Reactive Intermediates in the Gas Phase. Generation and Monitoring

360 Pages·1979·5.005 MB·English

by  D. W. Setser (Eds.)

Contributors
LARRY I. BONE J. H. KOLTS
MICHAEL A. A. CLYNE M. C. LIN
N. DJEU j. R. MCDONALD
WILLIAM FELDER WING S. NIP
ARTHUR FONTIJN D. W. SETSER
REACTIVE INTERMEDIATES
IN THE GAS PHASE
Generation and Monitoring
Edited by
D.W. SETSER
Department of Chemistry
Kansas State University
Manhattan, Kansas
1979
ACADEMIC PRESS
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COPYRIGHT © 1979, BY ACADEMIC PRESS, INC.
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United Kingdom Edition published by
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Library of Congress Cataloging in Publication Data
Main entry under title:
Reactive intermediates in the gas phase.
Includes bibliographies and index.
1. Chemical reaction, Conditions and laws of—
Addresses, essays, lectures. 2. Chemistry, Physical
organic—Addresses, essays, lectures. I. Setser, D. W.
QD501.R346 541\39 79-51698
ISBN 0-12-637450-3
PRINTED IN THE UNITED STATES OF AMERICA
79 80 81 82 9 8 7 6 5 4 3 2 1
List of Contributors
Numbers in parenthesis indicate the pages on which the authors' contributions begin.
LARRY I. BONE* (305), Department of Chemistry, Appalachian State
University, Boone, North Carolina 28608
MICHAEL A. A. CLYNE (1), Department of Chemistry, Queen Mary
College, University of London, London El 4NS, United Kingdom
N. DJEU (323), Laser Physics Branch, Naval Research Laboratory,
Washington, D.C. 20375
WILLIAM FELDER (59), AeroChem Research Laboratories, Inc.,
Princeton, New Jersey 08540
ARTHUR FONTIJN (59), AeroChem Research Laboratories, Inc.,
Princeton, New Jersey 08540
J. H. KOLTSt (151), Department of Chemistry, Kansas State University,
Manhattan, Kansas 66506
M. C. LIN (233), Chemical Diagnostics Branch, Chemistry Division,
Naval Research Laboratory, Washington, D.C. 20375
J. R. McDONALD (233), Chemical Diagnostics Branch, Chemistry
Division, Naval Research Laboratory, Washington, D.C. 20375
WING S. NIP (1), Division of Chemistry, National Research Council of
Canada, Ottawa, Ontario K1A OR6, Canada
D. W. SETSER (151), Department of Chemistry, Kansas State
University, Manhattan, Kansas 66506
* Present address: Dow Chemical Co., Freeport, Texas 77541.
tPresent address: Research and Development Department, Phillips Petroleum Company,
Bartlesville, Oklahoma 74004.
vii
Preface
An understanding of the time evolution of complex chemical systems
can be achieved only from knowledge of the individual elementary reac-
tion steps comprising the system. Frequently some of those elementary
steps involve reactive intermediate chemical species. To study the reac-
tive, transient intermediates, methods must be developed for isolating
the species and observing their thermodynamic, kinetic, and structural
properties.The need for knowledge of the macroscopic chemical proper-
ties of reactive intermediates frequently coincides with a desire for obser-
vation of state-to-state chemical dynamics of their elementary reactions.
Fortunately, sensitive analytical tools for measuring the properties of a
given chemical species are available. The limitation to the study of reac-
tive intermediates is often the development of methods to generate the
species in an environment favorable for quantitative measurements. A
description of such methods for reactive intermediates in the gas phase is
the central theme of this book.
The book was planned so that the chapters would enable readers to
learn about techniques for generating reactive intermediates, as well as to
review the literature on various reactive species. Insofar as possible, per-
tinent experimental detail also will be emphasized in the chapters. Writing
on these topics is not as glamorous as applying modern dynamical theory
to state-to-state data or interpreting a mechanism of reaction. However,
this book was intended to provide insight into production and isolation of
reactive intermediates, rather than to discuss reaction dynamics. The
need for a book emphasizing experimental techniques is accentuated by
the lack of experimental detail given in most journal articles.
A balance regarding presentation of techniques and review of interest-
ing species was sought. The first three chapters deal with flow techniques,
and the discussion proceeds from ground state atoms and radicals, nor-
mally at room temperature, to high temperature conditions and species,
and finally to long-lived electronically excited states. The fourth chapter
is devoted to discussion of pulsed excitation systems with emphasis on use
of lasers for both generating and monitoring the chemical species. The
ix
X PREFACE
authors of this chapter had an especially challenging task because of the
revolution occurring with the introduction of lasers into the field. The fifth
chapter discusses positive ions with an emphasis on generation by photo-
ionization methods. The last chapter discusses rare gas halide discharge
lasers and their applications.
I wish to thank the authors for their efforts in presenting pertinent ex-
perimental detail and yet remaining sufficiently general in scope to cover a
variety of chemical species.
REACTIVE INTERMEDIATES IN THE GAS PHASE
1
Generation and Measurement
of Atom and Radical Concentrations
in Flow Systems
MICHAEL A. A. CLYNE WING S. NIP
Department of Chemistry Division of Chemistry
Queen Mary College National Research Council of Canada
University of London Ottawa, Ontario
London Canada
United Kingdom
I. Introduction 2
II. Description of Discharge Flow Systems 2
A. Measurements of Pressure and Flow Rates 4
B. Mixing of Reagents 5
C. Diffusion 5
D. Analysis of Kinetic Data 5
E. Effects of Wall Coatings 6
F. The Discharge Bypass System 7
III. Production of Atoms 8
A. H(2S) 8
B. OC?j) 10
C. N(4S) 11
D. S^P,) 12
E. C(3P7) 13
F. F(2P,) 13
G. C1(2P7) 16
H. Br(2P,) 19
I- I(2Py) 20
IV. Production of Molecular Radicals 20
V. Detection of Atoms and Radicals 22
A. Optical Spectrophotometry, Including Resonance Fluorescence and
Absorption 22
B. Mass Spectrometry 38
1
Copyright © 1979 by Academic Press, Inc
All rights of reproduction in any form reserved
ISBN 0-12-637450-3
2 MICHAEL A. A. CLYNE AND WING S. NIP
C. Electron Paramagnetic Resonance (EPR) and Laser Magnetic
Resonance (LMR) 44
D. Chemiluminescence 47
References 50
1. INTRODUCTION
The discharge-flow method is established as one of the main quantitative
techniques for kinetic studies of gaseous elementary reactions. Since the very
innovative work of Kaufman (from 1958 onward), which essentially est-
ablished the method, a series of major developments of the discharge-flow
method have taken place. These developments included the extension of the
scope of the method from the three common atoms O, H, and N to many other
atoms, and to molecular free radicals including OH, CIO, BrO, FO, and SO.
In parallel, the first radical detection methods employed—chemiluminescence,
thermal gauges, etc., have been gradually superseded by novel detectors, or by
ones adapted from other branches of experimental chemical physics. Fore-
most among the direct and highly sensitive detection techniques now available
are atomic resonance in the vacuum ultraviolet, mass spectrometry, EPR and
laser magnetic resonance, and laser-induced fluorescence.
Possibly the most well-known recent application of the discharge-flow
technique has been to the kinetics of atmospherically significant elementary
reactions. Virtually all the direct measurements of rate constants for such
reactions have been and are being carried out using either the discharge-flow
or flash-photolysis methods. For instance, the first direct determinations at
298 K of the rate constants for the following and many other stratospheric
reactions were made using the discharge-flow technique: CI + 0 -> CIO +
3
0, 0 + C10-+Cl + 0, CIO + N0 + M->C1N0 + M, H0 + NO->
2 2 2 3 2
OH + N0, H + 0 OH + 0. There are numerous other major
2 3 2
applications areas to which the discharge-flow method is currently contribut-
ing fundamental kinetic data. These include: chemical lasers, excimer lasers,
combustion chemistry, and free-radical spectroscopy. In the present article,
we survey the scope of the discharge-flow method. Because of its particular
usefulness as a highly sensitive detection method, we include a more detailed
account of atomic resonance in the vacuum ultraviolet. Other major detection
techniques are considered in less detail.
II. DESCRIPTION OF DISCHARGE FLOW SYSTEMS
The basic components of a discharge-flow system are shown in Fig. 1.
Atoms and radicals are formed either directly from a microwave (or radio-
frequency) discharge in tube D, or by rapid reaction of an atom with a
1. ATOM AND RADICAL CONCENTRATIONS IN FLOW SYSTEMS 3
B
O
Fig. 1. Resonance absorption and fluorescence detection of atoms in a discharge-flow
system. Note discharge-bypass system P, comprising needle valves NV1, NV2 for carrier gas,
molecular gas inlet X, microwave discharge D, recombination volume B, joints M for poisoning
2
discharge tube with H,P0. Flow tube includes reagent inlets L. Absorption (A) fluorescence
4
(F) cell includes LiF windows G, G', H, microwave discharge I, connecting tube C to 1 -m vacuum
monochromator (Hilger E760), K to pump. [After Clyne and Cruse (1972a).]
molecule. The active species are passed along the flow tube (of cylindrical or
rectangular cross section) where they may be mixed with another reagent from
fixed or moveable inlet jets L. Measurements of atom and radical concentra-
tions are made downstream of the mixing zone.
In this section, we indicate various parameters which govern the operation
of a discharge-flow system. (The methods of generation and detection of
atoms and radicals will be described later.) Stirred flow reactors have been
used to a limited extent, and their operation has been described elsewhere
(Mulcahy and Williams, 1961; Mulcahy et ai, 1967, 1969).
Under plug flow conditions, the elapsed time in a flow system is simply
equal to x/v, where x is the displacement along the flow tube and v is the flow
velocity. Flow tubes (of pyrex or silica glass) are typically 20-50 mm in
internal diameter and 10-200 cm in length. A total pressure P of 0.5-10 Torr
in the flow tube is maintained, usually by fast conventional rotary pumps
(typically 300-1000 liter min"l) using high-conductance cold traps. The
4
MICHAEL A. A. CLYNE AND WING S. NIP
partial pressure of each component p in the flow tube is then related to the
{
individual flow rates (F;) into the system by the expression p = PFJY,F,
{ {
where ZFj is the total mass flow rate (mainly carrier gas).
Although the flow system is versatile in the production of atoms and
radicals, several aspects of its operation must be carefully considered in order
to obtain accurate kinetic information.
A. Measurements of Pressure and Flow Rates
For a reaction which is first order in reagent A, the rate constant k' is
given by k! = -v d\n\_\~\ldx = — RT(LFJaP) d ln[A]/dx where v is the
plug flow velocity, a is the cross section area of the flow tube, and x is the
displacement along the flow tube. Thus, a realistic error of ±2% in P and
±3% in ZFj will lead to a ±5% uncertainty in the measurement of k'. The
uncertainty in higher order reactions will be progressively greater (Clyne,
1973). Thus, the uncertainty in the bimolecular rate constant of an atom (A) +
molecule (B) reaction determined under pseudo first-order conditions will be
±13%
Calibrations of flow rates are usually done by following the rate of fall in
pressure, —dP/dt, of gas in a calibrated volume K, F = —(V/RT) dP/dt. As
{
pointed out by Clyne (1973), P is a strongly nonlinear function of t; and it is
desirable that — dP/dt be measured from the almost linear plot of 1 jP versus t:
— dP/dt = P2 d(l/P)/dt. The mean flow rate of a condensable reagent can
also be determined by trapping material downstream of the needle valve over
a known period of time and measuring the total mass collected. "Bubble
meters " have also been used to calibrate flows from > 1 atm backing pressure.
It is realistic to expect an accuracy no better than ± 3 % using existing flow
calibration methods.
Errors in total pressure (P) measurements in flow systems arises from two
factors, (a) Errors in P itself. At a typical pressure of 1 Torr, a 2 % accuracy
implies measuring the pressure to 0.02 Torr, which is marginal using oil
manometers. This problem has been solved by the availability of sensitive
and accurate pressure transducers such as capacitance manometers, (b) The
pressure gradient along the flow tube. According to the Poiseuille flow
equation, (P, — P) = $rjRTlZ,F/Pnr4', where rj is the viscosity of the
2 i 1
mixture, the relative pressure drop (P — P)/P\ along a 2-cm-i.d. flow tube
1 2
would be 0.3 per 100 cm length with an argon flow of 2 x 10~4 mole sec"1
near 300 K and P « 1 Torr. The effect may be minimized by linear inter-
l
polation between the pressures measured at both ends of the flow tube.
According to the Poiseuille equation, the relative pressure drop increases
with increase in total flow rate and with decrease in pressure and, particularly,
with tube radius r.