Air pollution research report 40 - Organic peroxy radicals
172 Pages
English
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Air pollution research report 40 - Organic peroxy radicals

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172 Pages
English

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Kinetics, spectroscopy and tropospheric chemistry
Environment policy and protection of the environment
Environmental research

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ISSN 101B-5593
COMMISSION OF THE EUROPEAN COMMUNITIES
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Kinetics; spèctroseòpy and
I tropösphéric chemistry Commission of the European Communities
environment and
quality of life
Air pollution research report 40
ORGANIC PEROXY RADICALS
Kinetics, spectroscopy and tropospheric chemistry
1 2 3 1 P. D. Lightfoot, R. A. Cox, J. N. Crowley, M. Destriau,
4 4 3 5 G. D. Hayman, M. E. Jenkin, G. K. Moortgat, F. Zabel
1 Laboratoire de photophysique et photochimie moléculaire
Université de Bordeaux I
F-33405 Talence Cedex
2 Marine and Atmospheric Science Directorate
NERC HQ, Polaris House
North Star Avenue
Swindon SN2 1 EU
United Kingdom
3 Abt. Chemie der Atmosphäre
Max-Planck-Institut für Chemie
Postfach 3060
D-W-6500 Mainz
« AEA
Environment and Energy
Building B551
Harwell Laboratory
Didcot, Oxon 0X11 ORA
United Kingdom
5 Bergische Universität
Physikalische Chemie
Gauss-Straße 20
D-W-5600 Wuppertal 1
Directorate-General
Science, Research and Development
PARI ÉURÖP. ßibiioth. Environment research programme
N.C. EUR 14502 EN 1993
CM Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General XIII
Information Technologies and Industries, and Telecommunications
L-2920 Luxembourg
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting
on behalf of then is responsible for the use which might be made of
the following information.
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1993
ISBN 92-826-5681-0
© ECSC-EEC-EAEC, Brussels • Luxembourg, 1993
Printed in Belgium CONTENTS
Organic Peroxy Radicals: Kinetics, Spectroscopy and Tropospheric Chemistry
P.D. Lightfoot, R.A. Cox, J.N. Crowley, M. Destriau,
G.D. Hayman, M.E. Jenkin, G.K. Moortgat and F. Zabel
Preface v
I. Introduction 2
II. Laboratory experimental techniques 4
II. A. Methods of generating peroxy radicals S
II. A. 1. Generation of the organic fragment in the presence of Cs 5
II. A.2. Peroxy radical production in the absence of 0 10 2
II.B. Methods of detection 11
II.B.1. Peroxy radicals
II.B.2. Reaction products2
IET. Structure, spectroscopy and thermochemistry
m.A. Structure
EI. A.. Geometry of the ground state
ni.A.2. Electronic energy levels
and photolytic dissociation of peroxy radicals IS
m.B. Electronic (u.V.) spectroscopy 16
m.B.1. H08 2
m.B.2. Alkylperoxy radicals 20
III.B.3. Acylperoxys 32
m.B.4. Oxygen-substituted peroxy radicals 3
m.B.5. Halogen-substituted peroxys7
ni.B.6. Some comments regarding peroxy radical u.v. spectra 44
in.C. Near-i.r., i.r. and e.s.r. spectroscopy 46
III.C.l. Electronic (near i.r.) spectroscopy
ffl.C.2. Vibrational spectroscopy
ni.C.3. Electron spin resonance spectroscopy 51
ULD. Thermochemistry of peroxy radicals4
m.D.l. Alkylperoxy radicals 55
m.D.2. Acylperoxys6
m.D.3. Oxygen-substituted peroxy radicals7
m.D.4. Halogen-substituted peroxys
III.D.5. Unsaturated peroxy radicals8
III.D.6. Thermochemical parameters for the 0-(C)(0*) group 60
III.D.7. Summary 61
IV. Reactions of peroxy radicals
IV. A . Reactions between peroxy radicals
IV.A. 1. Alkylperoxy radicals6
IV.A.2. Acylperoxys 7
IV.A.3. Oxygen-substituted peroxy radicals 7IV.A.4. Halogen-substituted peroxy radicals 79
IV.A.5. Discussion 82
IV. B. Reactions of peroxy radicals with H0 87 2
IV.B.l. Alkylperoxys
IV.B.2. Acylperoxy radicals 91
IV.B.3. Oxygen-substituted peroxy radicals
IV.B.4. Discussion4
. IV.C. Reactions of peroxy radicals with NO
IV. C. 1. Alkylperoxy radicals7
IV.C.2. Acylperoxys 105
IV.C.3. Oxygen-substituted peroxy radicals
IV.C.4. Halogen-substituted peroxys6
IV.C.5. Aromatic peroxy radicals
IV.C.6. Discussion
IV.D. Reactions of peroxy radicals with N0 110 2
. rV.D.l. Alkylperoxys1
IV.D.2. Acylperoxy radicals4
IV.D.3. Oxygen-substituted peroxy radicals7
IV.D.4. Halogen-substituted peroxys
IV.D.5. Aromatic peroxy radicals 12
IV.D.6. Discussion0
IV.E. Reactions of peroxy radicals with other species
IV.E.1. Alkylperoxys
IV.E.2. Halogen-substituted peroxy radicals 13
IV.E.3. Summary 13
V. Peroxy radicals in the atmosphere
V. A. Production of peroxy radicals in the atmosphere1
V.B. Photochemical ozone formation
V.C. Reactions competing with photochemical ozone formation 132
V.D. Peroxy radicals in the boundary layer under low NO conditions6 x
V.E.ys in night-time oxidation chemistry 137
V.F. Peroxy radicals in halocarbonn9
V.G. Measurements of peroxy radical concentrations in the atmosphere 140
VI. Future work 143
VI. A. Laboratory experimental techniques
VLB. Structure, spectroscopy and thermochemistry 144
VI. C. Reactions of peroxy radicals
VI.C.l. Reactions between peroxy radicals
VI.C.2.s of peroxy radicals with H05 2
VI.C.3. Reactions of peroxys with NO and N0 146 2
VI.C.4.s of peroxy radicals with other species
VLD. Peroxy radicals in the atmosphere 14
VII. References 147
IV -PREFACE
P.D . LlGHTFOOT
Editor, Laboratoire de Photophysique et Photochimie Moleculaire, Université de Bordeaux I, Talence, France
Dagaut and Kurylo is restricted to the u.v. That the atmospheric degradation of organic
compounds is a complex process involving free spectroscopy and the kinetics and mechanisms
radicals is undisputed. In the atmosphere, of reactions of peroxy radicals, whereas we
organic molecules are transformed, after initial cover, in addition, the role of peroxy radicals in
free radical attack or photolysis, into peroxy the atmosphere and their detection in the field,
radicals, RO2. The important role thaty as well as their thermochemistry, electronic
radicals play as intermediates in the degradation structure and i.r. and e.s.r.spectroscopy.
of organic compounds has been recognised since The authors of this review have received
the early 1970's and has led to the development support for their research into peroxy radical
of a comprehensive theory of tropospheric chemistry from the Commission of the
photochemistry which is broadly consistent with European Communities, for example under the
observations of the distributions of relevant Science and Technology for Environmental
trace atmospheric constituents such as ozone, Protection (STEP) initiative, and also under the
hydrocarbons and nitrogen oxides. In recent former COST 611 programme. We are all very
years, a great deal of progress has been made grateful for this support, as well as the personal
towards an understanding of peroxy radical involvement of Dr. G. Angeletti of Directorate-
chemistry and a review of the situation seemed General XII of the CEC. The Commission
timely. In this review, we attempt to summarise provided the impetus for the preparation of the
the current knowledge of the chemistry and review and very generously provided funds for
physical properties of organic peroxy radicals, meetings of the review group.
with particular emphasis on their relevance to The review is being published both here,
tropospheric chemistry. We have tried to under the auspices of the CEC, and by the
include published literature and work in press journal Atmospheric Enviroment, where it will
thup to the end of 1991. The review team is made appear as the 10 issue of volume 26A in 1992.
up of representatives from several European The review closely follows the format of the
research groups which are active within the field review of the physics and chemistry of the
of peroxy radical chemistry. It has been an nitrate radical, edited by R.P. Wayne, which
honour to chair this group and to coordinate its appeared recently as a CEC publication and in
efforts, as well as to act as Editor for this Atmospheric Environment.
document that we have submitted to Much of the work reported in this review, by
Atmospheric Environment. the authors and by others, has been performed
as part of the LACTOZ (Laboratory Studies of Since undertaking this review, a second
Chemistry Related to Tropospheric Ozone) review of gas-phase peroxy radical chemistry,
subproject of the EUROTRAC programme. prepared by Drs. T.J. Wallington, P. Dagaut
and M.J. Kurylo, and which will be published Finally, I would like to take this opportunity
almost simultaneously with our own in to thank my co-authors for the enormous
Chemical Reviews, has come to our attention. amount of time and effort that they have
Although considerable overlap is of course devoted to the review, despite numerous other
unavoidable, the two reviews have somewhat commitments.
different goals. The review of Drs. Wallington,
V -ORGANIC PEROXY RADICALS: KINETICS,
SPECTROSCOPY AND TROPOSPHERIC CHEMISTRY
1P.D. LIGHTFOOT,*R.A. Cox, J.N. CROWLEY,* M. DESTRIAU,*
T G.D. HAYMAN,^ M.E. JENKIN,^ G.K. MOORTGAT* and F. ZABEL
»Laboratoire de Photophysique et Photochimie Moléculaire, Université de Bordeaux I, 33405
Talence Cedex, France. 1 Marine and Atmospheric Science Directorate, NERC HQ, Polaris
House, North Star Avenue, Swindon SN2 1EU, U.K.. #Abt. Chemie der Atmosphäre, Max-
Planck-Institut für Chemie, Postfach 3060, W-6500 Mainz, Germany. 9* AEA Environment and
Energy, Building BS51, Harwell Laboratory, Didcot, Oxon OXll ORA, U.K.. TBergische
Universität, Physikalische Chemie, Gauss-Strafie 20, W-5600 Wuppertal 1, Germany.
(Received for publication 5 April 1992)
Abstract- The present state of knowledge of organic, or carbon-based, peroxy radicals (RO^ is
reviewed. Data on the chemical and physical properties of peroxy radicals in the gas-phase is
considered, as well as the role of peroxy radicals in tropospheric chemistry and measurements of
their concentrations in the atmosphere. Where appropriate, peroxy radicals are grouped together
by type (allcyl, acyl, oxygen-substituted, halogen-substituted and aromatic radicals) to facilitate
comparison. Data on the hydroperoxy radical (HOj) is included where it is directly relevant to
measurements on organic peroxy radicals, eg. absorption cross-sections used in measurements
of RO, + HO, rate constants. The literature data is critically reviewed and recommendations for
absorption cross-sections, rate constants and branching ratios are made where considered
appropriate.
The laboratory experimental techniques which have been used for the generation and
detection of peroxy radicals and the products of their reactions are discussed. The structure,
spectroscopy and thermochemistry of the radicals are examined. Although the majority of
spectroscopic data concerns the u.v. spectra much used for kinetic studies, near-infrared,
infrared and electron spin resonancea are also considered. In many cases, peroxy radical
u.v. spectra are well-fitted by a Gaussian distribution function, enabling the cross-sections to be
easily calculated at any wavelength.
For the purpose of this review, the chemical reactions of peroxy radicals are divided into
reactions with organic peroxy radicals with HO,, with NO and N0 , and finally with other 2
species. Peroxy radical abstraction and addition reactions with closed-shell species are
sufficiently slow to be of negligible importance at temperatures pertinent to the atmosphere and
are consequently not covered. Data on both the kinetics and mechanisms of peroxy radical
reactions are considered.
The role of peroxy radicals as intermediates in the atmospheric degradation of volatile
organic compounds and in the production of ozone in the troposphere under both low and high
[NOJ conditions 'is discussed. The involvement of peroxy radicals in night-time oxidation
chemistry and the oxidation of halocarbons is also indicated. The techniques used for the
difficult measurement of peroxy radical concentrations in the atmosphere are described, together
with the results to date.
Finally, some tentative suggestions as to further avenues of research are made, based on the
data reviewed here and with particular reference to the solution of outstanding problems in
atmospheric chemistry. Although a great deal of progress has been made in recent years, it is
clear that additional work is needed in most areas covered by this review. New, sensitive and
selective laboratory techniques are required for studies of peroxy radical kinetics and high level
ab initio calculations would help design laser-based detection techniques. Further product studies
of photooxidation systems are needed, particularly as a function of temperature. Recent work
has shown that the rate constants for RO, + HO, reactions used in modelling studies may be
too low; if so, these reactions will be correspondingly more important than previously believed
in tropospheric oxidation. Recent kinetic studies of the potentially important reactions of
methylperoxy radicals with CIO and NO, need to be confirmed and mechanistic work is
necessary. Although substantial progress has been made towards the monitoring of peroxy
radical concentrations in the atmosphere, more work is needed, both on measurements and the
development of new techniques.
Key word index: peroxy radicals, atmosphere, troposphere.
1 -PHILLIP D. LIGHTFOOT et al.
I. INTRODUCTION RO2 at ambient temperature are those involving
odd­electron species, i.e. atoms, other radicals
and nitrogen oxides. Thus, in the atmosphere, The gas­phase oxidation of hydrocarbons and
other volatile organic compounds is an the important reactions are the reactions of RO2
important process in a number of systems of with H0 , NO, N0 , NO3 and other RO2 2 2
radicals. The central role of these reactions practical interest, and the mechanism and
within the overall scheme for atmospheric kinetics of the oxidation process have been the
subject of academic study for many decades. In oxidation of organic compounds is evident from
recent years, particular attention has been paid the schematic diagram illustrated in Fig. 1.1.
to the oxidative degradation of hydrocarbons in The reaction of peroxy radicals with nitric
oxide is of profound importance in the the atmosphere, following the discovery of their
role with nitrogen oxides in the formation of atmosphere, since it leads to the production of
photochemical smog. Earlier work on the gas­ ozone. The photostationary state between NO,
phase oxidation of hydrocarbons was stimulated N0 and O3 is maintained in the sunlit 2
atmosphere by the reactions: by low temperature combustion phenomena such
as autoignition. As a result of the elementary 3N0 + hv (X<420 nm)­* NO + 0( P) (1.2) 2
reaction rate studies and mechanistic
30( P) + 0 + M ­♦ O3 + M (1­3) investigations stimulated by these issues, the 2
broad features of the mechanisms of NO + O3 ­* N0 + 0 (1.4) 2 2
hydrocarbon oxidation in air have now been
Additional reactions leading to oxidation of NO
established.
to N0 , eg.:2
Hydrocarbon oxidation in the atmosphere is
NO + RC^ ­» N0 + RO (1.5) 2initiated by free radicals, mainly hydroxyl
gives rise to net production of ozone from the radicals, OH, which react by hydrogen
photolysis of N0 . Ozone production is abstraction or addition (in the case of 2
therefore influenced by the fraction of the R0unsaturated compounds). The second stage of 2
produced which reacts with NO to give N0 , in low temperature oxidation is the formation of a 2
competition with other pathways. peroxy radical, RO2, by addition of oxygen to
the radical formed in the primary step: The reactivity of a given peroxy radical
depends on the structure and nature of the R + 0 + M ­» RO2 + M (1.1) 2
organic moiety in the radical. There is a large
The kinetics and mechanisms of the subsequent variety of hydrocarbons and other organic
reactions of peroxy radicals determine the species from which R0 radicals can be derived. 2
primary oxidation products and thus the Systematic studies of the kinetics and products
characteristic chemical behaviour of the system. of R0 radical reactions carried out in recent 2
The subsequent reactions of RO2 often lead to years have served to provide reactivity patterns
the eventual regeneration of OH radicals and so and a mechanistic rationale for them.
the overall oxidation proceeds by a chain At low temperatures, R0 reactions tend to 2
mechanism. Moreover, in the atmosphere, proceed via association complexes, which can
where the initiating radicals are mainly lead to unusual temperature and pressure
generated photochemically, carbonyl compounds dependences for the reaction rate constants. The
and hydroperoxides are formed in the RO2 importance of peroxy radicals at higher
reactions; these products absorb light, giving temperatures is limited by their decomposition
rise to additional radical production. At higher to R + 0 , i.e. the establishment of the 2
temperatures in combustion systems, the equilibrium:
hydroperoxides produced from RO2 reactions
R + 0 + M «♦ RC^ + M (1.6,­6) 2can thermally decompose to give radicals,
For simple hydrocarbon radicals, the thereby acting as degenerate chain branching
equilibrium is only reached at temperatures in agents.
excess of — 750 K, but for some peroxy Peroxy radicals are relatively unreactive
radicals, (eg. phenylperoxy, allylperoxy, compared to OH and abstraction of hydrogen
trichloromethylperoxy) the R­0 bond is much atoms by RO2 has a sufficiently high activation 2
weaker. Thermochemical factors therefore energy to be negligibly slow at room
control the ceiling temperature temperature. The only known fast reactions of
­ 2