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Book Review Fundamentals of Photoinduced Electron Transfer. By G. J. Kavarnos

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BOOKS
A Wide Spectrum for Analysis
Spectroscopy in Catalysis. An Introduction. By J. K Niemantsverdriet.
VCH Verlagsgesellschaft, Weinheim/
VCH Publishers, New York, 1993.
288 pp., hardcover DM 148.00,
$95.00.--ISBN
3-527-28593-8/156081-792-5
Heterogeneous catalysis under real
conditions is an extremely complicated
process, not least
due to the complex nature of the
catalysts and the
structural changes
that they undergo
as a result of variations in the reacting environment
and other external
conditions. Ultimately one would
like to have experimental methods that
could reveal in situ on an atomic scale the
chemical processes occurring on catalyst
surfaces. As these do not exist, systematic
progress in understanding heterogeneous
catalysis can only be achieved by the combined use of as many analytical techniques as possible in conjunction with
model studies. The latter are important
for two reasons. Firstly, they enable one
to study systems under well-defined conditions with regard to surface structure,
surface composition, and external reaction conditions, so that the dependence on
individual variables can be investigated.
Secondly, they can be carried out under
conditions that are essential for many of
the analytical methods; for example, ultra-high vacuum conditions are necessary
for all types of electron spectroscopy.
This section contains book reviews and a list of
new books received by the editor Book reviews are
written by invitation from the editor. Suggestions
for books to be reviewed and for book reviewers
are welcome. Publishers should send brochures or
(better) books to Dr. Ralf Baumann, Redaktion
Angewandte Cheniie. Postfach 101 161, D-69451
Weinheim. Federal Republic of Germany. The editor reserves the right of selecting which books will
be reviewed. Uninvited books not chosen for
review will not be returned.
A n R m Chhein. lnf.Ed. Engl. 1995. 34, No. 4
XJ
Whereas on the one hand in-situ measurements under real conditions seldom
lead to unambiguous conclusions about
relationships, model studies on the other
hand suffer from the inherent danger of
oversimplification and the neglect of
“synergistic effects”. However, so far
there has been no way out of this dilemma
(the gap between surface science and
catalysis) other than to approach the
problem from the two sides simultaneously. With this in mind the book reviewed
here makes a particularly useful contribution in describing as wide a range of complementary methods as possible.
The book begins with an introductory
chapter outlining the basic features of heterogeneous catalysis, its enormous technological and economic importance, and
the scientific problems that it poses. This
is followed by seven chapters dealing with
the most important methods used to study
the physical properties of, and chemical
processes occurring at, catalyst surfaces,
as follows-Chapter 2: Temperature-programmed surface reaction and desorption
techniques; Chapter 3 : Photoelectron and
Auger electron spectroscopies ; Chapter 4: Ion scattering and ion spectroscopy
techniques, including mass spectrometry
of secondary ions and neutral particles;
Chapter 5 : Mossbauer spectroscopy;
Chapter 6: X-Ray and electron diffraction methods; Chapter 7 : Electron microscopy and scanning probe microscopy;
Chapter 8: Vibrational spectroscopies
with photon and electron excitation.
Whereas those methods that depend on
particles (electrons, ions, or atoms) for excitation or detection usually require vacuum conditions, the photon techniques can
be used in situ. The methods described
can provide answers to all the relevant
questions, such as those relating to the
atomic structure of surfaces, their qualitative and quantitative chemical composition and the distribution of the various
surface constituents, the electronic structure of surfaces and adsorbed reacting
species, and their vibrational properties as
an indication of the nature of the chemical
bonding.
The author is a physicist and an active
researcher in an institute devoted to catalysis research. Through a good choice of
VCH Verlugsgrsellxhafi mbH, 0-65451 Weinheim, 1595
subject matter and the way in which it is
presented he makes a valuable contribution to bridging the gap between surface
physics and catalyst research. The theoretical fundamentals of the individual
methods are treated in a clear and easily
understandable way, with just the right
amount of detail for an initial assessment
of the capabilities and limitations of the
techniques for specific applications in
catalysis research. Especially useful in this
respect are the discussions that have been
worked into the text in many places, comparing and evaluating the (mutually complementary) information given by the various methods when applied to a particular
problem. Also of special value is Chapter 9, in which three case studies (supported rhodium catalysts, Co/Mo sulfide
desulfuration catalysts, and alkali promotors on metal surfaces) are taken as examples, stretching each of the previously
described methods to its full capabilities
to yield the maximum information about
the catalytic system.
The book can be recommended equally
for the catalysis scientist, to extend his or
her knowledge of the methods, to the surface physicist to stimulate interest in the
analytical problems of heterogeneous
catalysis, and for the student wishing to
specialize in catalysis.
Klaus Wundelt
Institut fur Physikalische Chemie
der Universitat Bonn (FRG)
Fundamentals of Photoinduced Electron Transfer. By G. J. Kavarnos.
VCH Verlagsgesellschaft, Weinheim/VCH Publishers, New York,
1993.359 pp., hardcover DM 105.00,
$75.00.-ISBN 3-527-27856-1,’
0-89573-751-5
This volume is intended to serve as an
introduction and course book to accompany a lecture series on photochemistry
for students of all branches of the natural
sciences. Accordingly, special attention
has been given to presenting the subject in
a suitable form for teaching purposes. The
text is divided into six chapters. The
first two, “Introductory Concepts” and
0570-0833/5510404-0495 $ 10.00
+ .Xi0
49 5
BOOKS
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~
“Properties of Charge-Transfer Intermediates in Photoinduced Electron Transfer”, give a general introduction to elementary theoretical models and the
variables involved, and a survey of the
most important experimental methods for
studying electron transfer. The chapters
on “Electron-Transfer Photochemistry”,
“Intramolecular Photoinduced Electron
Transfer”, and “Photoinduced Electron
Transfer in Organized Assemblies and the
Solid State” convey a comprehensive picture of the many different types of photoinduced electron transfer processes and
the resulting chemical reactions, especially in organic molecules. The systems and
reactions in which the initiating step is a
photoinduced electron transfer range
from isomerizations, cleavage of carboncarbon bonds and other photochemical
transformations, electron transfer processes in micelles, vesicles, and semiconductor surfaces, through to applications
in photovoltaic cells and xerography.
Lastly Chapter 6, “Theories of Photoinduced Electron Transfer”, summarizes the
main theoretical approaches to the interpretation and quantitative description of
electron transfer rates. Each chapter begins with a useful introduction and ends
with an extensive list of references,
grouped under subject headings and partly annotated with comments, and a set of
exercise problems (without answers).
To summarize the overall impression,
the book serves its readers best when describing the many different phenomena,
systems, and methods of investigation.
Here the numerous figures and tables are
a valuable aid to understanding. On the
other hand, it is less convincing when
dealing with the fundamentals of photoinduced electron transfer. Many of the important concepts involved in the description of these reactions, such as the
through-bond and through-space couplings between electron donor and electron acceptor, the reorganization energy,
and aspects of the thermodynamics, are
treated in a misleading or even incorrect
way. One glaring example is the relationship between the electronic coupling He,,
the donor-acceptor separation, and the
validity of the “Golden Rule” for the rate
of electron transfer. On page 308 the dependence of He, on the separation is given
as follows: ‘$He,=
exp[-P(d- &)]
where /l
is an orbital parameter, d is the
actual separation distance . . . Since 8 is a
measure of the ability of an orbital to extend into space and interact with another
orbital, the magnitude of interaction between donor and acceptor orbitals is inversely proportional to 8.”This statement
does not make sense. Then, beginning in
el
496
C
the second line of page 328 : “Importantly, He, is proportional to A , , the distance
separating two reactants (as well as their
mutual stereoelectronic orientation), and
thus by inference is also proportional to
the probability that two states couple at a
fixed nuclear configuration. The Golden
Rule is valid for small A , . If A , % 0, the
rule breaks down.” This is clearly wrong.
Lastly, on page 329 we read the following
statement about A , : “The horizontal displacement between the minima of these
potential energy curves . . .” ( i t . the potential energy surfaces for reactants and
products as functions of a nuclear coordinate) “. . . is given by A , . ” This too is incorrect. Sadly, the list of such inconsistencies could be continued. Thus, the reader
who wishes to learn not just about the
phenomena but also about the mechanisms-in fact about the “fundamentals
of photoinduced electron transfer”would be better advised to go directly to
the secondary literature that is plentifully
cited in this book.
Hans Heitele
Institut fur Physikalische und
Theoretische Chemie
der Technischen Universitat Munchen
Garching (FRG)
Crystallography made Crystal Clear.
A Guide for Users of Macromolecular
Models. By G. Rhodes. Academic
Press, New York, 1993. 202 pp.,
paperback $34.95.--ISBN
0-12587075-2
With the rapid development of molecular biology, and especially of genetic engineering, there is
an increasing need
for knowledge of
the three-dimensional structures
of biological macromolecules in
full atomic detail.
This information
is essential in order to understand
the mechanisms
whereby they function and to achieve controlled biological action by the methods of
protein design and drug design. The two
methods currently used for structure
determination are NMR spectroscopy
and X-ray crystallography. NMR spectroscopy has the advantage that one can
work with solutions, but difficulties arise
with molecular masses above about
20000 u. Crystallography is not subject to
this limitation, but suffers from notorious
difficulties in obtaining single crystals,
VCH Verlugsgesrllschafl mhH, D-6945/ Wrinhrim. 1995
~
~
and from the necessity to prepare heavy
atom derivatives to overcome the “phase
problem”.
Up to now the molecular biologist or
biochemist wishing to learn about protein
crystallography has needed to rely either
on textbooks of biochemistry or biophysics which typically devote one or two
chapters to crystallography, or on textbooks dealing specifically with protein
crystallography, which are usually too detailed and contain too much mathematics
for easy reading. The book by Gale
Rhodes reviewed here exactly fills that
gap. In just over 200 pages with many illustrations and without many mathematical formulas, the author explains how a
crystal structure analysis is carried outfrom the crystallization of the protein
through to the final refinement of the
structure. The book includes comments
on the validity of models and, especially
useful in my view, a guide to reading publications on crystallography.
Chapter 1 introduces electron density
distributions in protein crystals with the
help of colored illustrations and discusses
their interpretation in terms of models, so
as to kindle the reader’s interest in discovering the results of crystallography. Chapter2 gives a general survey of protein
crystallography, and can be regarded as a
summary of the chapters that follow. In
Chapter 3 the reader learns about the
methods used for crystallizing proteins
and nucleic acids, and Chapter 4 explains
in detail the diffraction of X-rays by the
crystal lattice and the recording of the diffraction data using either cameras (film)
or modern surface counters. Chapters 5
and 6 explain how the phase angles needed for calculating the electron density distribution in a novel structure are obtained
through isomorphic substitution by heavy
atoms using Patterson methods, and on
the other hand how, in cases where similar
related structures are known, the method
of “molecular replacement” can be used.
The author also describes how the electron density distribution can sometimes
be modified (by “solvent flattening”), so
as to obtain a significant improvement in
signal-to-noise ratio. Chapter 7 is mainly
concerned with estimating the quality of a
claculated electron distribution, interpreting it in the form of a model displayed on
a graphics screen, and refining such a
model on the basis of the measured diffraction data. Chapter 8 discusses quality
criteria for models, such as Ramanchandran plots, temperature factors, disordered regions in models, and unexpected
electron density features which can be attributed to solvent molecules. The chapter
ends by analyzing a publication con-
0570-OR33ISSj0404-0496 $ fO.OU+ .2Sj0
Angen. Chem. Int. Ed. Engl. 1995, 34. N o . 4
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