 Cl( P .,^,^ ) + OCIO

Clyne, McKenny and

Watson29 1974.

< 2.2 X 10

•15

298 K (t)

(25)

(t) This is a provisional experimental upper limit. Using the

thermodynamic values reported by Domalski et.al.^S, 1974

in conjunction with the preferred value for k2 (CI + OCIO ^

2 CIO) , the following can be calculated. log]_o = -15.05±0.55. The limits are due to the uncertainty in the

value of AH^298 0^10. -d[C10]/dt = k24[C10]2.

^25'

^ t

> ci +0 CIO + CIO 2 2 25" t CI + ClOO'

24 Porter and Wright , 1953.

1.52 X lO"-"-^ X

exp (0±325/T)

293- 433 K (u)

J-J -L 0 iLU^ f i.NLJJ_J__Loii CXI L LA Thrushes, 19 56.

-13

1 9 3 V in

3.39 X 10-13

300 K

300 K

(u) (v)

(u) (w)

Edgecombe, Norrish and

Thrush23, 1957.

-14

6.57 X 10 300 K (u)

*Clyne and Coxon"'"^, 1966 . -14

2.33 X 10 300 K (u)

*Clyne and Coxon"""^, 1968. (1.2±0.3) X 10"^^

exp (-1260+150/T)

X

(294- 495) K

(u)

Clyne and Coxon"''^, 1968. (1.0±0.3) X 10~^^ X

exp (-1215±160/T) (294- 495) K (x)

Johnston, Morris and

Van den Bogaerde^^^ 1969.

-14

2.6 x 10 298 K (u) (y)

*Clyne and White^^, 1971. (1.3±0.1) X 10~^^ X

exp (-1150+50/T) (273-710)K (u) (x)

19 20 *Basco and Dogra ' , 1971a, b.

-14

4.4 X 10 300 K

*Walker-^'^, 1972- (1.9±0.6) X 10"^^

exp (-1300+120/T)

X (u)

*Clyne, McKenny and

Watson29, 1974.

-14 2.25 X 10 300 K (z)

t The high pressure (> 75 torr) results indicate that all the

reaction proceeds via channel 25, whilst the low pressure (1-3

torr) results indicate that the ratio k25'/k25" - unity.
2y

15

(u) Clyne and Coxon ' s (1968) value for the absorption cross

section was used to calculate k^^.

(v) k;25 was reported to be flash energy dependent, due to the

reaction mechanism being misinterpreted. Low flash energy.

(w) High flash energy.

56

(x) Clyne and White (1971) reanalysed the data to allow for any

third order decay of CIO.

(y) This value derived by combining their value of with the thermodynamic value of K2 - 109 = k6/0.5 k25. Neither kf: nor K2 xs accurately knov/n . (The value for = 22 7 in reference 26 was calculated incorrectly). Two likely values for K2 may be derived: 109 and 661, corresponding, respectively to AHf (C£00) = 86 and 96 kJ mol"!. See also remarks reaction 30.

(z) This is the only published study not to use an optical

spectroscopic method.

The preferred value for k„j- at 298 K is: -d[C10]/dt = k^ [ClO]^.

2 5 -14

(a) * (2.4±0.4) X 10 (low pressure)

-14

(b) ** 4.6 X 10 (high pressure)

The activation energy (for the overall process, which cannot be

described by one simple initial step) is:

(a) (10.3±1.0) kJ mol (low pressure)

(26) CIO + CIO + Cl ^ + 0 ^ + M

Johnston, Morris and 1.0 x

10~^"^ (0^) 298 K (A)

Van den Bogaerde26, 1969. ^ ^^-32 ^^^^ 298 K (A)

Wu^"^, 1970. k„,/k„^ = (B)

-17

1.1 X 10

Walker-"-"^, 1972. (2. 0±2.0) x 10~^^ (Ar) 298 K (C)(A)

(A) This value of k„, is defined as: -d[C10]/dt = k2g[C10]2 [M] .

(B) This value is only accurate to within a factor of two.

(C) This value was derived using data from; Clyne and Coxon

(1968); Clyne and White (1971) and Walker (1972). The

slope of -d[C10]/dt versus [M] , was positive but within

the error limits of being zero.
30

(27) 0 + CI, CIO + CK^P ^^,^)

Clyne and Coxon 32 1966b, (9.3±2.2) X 10 -"-^ X

exp (-1560±50/T)

-14

5.1 X 10

Niki and Weinstock 31 1967, (7.5±0.6) X 10

-14

(174-396)K

300 K

300 K

(D) These two studies, both performed using the discharge flow

technique, but with different detection techniques, are in

. - fair agreement at 300 K. Therefore, the preferred value

would be an average of the two at 300 K, and equal to 6.3 x

- 10-14.

k

(28) HO + HCl

Takacs and Glass

^^0 + C1(^P3^,1

46 1973.

Anderson, Zahniser and

Kaufman36^ 1974.

Smith and Zellner 39 1974,

Wilson, 0' Donovan, and

Fristrom^O, 1969.

Wong and Belles 58 1972

(6.4±1.5) X 10

2.0 X lO"-"-^ X

exp (-313/T)

-13

6.9 X 10

4.1 X 10"^^ X

exp (-500/T)

-13

7.5 X 10

1.3 X lO"-^-"-

Estimated E =

-13 295 K

(225-460)K

295 K

(210-460)

K

295 K

(1920-1940)

K

21 kJ mo

1

1'

(D)

(D)

(E)

(E)

(E)

(E' )

(F)

(E) There is good agreement between these three groups of workers

at 295K, and the preferred value at this temperature is an average of the three, and equal to (6.9±0.6) x lO"!-^, The

preferred Arrhenius expression for the (220-300) K temperature

range is:

k* = 2.8 x 10~^^ exp (-400±100/T)

Z o This yields values which are the average of references (36)

and (39).

(E') This value is greater than would be predicted at 1920 K from

the Arrhenius expressions of references (36) and (39).

(F) This value for the activation energy was estimated from the

production of H^O in the 0 + HCl study.
31

2 (29) 0 + HCl OH + Cl( P

j /p).

-12

Balakhnin, Egorov and (1.75±0.6) x 10 x Intezarova^^, 1971. exp(-2250/T) (295-371)K (G)

Wong and Belles^^, 1972. (1.9±0.27) x 10"^^x

exp (-3584±70/T) (356-628)K (G)

(G) These two results are at variance with each other and thus no preferred value is given.

(30) ClOO + M-—-V Cl( P 3^,o) + Op +

There is no experimental determination of k3o, but a value can be

calculated from the thermodynamic value of K2* = k3/k30 and from

the experimental value of k3. However, neither K2* nor k3 are particularly well knov/n. The value for AHf298 ClOO is possibly not

better known than (89 t5)kJ mol-K^O), However, if AHf298 ClOO is 84.2

kJ mol~l, as has been reported'''^, then the following values for K2*

can be calculated. This does not mean that the writer believes

that this is the best value for Allf ClOO, but this value is a lower

limit and calculations using this value v/ill predict the maximum

possible ratio of [ClOO] to [CI]

.

Elevation T (K) K

* (atm"^) ^30^^^ ^30^^^

0 298 9.3 1.5 X 10-15 4.5 X 10-1^

18 216.7 3007 6.3 X 10-" 1.9 X IQ-l^

23 219.6 2279 8.2 X 10-" 2.5 X 10-1^

28 224.5 1439 1.3 X 10-" 3.9 X 10-^^

33 231.1 823 2 . 2 X 10-" 6.6 X

10-^-^

38 244.8 279 6.0 X 10-" 1.8 X 10-1^

43 258.6 105 1. 5 X 10-" 4.6 X 10-1^

48 270.7 49 3.1 X 10-" 9.5 X 10-1^

10-^4

k-5 was taken to be temperature independent and equal to 5.6 X

Reference 11.

-33

(b) k3 was taken to be temperature independent and equal to 1.7 x 10

Reference 22.
32

XII. Reactions of OCIO and Cl

^ Q which involve CIO radicals

]^ (31) 0(^P) + OCIO-^ CIO + 0 ("^e")

r. ' *Bemand, Clyne and Watson^, (5^^) x lO""'"'^ 298 K (H)

1973.

Basco and Dogra^"^, 1971 5.0 x lO""^"^ 300 K

Clyne and Coxon'''^, 1966a. > 4.0 x lO""'""^ 300 K

(H) This value was determined from two separate direct studies.

-13

(a) mass spectrom.etry : -(4.7 + 1.6) x 10

- 1

3

. -

-

'

^ (b) atomxc resonance fluorescence: -(6.3±1.9) x 10

(32) NO + OCIO —^ NO ^ + CIO

*Bemand, Clyne and Watson^, (3.4±0.5) x lO""*"^ 298 K

1973.

Coxon/^ 1968. > 8.5 x lo"^-^ 300 K

k

(33) N(^S) + OCIO --^ NO + CIO

; - c;q ... • ' _ I

-5

, Watson , 1973. <_ 6 x 10 298 K (I)

(I) A preliminary experimental value, no detailed results.

(34) H(^S) + OCIO —^

OH + CIO

*Bemand, Clyne and Watson^, (5.7±1.2) x 10 29 8 K

1973.

(35) 0 + Cl

^ O CIO + CIO

*Freeman and Phillips^^, (1.410.2^) x lO""'"''' 300 K

1968,
33

IV Optical Absorption Cross Sections

r o (36) CI2 + hv—>2 CI (Seery and Britton , 1964).

Cl^ + hv~> CK^P,/^) + CK^P^/t); X < 483. 0 nm

A (nra)

21 2 — 1 10 X a (cm molecule ) A (nra)21 2 — 1 10 X a (cm molecule ) 240 0.8 350 189

250 1.2 360 131

260 2.3 370 83

270 8.8 380 49

2 80 27 390 33

290 65 400 19

300 120 410 13

310 185 420 9.9

320 236 430 7.3

330 256 440 5.3

340 236 450 3.4

(a) The authors reported that each measurement was only accurate to

within ±2.0 x 10~21 cm^ molecule-1.
34

(37 ) The Absorption Spectrum of HCl

74 *Romand, J. , 1949.

6 3 *Romand, J., Vodar, B. , 1948.

See figure 1.

HCl(^Z^) + hv ^ H( S-^^^) + CI ( P 3/2^

- H(2s3/2) + CI (^P 1/2^

A(nm) 10

2 _ X 0 (cm molecule 2 0

A (nm) 10 y a (cm molecule ) 206.8 0 . 72 172.9 105

203.5 1.6 171.3 129

201.2 1.9 166.9 186

199.0 2.8 163. 0 224

19 5.1 5.5 159. 7 - 269

19 3.1 7.8 157.9 275

, - ± J Jl * ~J ± D D . -) J J ^

189. 1 14 . 1 152.4 334

188.4 21.4 149. 5 298

183.0 31.6 146. 3 275

181.1 41.7 145. 6 224

179.2 57.5 141. 3 . 166

177. 4 64.5 139. 4 132

Also the spectrum has been reported by Myer and Samson , 1970.
35

(38) The Absorption Spectrum of ClOO

ClOO + hv C10{'^Y[^^^) + 0("'"D)

A < (267.9±3.0)nm

26 Johnston, liorris and Van den Bogaerde , 1969*. See figure 1. (a)

X 8 2 — 1 ]_ g

' 2 —

1

A (nm) 10 X a (ClOO) cm molecule A (nm) 10 x a (ClOO) cm molecule

225 2.6 255 12.4

230 4.9 260 10.0

235 7.8 265 7.3

240 10.5 270 5.1

245 12.7 275 3.4

250 13.3 230 2.3

(a) The spectral band width v/as 1.3 nm.
36

(39) The Absorption Spectrum of CIQ

C10(^n^^2, 12^^^^^ ^^^^^3/2^

A < 2 6 3.0 nm (C10(^n^^2) )

X < 265.0 nm (ClOC^n^^^) )

' cio(^n) —^

cii^p^^^) + o(^p)

CIO is known to predissociate to CK^p^^^) + o(^P) at wavelengths

> 280 nm. Durie and RamsaY^\ 1958 reported that the 7.0 band is

totally predissociated.

2 6 Johnston, Morris and Van den Bogaerde , 1969*. See figure 1. (b) __ _ __ __ _ __

A (nm) 10 X o(ClO) cm molecule A (nm) 10 x a (CIO) cm molecule

225 , .6^ 255 4.5

230 .., . . 8^ 260 5.3

235 - 1. 3

'• ' 265 5.7

240 1.9 270 5.6

24 5 2.7 275 4.9

250 3.6 280 4.7

(b) The spectral band width was 1.3 nm, and these values were placed on an absolute basis using Clyne and Coxon's-'-^ 1968 value for (CIO at

255.7 nm.
37

Porter and Wright , 1950.

Anm 10 X a (CIO) cm molecule Anm 10 x a (CIO) cm molecule

263.63 1.4 274.95 5.8

264.06 1.4 277. 16 7.2

264.58 2.2 279. 60 5.8

265.25 3.6 282.24 5.8

266. 10 3.6 285. 18 4.3

267. 12 3.6 288.40 3.6

268. 25 3.6 291. 80 3.6

269. 50 4.3 295.43 2.2

271. 11 5.0 299. 30 1.4

272. 94 5.0 30 3.4 5 0.7

(c) These values represent the absorption cross sections at the band

heads. The value at 2 7 7.16 nm was equated to that reported by

Clyne and Coxonl^^ 1968.

The vacuum ultra-violet spectrum of CIO has recently been reported

by Basco and Morse'^S, 19 73. Figures of the extinction coefficients for

the Z-X (160- 175nm) , and D^X (~ 145-160nm) transitions of CIO are shown.
38

Other Absorption Processes

(i) OCIO + hv

^

ClO(^n) + 0(^D) -—(Al ciO(^n) + 0(^P)

X < 2 76±3.nm (continuum)

a

Aj^ < 375. 3nm (predissociation)

Finkelnburg and Schumacher 1931.

6 7 Urey and Johnston , 1931.

6 8 Goodeve and Stein , 1929.

69 Coon and Ortiz , 1957.

70 Coon, DeWames and Loyd , 19 62.

e(351.5 ) = 1.14 X lO""'"'^ cm^ molecule""'". (Clyne and Coxon"'"^, 1968, nm ^

' 79 Basco and Morse , 1974, reported the extinction coefficients for

the C^X; D^X and E^X systems of OClO in the region ~145nm-184nm.

(ii) CVAO

^

CI + NO

Continuous spectruia from (-630 - < 200) nm

71 Goodeve and Katz , 1939.

(iii) Cl^O hv ^ CIO + Cl

Continuous absorption from (850-220) nm. See "Photochemistry",

Calvert & Pitts, p. 232(72).

(iv) The absorption coefficients for CFCI3 and CF2CI2 (freons)

have been measured between 200 nm and 120nm. Doucet,

Sauvagea, and Sandorfy, 1973(73), Molina and Rowland 77^ 1974

have measured the absorption coefficients between 226nm and

186nm- their data is tabulated below:
39

The Absorption Spectra of CFC1 -, and CF

^ Cl ^.

CFCl^ + hv -

CF^Cl^ + hv

CFCI2 + CI

CF^Cl + CI

Molina and Rowland 77 (1974).

Anm

,^20 2 , , -1

10 X a cm molecule

CFCl. C^2^l2

226. 0 i 0.9

1

i ~

1 223. 5

1

! 1.

j

221. 0

1

i 1.8

!

I

I

1

i 218. 6 3.4

i

216. 2

j

j 5.1 j _

i 213. 9 1 8.0 i 0.3

j 211. 6 1 12.

0.7

i 209.4 1 18.5

! 1.4

207.3 i 25.0 1

j 1.8

205.1 1 34.

i 3.

203. 0 1 46 4.9

201.0

i

1 59

1 7.2

199. 0

i

1 73

1 11. 3

197. 0 89 16. 4

195.

113 24.5

193. 2 137 34. 5

191. 4 164 49

189. 6 197 66

187. 8 227 87

186. 0 241 91
40
41

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43

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44

54. Harker, A. B. & Johnston, H. S., J. Phys. Chem, 1153, 1973.

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45

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CHEMICAL KINETICS DATA SURVEY PUBLICATIONS

Chemical Kinetics Data Survey are listed below. Several

of the Survey reports have been superseded by later issues.

They are not listed, but are referred to in the citations of

the current work.

a. "Photochemical and Rate Data for Twenty-eight Gas

Phase Reactions: A Survey of the Data," R. F. Piampson,

Ed., J. Phys. Chem. Ref. Data 2_, No. 2, 26? (1973);

These are slightly revised versions of data sheets

originally distributed as NBS Reports 10692 and 10828.

b. "Chemical Kinetics Data Survey III. Selected Rate

Constants for Chemical Reactions of Interest in

Atmospheric Chemistry," NBS Report No. 1086? (1972),

48 p. D. Garvin and L. H. Gevantman * Supersedes NBS Report 9884 (I968) which is out of

print and out of date

c. "Chemical Kinetics Data Survey V. Sixty-six

Contributed Rate and Photochemical Data Evaluations

on Ninety-four Reactions," NBSIR 73-206 (1973), 115 p.,

D. Garvin, Ed., (National Technical Information Service,

Springfield, Va. 22151, Order as COM-73-11262 $8.25 hard

copy, $1.45 microfiche)

d. "Chemical Kinetics Data Survey VI. Photochemical

and Rate Data for Twelve Gas Phase Reactions of

Interest for Stratospheric Chemistry," NBSIR 73-207

(1973), 124 p., R. F. Hampson, Ed ., (National

Technical Information Service, Springfield, Va. 22151,

Order as COM-73-11862 $4.50 hard copy, $1 . 45 ' microfiche

)

e. "Chemical Kinetics Data Survey VII. Tables of Rate

and Photochemical Data for Modelling of the Stratosphere

(Revised)," NBSIR 74-430 (1974), 101 p., D. Garvin and

R. F. Hampson, Ed., (National Technical Information

Service, Springfield, Va. 22151, Order as COM-74-10724

,

$8.25 hard copy, $1.45 microfiche); Supersedes Part IV

NBSIR 73-203 (1973)

f. "Chemical Kinetics Data Survey VIII. Rate Constants of

ClO^ of Atmospheric Interest," NBSIR 74-516 (1974), 45 p.,

R. T. VJatson

^5 Available from: Chemical Kinetics Information Center

NBS, Bldg. 222, A-I58, Washington, D. C. 20234

NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 74-5162. Gov't Accession

No.

3. Recipient's Acces.sion No.

4. TITLE AND SUBTITLE

Chemical Kinetics Data Survey

VIII. Rate Constants of CiO^ of Atmospheric Interest

5. Publication Date June 1974

6. Performing Orgsniziition Code

Dr. R. T. Watson California, Berkeley, Calif.

8. Performing Organ. Report No. NBSIR 74-516

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 202^4

10. Project/Task/Work Unit No. 3 160409

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Department of Transportation

Washington, D. C. 20590

National Bureau of Standards, Wash, D. C. 20234

13. Type of Report & Period

ov & re d Interim

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) The quantitative data on reactions of chlorine atoms and the chlorine oxides

are compiled. Preferred values for rate constants are given where possible.

Optical absorption cross sections are given for the species that may photolyze

in the atmosphere. The experimental techniques used to obtain the rate data

are discussed.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Absorption cross sections; atmospheric chemistry; chemical

kinetics; chlorine, chlorine dioxide; chlorine monoxide; data evaluation; gas phase;

optical; rate constants; stratospheric chemistry

18. AVAILABILITY [x^ Unlimited

1 For Official Distribution. Do Not Release to NTIS

1 Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. CM

r

• Order From National Technical Information Service (NTIS)

Springfield, Virginia 221'51

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

45

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-OC 29042-P74