Table Of ContentAdvances in
Photochemistry
Volume 1
Advances in Photochemistry
Volume 1
Editors
NOYES, Department of Chemistry,
W.A LBERT JR.,
University of Rochester, Rochester, New York
GEORGE S. HAMMOND, Department of Chemistry,
California Institute of Technology, Pasadena, California
N. PITTS, Department of Chemistry,
J. JR.,
University of California, Riverside, California
SHIGERU NAKATA
1963
Interscience Publishers
a division of John Wiley & Sons New York London
Copyright @ 1963 by John Wiley & Sons, Inc.
All rights reserved
Library of Congress Catalog Card Number 63-13592
Interscience Publishers, a division of John Wiley & Sons, Inc.,
605 Third Avenue, New York 16, New York
PRINTED IN THE UNITED STATES OF AMERICA
BY MACE PRINTING CO., EASTON, PA.
lntroduction to Series
the
Photochemistry first received any systematic attention well over one
hundred years ago and it did not receive any great attention until after
World War 11. Free atoms and free radicals produced by photochemical
means have been used for many years to study single steps which may
form part of complex mechanisms, but, in a way, the more fascinating
problems of complex molecules which undergo reaction after absorption
of radiation, without at any time passing through the stage of atoms
and radicals, have only occupied the attention of chemists during recent
years.
Progress in photochemistry could only be made following progress in
spectroscopy and, in particular, the interpretation of spectra in at least
semiquantitative terms, but history has shown that this was not enough.
The arrival of new methods of analysis which permit determination of
small amounts of products, the development of flash photolysis, nuclear
magnetic resonance, and electron spin resonances which can yield
valuable information about the natures of intermediate excited states,
as well as atoms and radicals, all have permitted the photochemist to
approach the truly fundamental problem of photochemistry: What is
the detailed history of a molecule which absorbs radiation?
This series of books entitled Advances in Photochemistry will have
chapters written by pioneers who are experts. As the late Professor
Fermi once said “a state of confusion exists at any frontier.” Frontiers
in photochemistry are at times full of confusion and for this reason
they are fascinating. The editors have in no sense acted as censors. The
authors are free to make any statement they wish which cannot be
proved wrong. We hope these volumes will be stimulating and that
they will serve the useful purpose of bringing together points of view of
photochemists who do not, at present, even always use the same vo-
cabulary.
The first two volumes of Advances in Photochemistry will appear in
quick succession but, thereafter, about one volume should appear
each year.
W. Albert Noyes, Jr.
George S. Hammond
J. N. Pitts, Jr.
V
Contributors to Volume 1
E. J. BOWEN
Laboratory of Physical Chemistry, University of Oxford, Oxford,
England
0. L. CHAPMAN
Department of Chemistry, Iowa State University, Ames, Iowa
R. J. CVETANOVI~
Division of Applied Chemistry, National Research Council of
Canada, Ottawa, Canada
R. DESSAUER
Organic Chemicals Department, E. I. du Pont de Nemours and
Company, Inc., Experimentgl Station, Wilmington, Delaware
HARRY E. GUNNING
Department of Chemistry, University of Alberta, Edmonton,
Alberta, Canada
G. S. HAMMOND
California Institute of Technology, Pasadena, California
J. P. PARIS
Radiation Physics Laboratory, E. I. du Pont de Nemours and
Company, Inc., Experimental Station, Wilmington, Delaware
J. N. PITTS, JR.
University of California, Riverside, California
R. SRINIVASAN
Thomas J. Watson Research Center, International Business
Machines Corporation, Yorktown Heights, New York
OTTO P. STRAWS2
Department of Chemistry, University of Alberta, Edmonton,
Alberta, Canada
D. H. VOLMAN
Department of Chemistry, University of California, Davis,
California
F. WILKINSON
Oxford University, Oxford, England
H. E. ZIMMERMAN
Chemistry Department, University of Wisconsin, Madison,
Wisconsin
vii
Contents
The “Vocabulary” of Photochemistry
By J. N. Pitts, Jr., F. Wilkinson, and G. S. Hammond.. .... 1
The Photochemistry of Aromatic Hydrocarbon Solutions
By E. J. Bowen.. ...................................... 23
Photochemical Gas Phase Reactions in the Hydrogen-Oxygen
System
By D.H.Volman ...................................... 43
Photochemistry of the Cyclic Ketones
By R. Srinivasan.. .................................... 83
Addition of Atoms to Olefins in the Gas Phase
By R. J. Cvetanovii:. .................................. 115
A Sew Approach to Mechanistic Organic Photochemistry
By H. E. Zimmerman. ................................. 183
Isotopic Effects and the Mechanism of Energy Transfer in
Mercury Photosensitization
By H. E. Gunning and 0. P. Strausz. .................... 209
Photochromism
By R. Dessauer and J. P. Paris. ......................... 275
Photochemical Rearrangements of Organic Molecules
By0.L. Chapman ..................................... 323
Author Index.. ............................................ 42 1
Subject Index. ............................................. 437
ix
Advances in Photochemistry, Volume 1
Edited by W. Albert Noyes, Jr., George S. Hammond, J. N. Pitts, Jr.
Copyright © 1963 John Wiley & Sons, Inc.
The “Vocabulary” of Photochemistry
JAMES N. PITTS, JR.,* University of California,
Riverside, California
FRANCIS WILKINSON, Oxford University, Oxford, England
GEORGE S. HAMMOND,t California Institute of Technology,
Pasadena, California
In any rapidly expanding field of science the vocabulary, which
constitutes one of the working tools of the researcher, is likely to include
many ill-defined, and frequently inappropriate terms. Much of the prob-
lem arises from the fact that increased understanding on the part of the
researchers in such an area renders diffuse, and, indeed, sometimes in-
exact, terminology which once was adequate to describe accurately and
concisely the theoretical and experimental aspects of the subject. Also,
on expansion, a subject which initially may have been relatively re-
stricted in scope often begins to involve other disciplines each having its
own specialized jargon. The consequent mixing of terminologies may
sometimes plague the “experts” and confound the novices. If the
vocabulary of such an area is not periodically redefined or recreated,
both classes of researchers are left struggling along and attempting to
communicate with heavy dependence on words which are no longer
entirely appropriate for expression of ideas which may be currently of
greatest interest.
We feel such a situation prevails today in photochemistry. Although
the relationship between, and the great importance of, spectroscopy to
photochemistry was recognized early, traditionally these two fields of
physical chemistry have developed almost in parallel, with useful over-
lap occurring only in the study of some relatively simple systems. Now
these fields have almost merged and are joined by a strong contingent
* This author gratefully acknowledges Grant AP-109 from the Division of Air
Pollution, Bureau of State Services, Public Health Service, and an American Chemi-
cal Society-Petroleum Research Fund Grant 278-A4.
t Contribution No. 2918 from the Gates and Crellin Laboratories of Chemistry,
California Institute of Technology.
1
2 J. N. F. WILKINSON, AND G. S. HAMMOND
PITTS,
of organic chemists. Amalgamation of their respective languages ib
difficult but of great importance.
We hope to accelerate the process of amalgamation by a discussion of
terms which, from our point of view, are useful. In particular we have
selected those which may cause confusion and also those which are not
selfexplanatory to the average chemist. Consideration has been given
to the “common usage” factor in language, to our own prejudices, and
to certain recognized conventions. These factors are not always com-
patible and some confusion or contradiction still cannot be avoided
(e.g., in definitions of fluorescence and phosphorescence).
As a purely pragmatic device we have chosen operational criteria as
a rather loose basis for organization. Since we are not attempting a
literature survey, references are limited, but include the by
Herzberg(l,2) Noyes and Leighton (3), and Pringsheim (4), key articles
and reviews by Mulliken (5,6), Platt (7,8), and Kasha (9) and three
recent books on various aspects of spectroscopy in which nomenclature
is discussed and terms are defined (10-12). We also have utilized a
manuscript in preparation by Calvert and Pitts (13). .
The Overall Photochemical Process
The interdisciplinary nature of spectroscopy and photochemistry
is well illustrated by the following definition of the primary photo-
chemical process given by Noyes et al. (14) :
“The primary photochemical process comprises the series of events
beginning with the absorption of a photon by a molecule and ending
either with the disappearance of that molecule or with its conversion to
a state such that its reactivity is statistically no greater than that of
similar molecules in thermal equilibrium with their surroundings.”
In the primary photochemical process there are usually a variety of
paths for degradation of the electronic energy of excitation. Chemical
paths include intramolecular rearrangements or the formation of free
radicals and excited molecules which may react in secondary processes
to form new products of chemical interest. However, also usually in-
cluded in the overall photochemical reaction are radiative and nonradia-
tive photophysical processes which do not lead to a net chemical change,
yet are alternate paths for loss of the absorbed energy (e.g., fluorescence
and intersystem crossing; cf. Fig. 1). Such processes involving electron-
ically excited states are of great interest to the spectroscopist and
photochemist alike. Thus as Noyes et al. point out, “The complete
VOCABULARY OF PHOTOCHEMISTRY 3
Fig 1. Sample from the manifold of excited states of a representative organic
molecule. Straight lines represent radiative and absorptive processes; wavy lines
show nonradiative processes; chemical transformations are not shown in this figure.
elucidation of a primary photochemical process must include an under-
standing of all that transpires, whether or not a chemical reaction
occurs” (14).
It is clear from the above discussion that most of the glossary of
photochemistry can be classified in terms of the three operations: (I)
production of exciting light, (2) absorption of the light, and (3)t he vari-
ous transformations of electronically excited states, including the chem-
istry of radicals and other reactive species produced therefrom. We shall
follow approximately this order of presentation.
Light Sources
Many kinds of light sources are currently in use. Those which give a
constant light output over an extended period of time, such as the mer-
cury arcs mentioned below, are often referred to as constant or steady
light sources. However, in recent years lamps which give a very high
light intensity for a very short period of time have found many applica-
tions. Thus, a photdysisjlash lamp is used to produce a photochemical
reaction and a spectro-$ash lamp is employed to determine the absorp-
tion of the transient species produced by the high intensity photolysis
flash.
4 J. N. PITTS, F. WILKINSON, AND G. S. HAMMOND
Pressure Broadening.-Increase in width of emission or absorption
lines caused by increase in the total pressure of a system.
High Pressure Lamp.-A lamp filled with mercury or xenon vapor
(or a mixture of them) operated at about 10 OF more atmospheres pres-
sure. Although the total output is very high, the emission is largely
above 3000 A. In mercury or mercury-xenon lamps it consists of broad
(Il ines’’ superimposed on a strong continuum. The 2537 A. line is almost
entirely lost hy self-absorption in the lamp.
Medium Pressure Lamp.-A lamp filled with mercury vapor and oper-
ated at a pressure of about 1 atmosphere. The total intensity in the
near ultraviolet and visible is lower than that of high pressure lamps,
but photochemically useful light at wavelengths less than 3000 A. is
produced. The ultraviolet spectrum consists of reasonably narrow
“lines” with only a weak continuum. Hence, in conjunction with a
filter or monochromator it is a good source for monochromatic radiation
including “reversed” 2537-A. radiation (see definition below).
Resonance Lamp.--Such lamps (sometimes called “low pressure”
lamps) are often used as line sources in photochemical studies. These
usually contain a small amount of a metal vapor (e.g., mercury, cad-
mium, zinc, etc.) and several mm pressure of a rare gas. They operate
at relatively low current (ca. 100 ma.) and high voltages (several thou-
sand volts). This is in contrast to a typical medium pressure lamp which
may operate off a 110-220 v. power supply delivering ca. 3-5 amp. The
most common example in photochemistry is the mercury resonance lamp
which has strong emission of the unreversed “resonance” lines at 2537 A.
and 1849 A. (ca. 90% or more of the total) along with other, much weaker
lines (“resonance lines” are those which appear both in absorption and
emission). There is little continuum. Sources of this type are widely used
for photosensitized reactions.
Self-Reversal.-Disappearance of an expected source line at high
pressures because of reabsorption in the lamp.
Reversed Radiation.-A pressure broadened resonance emission line
with radiation in the middle of the ‘(line” virtually absent because of
self-absorption near the walls, (e.g., the 2537 A. “line” obtained from
medium pressure mercury lamps).
Wave Number.-A unit of energy with dimensions of em.-’ and
symbolized as v according to Mulliken’s “Report on Notation for the
Spectra of Polyatomic Molecules,” (6) and Herzberg (1). It is also often
symbolized by F. Herzberg uses v‘ to symbolize frequency in see.-’.
