Showing papers in "Annual Review of Physical Chemistry in 1976"

Electronic Energy Transfer in Molecular Crystals

Abstract: In the last ten years, a number of reviews of properties of molecular crystals have appeared in this series (1, 2, 3). The spectral properties of insulating molecular crystals [which are often described within the framework of the Frenkel theory of excitons (4, 5)] have been reviewed by Hochstrasser (I) and by Robinson (2). The properties of charge transfer crystals have been reviewed recently by SODS (3). In addition, the review by El-Sayed (6) on double resonance techniques applied to triplet states of organic molecules concerned itself with organic molecular crystals. In the present review, I focus on a different aspect of molecular crystals: the transport of electronic excitation from molecule to molecule. This process, and its effect on optical spectra, electron paramagnetic resonance spectra, and fluorescence, has been of great interest in the last few years; I present a critical review of the theoretical developments in this field. Specific attention is paid to the theory of exciton transport in molecular crystals, the theory of exciton-molecular vibration and exciton-phonon interactions, and finally, to a quite different topic, the theory of excitation transport across surfaces. Those works dealing with the optical properties of molecular crystals within the coupled oscillator model are not dealt with. It will be assumed that the interaction of light with the system is weak enough so that retardation effects and the formation of polariton states may be neglected. Only work dealing with excitation transport is dealt with herein. We begin by discussing the simplest type of system in which excitation transfer takes p lace in order to define the terms necessary to treat excitation transfer in more complex systems.

History of H3 Kinetics

Abstract: Equilibrium and steady-state rates of reaction are considered as studied experimentally by thermal conversion, flow tubes, and shock tubes, and theoretically using transition-state theory. Non-Boltzmann experiments (hot atom studies and molecular beam reactions) and the interpretation of these experiments as well as the equilibrium ones in terms of state-to-state cross sections are also considered. 302 references (GHT)

The Calculation and Measurement of Cross Sections for Rotational and Vibrational Excitation

Abstract: A review is presented of the conversion of translational into internal rotational and vibrational energy given by: A + BC(j/sub i/, m/sub i/, n/sub i/) ..-->.. A + BC(j/sub f/, m/sub f/, n/sub f/) +- ..delta..E where A is a ground state atom, another molecule--considered to be structureless--or an ion, and BC is usually a diatomic molecule with rotational quantum states j and m and vibrational state n; i and f denote initial and final states, respectively, and ..delta..E denotes the change in relative translational energy. 4 figures, 4 tables, 341 references (GHT)

X-ray Crystallographic Studies of Proteins

Abstract: Following the dramatic impact of the first protein structure determinations, the field of macromolecular crystallography has expanded with ever-increasing rapidity. In many cases the techniques used are the same as those pioneered by Perutz and Kendrew and their disciples, but the preoccupation has shifted from methods to results. The first glimpses of protein architecture, provided by myoglobin (1) and hemoglobin (2), showed that proteins with related function could have structural similarity. In general, however, the early structure determinations, of lysozyme (3), ribonuclease (4), carboxypeptidase (5), chymotrypsin (6) and so on (Table 1), demonstrated most strikingly the enormous variability of protein structure. Recent structure analyses have provided further examples of diversity in structure, but at the same time have shown the existence of similar patterns of folding within different proteins that may or may not have related function and may or may not have recognizably homologous amino acid sequences. The origin of these recurrent "domains" or "super secondary structures" is the subject of considerable debate. Not only has the number of known structures continued to grow, but also the quality of a few structure determinations has been increased by improved data collection and refinement techniques to the point that relatively precise statements can be made about protein conformation and function. In a review of this type it is impossible to discuss all the protein structures known at the present time. Rather, the intention is to describe those areas of protein crystallography within which significant advances have been made during the last few years. Table 1 lists those protein structures for which the polypeptide chain has been traced, and includes selected references to the more recent literature. Also a number of relatively recent reviews are available (7-13).

Chemical Reactions at Very Low Temperatures

Abstract: INTRODUCTION The Arrhenius law, k = Aexp[ -ElkB T], predicts that the rate of a chemical reaction vanishes in the limit T -+ O. Here, k is the rate constant, E the activation energy, and kB the Boltzmann constant. However, as early as 1903 Moissan & Dewar (1) observed rather fast fluorination of hydrocarbons at low temperatures. The next thirty years were marked by observations of efficient oxidation of nitric oxide (2), chlorination of ammonia (3), and NO-NCI3 interaction (4), as well as of hctcrogencous hydrogcn ortho-para convcrsion (5) and hydrogcn-dcutcrium exchange (6) at low temperatures, up to the boiling point of nitrogen (77°K). The interest in chemical reactions at low temperatures increased in the 1950s, when new experimental methods for studying them emerged, e.g. tracer methods, radiospectroscopy, etc. Attention was drawn to high free-radical concentrations in solid matrices and to various solid-phase radiation reactions involving electrons and ions, radicals and molecules. Gradually it became clear that most low-temperature reactions actually proceed faster than would be predicted on the basis of an Arrhenius-type extrapolation. There exist two main reasons for such deviations from the Arrhenius law at low temperatures. First, if the conversion of a certain species A can proceed via several parallel channels-I. A -+ B, 2. A -+ C, 3. A -+ D, etc-each with its own activation energy E" at low temperatures the channels with higher activation energies are suppressed and only one single channel of the least (Ei)min magnitude remains. For example, since ion-molecule reactions are activationless, the ionic mechanism of the polymer chain growth often prevails over the radical one at low temperaturcs, and this can even alter the polymer structure. Second, even for single-channel systems quantum-mechanical tunneling can result in very strong deviations from the Arrhenius law and an apparent decrease of activation energy with the decreasing temperature, and in high observable reaction rates at very low temperatures. The definitions of low and very low temperatures are quite conditional and their sense in physics, chemistry, and biology differs. Our paper is devoted to "very low"

Molecular Dynamics and Monte Carlo Calculations in Statistical Mechanics

Abstract: Monte Carlo and molecular dynamics calculations on statistical mechanical systems is reviewed giving some of the more significant recent developments. It is noted that the term molecular dynamics refers to the time-averaging technique for hard-core and square-well interactions and for continuous force-law interactions. Ergodic questions, methodology, quantum mechanical, Lorentz, and one-dimensional, hard-core, and square and triangular-well systems, short-range soft potentials, and other systems are included. 268 references. (JFP)

Reactions of electronically excited-state atoms

Abstract: Gas-phase electronic energy transfer processes are reviewed for excited atoms with sufficient energy to yield electronically excited products in encounters with other atoms or molecules. Attention is focused on excited states that result from orbital excitation rather than low-energy spin-orbit states that have the same electronic configuration as the ground state. 281 references (GHT)

Molecular Electronic Structure Theory: 1972-1975

Abstract: Emphasis is placed on the most important contributions to electronic structure theory since 1972. Theoretical developments in the Hartree--Fock problem and the electron correlation problem are covered, as are applications of the theory. 269 references (GHT)

Phase Transitions-Beyond the Simple Ising Model

Abstract: In the modern development of the subject of phase transitions there has been an emphasis on spin models, such as Ising or Heisenberg models, and their lattice gas analogues. Also, for obvious reasons, the simplest models with a single kind of interaction term have been studied most. There has been a tendency to emphasize the critical point somewhat to the exclusion of the rest of the phase diagram. However, current papers, in increasing proportion, are now treating a variety of more complex systems with a modern degree of theoretical rigor and experimental precision. This modern development is reviewed. Subject areas covered include: multiterm systems; multicritical points; and lattice systems with hard interactions. (GHT)

Liquid Theory and the Structure of Water

Abstract: This review is largely devoted to developments in the theory of liquids in connection with the structure and properties of liquid water and to aqueous solutions. Recent detailed reviews and discussions of models of water are considered. Eisenberg & Kauzmann (1) have written a book on The Structure and Properties of Water, and Horne (2) has edited a book containing papers on water and aqueous solutions. Very recently, Franks (3) has completed an extensive review on water covering the period starting in 1972. During the past two years, several reviews on water have been written. Perram (4) reviewed the progress made toward a liquid state theory of water, particularly as regards an interstitial model of water. Hornung & Choppin (5) wrote a review concerning the spectroscopic properties of water. To explain the interaction of water with proteins, various models have been reviewed and compared by Hagler, Scheraga & Nemethy (6). Jhon & Andrade (7) wrote a review of water in gels. A review on structure models, spectroscopic studies, static and dynamic properties, and the role of water in solutions and membranes h�s been discussed by Salford & Leung (8).

High Temperature Chemistry: Modern Research and New Frontiers

Abstract: This monograph is restricted primarily to a discussion of a select group of techniques and frontier areas that have developed since 1970. This review discusses primarily gas- and liquid-phase phenomena but also focuses on some aspects of solid-state high temperature chemistry. (3 tables, 4 figures, 117 references)

Effects of Molecular Mobility on Reaction Rates in Liquid Solutions

Abstract: This review concerns molecular motions, which include motions of geminate pairs (cage effects), the formation of encounter complexes, motions of solvent molecules, and conformational changes of the reagent molecules. With rare exceptions, we discuss these motions only to the extent that they have an effect on the rates or kinetically-controlled product distributions of chemical reactions. Because they are covered in separate reviews in this or preceding volumes, we have intentionally neglected picosecond processes, electron tunneling, and reactions in liquid crystals. Although theoretical and experimental tools have been developed to the point where they are useful for studying the effects of molecular motions in liquid-phase reactions, thest: tools are being continually refined and new ones developed. In particular, we noted a continued concern with cooperative effects in diffusion (1-3), effects of local depletion of the reagent (1, 3), possible effects of excess energy in a caged pair (1), and hydrodynamic models (4-8). Physical methods currently in use for probing rotational motions of solutes in liquids include studies of infrared and Raman lineshapes (9), studies of NMR relaxation times (10-13) and picosecond flash experiments (13a). In addition, considerable information about molecular rotation is obtained from studies of caged pairs in which one or both of the partners are chiral. Information about fast conformational changes is obtainable from picosecond flash experiments and from NMR, ultrasonic, and other forms of relaxation spectrometry. Heywood et al (14) have described a useful statistical method of analyzing data from ultrasonic relaxation experiments on molecules undergoing conformational changes, and Strehlow & Frahm (15) have presented a theory for obtaining, from NMR Tl relaxation times, not only rates of conformational inter­ conversion but exchange rates in general.

Solutions of Nonelectrolytes

Abstract: We introduce, with this chapter, the physical chemistry of systems for which composition is a state variable. A solution is a mixture at the molecular level of two or more chemical species; it may be gaseous, liquid, or solid. If clusters of molecules are present, the situation becomes more complex. If the clusters are of the order of 100 A to a few thousand A or around 10\" 4 cm in size, the system is colloidal in nature. If they reach to 10\" 4 to 10\" 3 cm, we speak of the mixture as a suspension, an emulsion, or an aerosol. Beyond this, we simply have a mechanical mixture of two or more bulk phases. There are no sharp natural boundaries in this sequence, but there are practical ones. Most physical chemists have concentrated on the extremes, that is, on molecular solutions or on systems having well-defined bulk phases which themselves may be solutions. On the other hand, much of the biological and physical world involves mixtures of the colloidal or intermediate type of dispersity. The physical chemistry of these last systems is difficult, and its introduction is reserved for Chapter 21. We confine ourselves here to the simpler case of molecular solutions. A solution or mixture of gases presents little problem, at least at the level of complexity of this text. Unless very dense, gases are always fully miscible and, in the usual laboratory pressure range of around 1 atm, form essentially ideal solutions. Dalton's law of partial pressures [Eq. (1-18)] is well obeyed. We will consider the entropy and free energy of formation of gaseous mixtures in Section 9-4. Solid solutions, that is, molecular dispersions of two or more species in a solid phase, are quite common. Alloys are one example; also, many ionic crystals are able to substitute one type of ion for another (of the same charge) almost randomly within their crystal lattices. Solid solutions are more difficult to study experimentally, however, and are less studied than liquid ones. Their behavior is also more subject to eccentricities. Most of our data are from, and most of our common experience is with, liquid solutions. For these reasons, the material that follows