Fluorine-Containing Free Radicals. Kinetics and Dynamics of Reactions

416 Pages·1978·7.06 MB·English

by  John W. Root (Eds.)

Fluorine-Containing Free Radicals
Kinetics and Dynamics of Reactions
John W. Root, EDITOR
University of California, Davis
Based on a symposium sponsored
by the Division of Physical
Chemistry at the 169th Meeting
of the American Chemical Society,
Philadelphia, Pennsylvania,
April 7-11, 1975.
66
ACS S Y M P O S I U M SERIES
AMERICAN CHEMICAL SOCIETY
WASHINGTON, D. C. 1978
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Library of Congress CIP Data
Main entry under title:
Fluorine-containing free radicals.
(ACS symposium series; 66 ISSN 0097-6156)
Includes bibliographical references and index.
1. Radicals (Chemistry)—Congresses. 2. Fluorides
—Congresses.
I. Root, John W., 1935- . II. American Chemical
Society. Division of Physical Chemistry. III. Series:
American Chemical Society. ACS symposium series; 66.
QD471.F67 546'.731 77-26667
ISBN 0-8412-0399-7 ACSMC 8 66 1-423
Copyright © 1978
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PRINTED IN THE UNITED STATES OF AMERICA
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
ACS Symposium Series
Robert F. Gould, Editor
Advisory Board
Kenneth B. Bischoff Nina I. McClelland
Donald G. Crosby John B. Pfeiffer
Jeremiah P. Freeman Joseph V. Rodricks
E. Desmond Goddard F. Sherwood Rowland
Jack Halpern Alan C. Sartorelli
Robert A. Hofstader Raymond B. Seymour
James P. Lodge Roy L. Whistler
John L. Margrave Aaron Wold
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
FOREWORD
The ACS SYMPOSIUM SERIES was founded in 1974 to provide
a medium for publishing symposia quickly in book form. The
format of the SERIES parallels that of the continuing ADVANCES
IN CHEMISTRY SERIES except that in order to save time the
papers are not typeset but are reproduced as they are sub
mitted by the authors in camera-ready form. As a further
means of saving time, the papers are not edited or reviewed
except by the symposium chairman, who becomes editor of
the book. Papers published in the ACS SYMPOSIUM SERIES
are original contributions not published elsewhere in whole or
major part and include reports of research as well as major part and include reports of research as well as reviews
since symposia may embrace both types of presentation.
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
PREFACE
This volume was conceived originally after a symposium that I organized
for the Physical Chemistry Division. The symposium consisted of two
half-day invited-lecture sessions presented in Philadelphia in April, 1975.
Chemical kinetics and dynamics research with fluorine-rich radicals
(including atomic fluorine) has become increasingly popular during the
seventies. Since the Philadelphia symposium was not comprehensive,
this subject received further discussion one year later during three half-
day invited-lecture sessions that were given in New York City. This
second symposium, whic
Chemistry and Fluorine Chemistry and Technology, included topics on
the kinetics, reactivities, and structures of fluorine-containing radicals.
The present volume was intended initially to provide the first com
prehensive treatment of the kinetic and dynamic characteristics of these
interesting chemical species. Following acceptance of this proposal by
the A.C.S. Symposium Series Editorial Advisory Board, invitations to
participate were sent to many individuals throughout the English-speak
ing scientific community. All of the contributors in kinetics and dynamics
at both American Chemical Society symposia were included, but multiple
coverage of individual topics was strongly discouraged. Participants were
requested specifically to aim for comprehensive state of the art reviews
and to prepare theoretical presentations addressed mainly to experimen
tally oriented readers.
Partially because of last minute cancellations, several important
topics have not been included in the present work. Papers on fluorine
atom chemical lasers, high-energy fluorine atom reactions, and classical
trajectory simulations of hydrogen transfer reactions by atomic fluorine
are conspicuously absent. Other more specialized topics not included
were olefin addition reactions by fluorine-rich carbenes, H /F explosions,
2 2
unimolecular reaction dynamics of fluorine-containing aliphatic radicals,
and high-power fluorine atom lasers in energy research.
Hopefully these omissions will not detract too severely from the
usefulness of this work. The above unpublished topics and the rapid
pace of ongoing research in this exciting field clearly suggest the need
for another project of this type in the near future.
I wish to thank the participants for their excellent work, the many
individuals with whom this project has been discussed during the past
vii
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
two years, Mr. Robert Gould of the A.C.S. Books Department for guid
ance and assistance, Ms. Cecelia Damian who performed the bulk of
the minor editorial revisions, and Dr. James Muckerman of Brookhaven
National Laboratory for the materials used in the design of the dust
jacket.
University of California, Davis JOHN W. ROOT
Davis, California
November 1, 1977
viii
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
1
Elementary Reaction Kinetics of Fluorine Atoms,
FO, and NF Free Radicals
E. H. APPELMAN
Argonne National Laboratory, Argonne, IL 60439
M. A. A. CLYNE
Department of Chemistry, Queen Mary College, London E1 4NS, England
The study of reactions o free fluorine atoms i thei ground
2p5 2P states has progressed rapidly in the last several
years. 3/2,M1/a2jor technique difficulties have been gradually overcome,
and several methods of determining F atom concentrations are now
available. Elementary reaction kinetics in low pressure flow
systems have received particular attention, probably because of
the relative ease of interfacing flow systems to F-atom detection
devices such as mass spectrometers and vacuum-uv atomic resonance
spectrometers. Photolytic production of F atoms is less easy -
F, for instance, absorbs only very weakly in the near ultraviolet
or2 visible regions. Similarly,detection of F 2P atoms in static
systems presents problems that have not been fullJy solved at the
present time. Therefore, the work described in this article has
mostly been carried out in flow systems.
Current interest in fluorine-based chemical lasers extends
from HF and DF lasers based on F atom reactions into the area of
elementary processes involving electronically excited radicals
such as NF. Also, the chemistry of NF and OF radicals is of con­
siderable interest, and we summarize here selected aspects of the
kinetic behaviour of these species.
Production of F 2P atoms
J
Ground state 2P halogen atoms may be produced by direct
J
dissociation of the molecular halogens, F , Cl, Br and I. A
2 2 2 2
convenient and commonly-used technique is to pass a mixture of
molecular halogen diluted with argon or helium through a microwave
discharge at a total pressure near 100-200 Ν m""2. This is the
simplest technique for forming F 2Pj atoms (1-4); the typical
degree of dissociation of F using an uncoated silica discharge
2
tube is 70-80%(1). Mixtures of fluorine diluted with inert gas
can be handled in a conventional glass flow vacuum system. A
© 0-8412-0399-7/78/47-066-003$10.00/0
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
4 FLUORINE-CONTAINING FREE RADICALS
problem with microwave-excited dissociation of F is that SiF^
2
and molecular and atomic oxygen are produced as a result of attack
on the Si0 material of the discharge and flow tubes (1, 2, 5) .
2
Rosner and Allendorf (3_) have replaced the silica discharge tube
by one fabricated from pure fused alumina, which is inert to
attack by fluorine atoms. However, this reduces the efficiency of
dissociation somewhat. The pyrex or silica flow tube may be pro­
tected from attack by F atoms by application of a thin coating of
teflon or Kel-F fluorocarbon polymer (2). A technique for coating
with a thin fused layer of teflon has been described (6). The
presence of undissociated F in the discharge products causes
2
complications in kinetic studies involving hydrogen atoms or
alkyl radicals, which react rapidly with F . However, F appears
2 2
to be unreactive toward h othe atom Ο 3P Ν **S
CI 2P and Br 2P (2,7) .
T T
d J — —
Production of F 2Pj atoms can also be achieved by microwave
dissociation of fluorides such as SF and CT% (8). These sources
6
are satisfactory for spectroscopic studies such as the epr spect­
roscopy of F 2Ρ3/2 an^ F 2pi/2 (§)r or for the formation of ex­
cited BrF and IF from the recombination of Br + F or I + F in the
presence of singlet oxygen (9). However, they are in some degree
suspect for systematic quantitative kinetic studies, in as much as
the nature and reactivity of the discharge products other than
F 2Pj are very incompletely known.
In principle, a cleaner source of F 2Pj atoms than any of the
preceding ones is the rapid reaction of Ν **S atoms with NF
2
radicals produced by thermolysis of N F^ (10,11). This reaction
2
occurs either directly to give F atoms Ν + NF -> 2F + N (10) or
2 2
via the formation of NF radicals; Ν + NF -> 2NF (11), followed by
2
NF + NF -> N + 2F or Ν + NF -»· N + F. In practice, the instabil­
2 2
ity of N^LJ. and its expense have precluded extensive use of the
Ν + NF method for forming F atoms.
2
The formation of F 2Pj atoms from the bimolecular reaction of
NO with F is extensively used in HF, DF chemical lasers:
2
NO + F -> FNO + F. The kinetics of this reaction have been studied
2
(12,13).
Measurement of Atom Concentrations : absolute concentrations.
The range of methods that have been employed to measure atom
concentrations is wide, and includes thermal and diffusion methods
which are not considered here. Most of the physical methods avail­
able are suitable only for relative concentration measurements,
and require calibration by chemical means in order to yield
absolute concentrations. This need not always be a disadvantage;
for instance, in kinetic studies of simple atom reactions under
pseudo-first order conditions it is sometimes only necessary to
monitor relative changes in atom concentrations.
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
1. APPELMAN AND CLYNE Elementary Reaction Kinetics of F Atoms 5
Titration reactions based on simple stoichiometry, and pro­
ceeding extremely rapidly, have been developed for the measurement
of absolute concentrations of ground state atoms in flow systems.
Well-known examples for Ν, Ο and Η atoms include the reactions,
Ν + NO -* N + Ο, Ο + N0 -> NO + Ο , H + ClNO -> HCl + NO and
2 2
Η + N0 -> OH + NO. These reactions permit the determination of
2
absolute atom concentrations by the addition of known quantities
of a stable reagent. Several similar titration reactions have been
characterized and used to determine F atom concentrations, namely
F + ClNO -*· NO + GIF (1) ,
F + Cl -* C1F + CI (2) ,
2
F + Br + BrF + Br (3),
F +
The reaction of ClNO with F has been used to determine F atom
concentrations by mass spectrometric measurement of the C1F
produced after the addition of excess ClNO. The C1F+ ion peak was
calibrated by the addition of measured quantities of ClF from a
cylinder (14). To eliminate the need to work with the reactive
ClF gas, a variant of this method was developed in which the F
atoms were titrated with ClNO to an endpoint identified as the
amount of added ClNO beyond which the C1F+ peak no longer incr­
eased (1). The ClNO methods are less well established than some
other, inasmuch as the rate constant of reaction (1) has not been
determined. Also, at high concentrations, the reaction
NO + F ->· FNO + F, which has a rate constant of 8 χ 10~15 cm3
2
molecule"1 s-1at 300 K, may occur to an appreciable extent, lead­
ing to an erroneous value for [F].
The reaction of F atoms with Cl appears to be an excellent
2
means of determining absolute F atom concentrations. Reaction (2)
has a forward rate constant around 1 χ ΙΟ"10 cm3 molecule"1 s"1 at
300 Κ and an equilibrium constant greater than 5 (1_, 7,15) . Hence
the reverse reaction is unlikely to be important under the correct
conditions. Furthermore, the possible secondary reaction
CI + F •> ClF + F
2
has a rate constant less than 5 χ 10~ll+ cm3 molecule""1 s"1 at 300
Κ (1_) and may be safely neglected. Reaction (2) has been used to
determine F atom concentrations mass spectrometrically by addition
of a measured excess concentration of Cl and subsequent deter­
2
mination of either the amount of Cl consumed (7_) or the amount of
2
ClF produced (4) . In the latter case the C1F+ ion current was
calibrated by measuring its intensity when an exces-s of F atoms
was added to a known amount of Cl . Measurement of ClF production
2
is more sensitive than measurement of Cl consumption and is also
2
free from errors that might result from recombination of CI atoms
or reaction of Cl with other species such as H or O. Reaction
2
(2) has also been utilized as a titration of F atoms, with the
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
6 FLUORINE-CONTAINING FREE RADICALS
endpoint estimated to be the point of critical extinction of the F
atom fluorescence (7) . In an alternative titration procedure, the
chemiluminescenee accompanying the recombination of the CI atoms
formed in reaction (2) is monitored, and Cl is added until the
2
intensity of this emission reaches a plateau. At low pressures,
this method yields a sharp endpoint (16).
Concentrations of F atoms have also been determined by
titration with Br to the point of extinction of the F atom fluor­
2
escence (7). Reaction(3) has a rate constant of about 2 χ 10"10
cm3 molecule"1 sec"1 at 298 K, and an equilibrium constant in ex­
cess of 109 (7). Hence its back reaction is of no concern and in
this respect it should be superior to reaction (2) as a means of
determining F atom concentrations. On the other hand, the rate of
the reaction (5),
Br + F
does not appear to be known. In practice reactions (2) and (3)
appear to be equally satisfactory for the determination of F atom
concentrations.
Reaction (4) has a rate constant of about 2 χ 10"11 cm3
molecule"1 s"1 at 300 Κ (10) and has been used to determine F atom
concentration. However, the secondary reaction
F + H + HF + F (6)
2
has a rate constant of about 3 χ 10"12 cm3 molecule"1 s"1 at 300 K.
(17). Inasmuch as reaction (6) regenerates the F atoms consumed
in reaction (4), when a substantial amount of F is present the
2
added H will first consume the F and only subsequently will it
2 2
consume the F. Advantage has been taken of this effect to titrate
both F and F with hydrogen mass spectrometrically, using an inlet
2
system that incorporates an inhomogeneous magnetic field to permit
monitoring of both the F and F (2).
2
Measurement of Atom Concentrations: Atomic Resonance
The method of atomic resonance spectrometry in the vacuum
ultraviolet, with detection of either absorption or fluorescence,
has become one of the most useful direct methods for the measure­
ments of reaction rates of ground (18)(and metastable excited
(19/20) ) state atoms. The sensitivity and scope of atomic res­
onance in this respect rivals, and possibly surpasses, that of
other methods such as epr and mass spectrometry. Recently, it has
been shown that atomic resonance may be used to measure fluorine
atom concentrations in a flow system (7,21). The sensitivity for
F is not yet as high as for atoms such as CI, whose resonance
transitions lie in the Schumann region of the vacuum ultraviolet.
The essence of the method is as follows. The source of atom­
ic resonance radiation, usually a microwave-excited discharge in a
low-pressure flowing gas, is incident on the reaction vessel,
either static or flowing, in which ground state atoms are present.
In Fluorine-Containing Free Radicals; Root, J.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1978.