Showing papers in "Annual Review of Physical Chemistry in 1970"

Magnetic and Electric Effects on Transport Properties

Abstract: with an introdu ctory section in which the recent history of the subject is presented along with a short outline of the underlying physics. A survey of the principal developments will then be given. In order to facilitate the review procedure, the subject matter will be sub divided in the following manner: the influence of a static magnetic field on the transport properties of dilute gases is split into two parts, one treat­ ing theory and the other experiment; this is followed by a section dealing with the analogous situation in a rarefied gas; then, related work concerning the influence of a static electric field on the transport properties of polar gases is described; and, finally, the influence of alternating fields on the trans­ port properties is covered briefly.

Photoreactivity of n,π* Excited States of Alkyl Ketones

Abstract: ion of a hydrogen from the solvent are often observed (vide infra). Having discussed the energies and primary photophysical processes of the $1 and T1 states of alkyl ketones, we can now construct an energy diagram as shown for acetone in Figure 3. We shall assume that acetone is a good model for the photophysical behavior of other aliphatic ketones. The important features of this diagram are: (a) the $1 (n,~r*) state of acetone relatively low in energy compared to other simple chromophores; (b) the Tx (n,~r*) state of acetone is relatively high in energy compared to most chromophores (only simple benzene derivatives possess a higher triplet energy); (c) the lifetime of 31 is long enough to allow participation in bimolecular processes of low activation energy. Because of their high triplet energy, aliphatic ketones are commonly used as photosensitizers for unsaturated compounds possessing low absorption above 2800 QVANTUM YIELDS AND PtEAiCTIVITIE$ The quantum yield (~b) for formation of a photoproduct, a measure of the efficiency with which the photoreaction proceeds, is by definition equal to the number of molecules of photoproduct formed per photon of light absorbed. It can also be viewed as the probability that the initially generated electronically excited state will yield the given photoproduct. Take, for example, a unimolecular photochemical isomerization of A to B occurring www.annualreviews.org/aronline Annual Reviews A nn u. R ev . P hy s. C he m . 1 97 0. 21 :4 99 -5 60 . D ow nl oa de d fr om a rj ou rn al s. an nu al re vi ew s. or g by C ol um bi a U ni ve rs ity o n 03 /0 6/ 09 . F or p er so na l u se o nl y. PHOTOREACTIVITY OF n,,r* EXCITED STATES 505 exclusively from the Tt state of an alkyl ketone (sA). If every photon absorbed is assumed to result in the formation of the St state of A (1A), the quantum yield for formation of B ($B°) will be the product of the probability of 1A leading to 3A times the probability of 3A proceeding on to B. ~B° will then be given by Equation 5 where kr is the unimolecular rate constant for the photolsomerizatlon of 3A to B. ~B° = = dpsTkrrT 5. ~ + kS T + k~ kr + ka + k The reactivity of an electronically excited state toward a primary photochemical process is defined as the rate constant for that process. Thus for the photoisomerizafion of A to B, kr is a measure of the reactivity of aA toward the photoisomerization (or the primary photoprocess which proceeds the isomerization). The quantum yield for a photoreaction, which depends upon the relative rates of the various competing processes occurring from the electronically excited states (Equation 5), is not in general a good measure of either the absolute or the relative photoreactivity of excited states (27-29). The use of energy transfer quenching (30) has become an extremely popular technique for estimating the reactivity of excited states toward various primary photochemical processes. In this method the yield of product formation is decreased by the addition of a compound which is ~nown to quench a specific excited state by electronic energy transfer. For example, the quantum yield for the triplet state isomerization of A to B will be lowered by the introduction of the additional pathway for deactivation of *A shown in Equation 6, resulting in the expression for $~ given in Equation 7. Division of $~0 (Equation 5) by $~ (Equation 7) yields

Electronic and Vibrational Excitons in Molecular Crystals

Abstract: This review is the third in the Annual Reviews of Physical Chemistry (1) during the past eight years that treats the optical properties of molecular crystals. Two other reviews (2) on this same type of subject matter, but with slightly different emphasis, have appeared recently in the literature. Besides these review articles, other recent treatises are strongly recommended to the reader. Knox's book (3) , for example, presents a much broader presentation of exciton theory and practice than this review attempts to do. For those readers who are interested in a unified approach, not just to the Frenkel exciton problem in organic crystals, but also to other kinds of excitons in other kinds of crystals, Knox's book is strongly recommended as a starting point. A condensed but rather clear discussion of many of the important aspects of space group theory and time reversal symmetry as they apply to the general energy band problem is to be found in the book by Callaway (4) . Other good sources for this type of information are the classic paper by Koster (5) and the books on applied group theory by Tinkham (6) and Mariot (7) . Craig & Walmsley (8) outline the fundamental theoretical material neces­ sary for an understanding of the subject matter presented in this review. A fairly recent paper by Davydov (9) corrects, clarifies, and extends Davydov's own classical work on the molecular exciton problem. Because of the exist­ ence of this publication, the original book in Russian ( 10) and its later English translation ( 1 1 ) , which describes Davydov's early work, are now to be valued more for their historical content and bibliographies than as a source for study of modern molecular exciton theory. Since Hochstrasser's review, a large number of new experimental and theoretical techniques have been introduced leading to many significant advances in molecular exciton theory. The field is a much more polished one than it was in 1966, and its scope is considerably broader. The field too has taken on new significance in recent years because of the partial shift of

Elementary Gas Reactions

Abstract: In agreeing to undertake a review with the above title, we did not reckon with the formidable energy and talent of our immediate predecessor in this area, F. Kaufman, nor with the coverage offered in two companion chapters in Volume 20, one on Shock Tube Technique in Chemical Kinetics by Belford & Strehlow and one on Isotope Effects by Wolfsberg. What appears below then are some vignettes in the field of gas kinetics arbitrarily selected for their interest-to us. We begin by discussing the effects of translational as well as internal energy in "driving" bimolecular reactions. We then focus atten­ tion on intramolecular vibrational relaxation in activated molecules and the role of centrifugal effects in decomposition of such activated species. Finally we look briefly at the hydrogen-iodine reaction and the implications of recent work involved with this long misunderstood system.

Excited states and reactions in liquids.

Abstract: In the beginning was the three star, one smile of ligh t across the empty face. Dylan Thomas (1) Excited states provide the chemist and biologist alike with suitable scapegoats with which they can at least qualitatively explain unusual data. T he central position occupied by excited states in photosynthesis, vision, laser technology, etc, cannot be denied, while the implication of energetic states in electro and chemiluminescence and oxidation reactions is only just coming to the forefront. From early beginnings, dealing mainly with the long-lived triplet state, it is now possible to observe excited states down to 10-12 sec by utilizing the advances made in technology over the past two decades. The object of this short review is to point out the advances and con­ tributions made by new techniques such as lasers and pulsed accelerators to both the mode of production and the understanding of the nature of excited states. The review will deal mainly with excited states in the liquid phase, with occasional reference to the gas and solid phases when these are ap­ propriate. Several books dealing with various aspects of excited states have been recently published : Theory and Interpretation of Fluorescence and Phos­ phorescence, R. Becker; Excitons in Molecular C rystals, D. P. Craig & S. H. Walmsley; Molecular Spectroscopy of the Tri plet State, S. P. McGlynn, T. Azumi & M. Kinoshita; Photoluminescence, C. A. Parker; the papers of two luminescence conference have also been published: Molecular Luminescence, edited by E. C. Lim; the Proceedings of the International Conference on Luminescence, edited by G. Szigeti; and The Chemistry of Ionisation and Ex­ citation, edited by G. R. A. Johnson & G. Scholes.

Nuclear Magnetic Resonance

Abstract: The technique of nuclear magnetic resonance spectroscopy originated in 1946. Four years later, in the first volume of the Annual Review of Physical Chemistry (1), only two references were made to N M R. By 1954, a review on N M R by Gutowsky (2) contained 200 references. He stated that about 400 articles on N M R had been published between 1946 and December 1953. In 1964, Grant (3) indicated that about 1000 papers a year were being published in high resolution NMR alone. At the end of 1967, Jonas & Gutow­ sky (4) estimated that N M R publications were appearing at the rate of over 500 per month! This review covers the years 1968-1969. Even if we omit aU the system­ oriented studies in which well-known NMR techniques were used, we are left with too much material for the limited length of this review. Thus, as other authors have done previously (4, 5), we have selected a few topics for comprehensive coverage, namely biological applications of NMR and carbon13 N M R. Our selection is based on the recent spurt of growth of these areas. The possibilities of N M R in biology have been greatly increased with the advent of spectrometers which use high-field superconducting magnets, both commercial (6) and \"home-made\" (7,8). Other instrumental improve­ ments have now made natural abundance carbon13 NMR a practical tool for chemists. Unhappily, our coverage of other important areas will have to be very brief, and many interesting topics will not be mentioned at all. How­ ever, we are making available a supplementary bibliography.1 Several books on N M R have appeared in 1968-1969. The books by Bovey (9) and Becker (10) deal mainly with high resolution proton NMR. Jackman & Sternhell ( 1 1) have greatly expanded and updated an earlier book by Jackman on proton N M R in organic chemistry. Memory ( 12) has written an introduction to quantum mechanical calculations of chemical shifts and coupling constants. Eaton & Lipscomb (13) have made a comprehensive compilation of N M R studies of boron hydrides and related species. The third volume of Advances in Magnetic Resonance (14) has appeared.