Showing papers in "Annual Review of Physical Chemistry in 1984"

Variational Transition State Theory

Abstract: In recent years our research group has made a systematic effort to study the validity of transition state theory (TST). We have found that the conventional theory is sometimes remarkably accurate, but in many other cases it leads to large errors. Fortunately we have found that a much more reliable theory that has many of the advantages of conventional TST can also be formulated, and it can be applied to practical problems with an effort that is much closer to that required for conventional transition state theory than to that required for quantal dynamics calculations. The two most important features in the improved approach to transition state theory state theory are the variational determination of the transition state and the incorporation of tunneling contributions by multidimensional semiclassical approximations. 13 refs.

Electron Transfer Reactions in Condensed Phases

Abstract: Despite the multitude of formalisms, there is general agreement that the crux of the electron transfer problem is the change in equilibrium nuclear configurations that occurs when a molecule or ion gains or loses an electron. In recent developments attention has been focused on the dynamics of these nuclear configuration changes, and, in addition, on the electronic factors, determining the electron transfer rate. In parallel with these theoretical developments there have been a large number of experimental studies. These studies have not only elucidated the factors determining electron transfer rates, but have in many cases directly tested the theories and suggested modifications to the theories where appropriate. It is impossible in an article such as this to cover the entire area of electron transfer reactions. Instead the authors shall concentrate on those aspects of the problem in which they are particularly interested. Steady-state schemes for the diffusion, activation, and electron transfer steps in bimolecular reactions are discussed first. This is followed by a description in terms of Born-Oppenheimer states and surfaces and a discussion of the classical, semiclassical, and quantum mechanical formalisms. Recent experimental studies bearing on the questions raised are presented in the final section. Throughout the discussion tomore » localized or trapped systems is restricted, specifically to systems in which the electronic interaction of the initial and final states is sufficiently small so that the electron transfer can be described in terms of the electronic properties of the unperturbed reactants and products.« less

The electromagnetic theory of surface enhanced spectroscopy

Abstract: Surface enhanced spectroscopy (SES) was born in 1 974 with the measure­ ment of Raman spectra of molecules adsorbed on a roughened electrode surface (la-e). Given the smallness of the Raman cross section, the detection of a Raman signal should have generated some excitement. This did not happen, but was delayed until 1977 when Jeanmaire & van Duyne (2) and Albrecht & Creighton (3) showed that the rough silver surface enhances the Raman cross section by a factor ranging between 104 and 106. This started a great outpouring (4a-i) of theoretical and experimental work, whose central theme is to understand how the presence of a solid surface modifies the spectroscopic and photochemical properties of a molecule located nearby. We divide the effects of the surface in two categories: electromagnetic, which can be described by solving Maxwell's equations, and chemical, which belong to quantum chemistry. The enhancement of the local laser field due to the polarization of the surface is an example of an elec­ tromagnetic effect. The appearance of a new excited state caused by chemisorption, leading to an enhancement of the Raman cross section through resonance Raman scattering (which would not be expected on the basis of the gas phase properties of the molecule), is an example of a chemical effect. While there is no doubt that chemisorption modifies the optical response of adsorbed molecules, the magnitude of the modification is still a subject of controversy. We don't know whether we should expect large modifications for all molecules, or for a small class (e.g. those with 11: orbitals); or whether

Transition metal molecules

Abstract: What is our present experimental knowledge of the electronic, magnetic, and structural properties of transition-metal molecules and clusters? How do these properties compare with theoretically-computed ones? These are the questions that we try to answer here.1 Although the distinction between a molecule and a cluster remains undefined, in this context the word "cluster" hardly seems appropriate sinee all of the presently characterized species, with only one exception, contain less than six metal atoms.2 The enthusiastic and welcome interest of theoreticians has produced a literature that has more than compensated for the dearth of experimental data on transition-metal molecules: They have not been daunted by the difficulties involved in making ab initio all-electron calculations on these heavy-atomed molecules, and even on clusters as large as CU13' The numerous semiempirical and ab initio methods continually being intro­ duced by quantum chemists are now customarily abbreviated to initials which are proliferating beyond perhaps even the theorists' memories. A glossary of such abbreviations used throughout this chapter is given in the footnote below.3

Dielectric Properties of Polyelectrolyte Solutions

Abstract: Dielectric properties of polyelectrolyte solutions have been studied experi­ mentally for more than thirty years, but progress in understanding these properties has been rather slow. A particularly unfavorable combination of experimental difficulties and unsettled theoretical problems has noticeably influenced both the extent of the work devoted to this subject and the pace in which explanation and prediction proceeds in this field. The latter is not surprising in view of the difficulties encountered in dielectric theory in general (see e.g. Refs. 1 , 2) and taking into account the additional problems arising through the presence of free charges in the solution. Moreover, in the complementary field of the dielectric properties of ordinary electrolyte solutions, the situation is hardly better, and even there many problems remain unsolved in spite of recent progress (3). In recent years new theoretical work directly or indirectly related to the dielectric properties of aqueous polyelectrolyte solutions has appeared (see the section on Theory), focusing attention again on the rather un­ satisfactory state of affairs in this field. Hence, the appearance of this review seems justified and we hope it will stimulate further investigations, theoretical as well as experimental, in which new possibilities available through developments in other areas might be exploited. A further justification is that in recent times, only a few review articles of limited extent in this field have appeared (4-7). We shall limit this survey to aqueous solutions of true polyelectrolytes,

Simulation of polymer motion

Abstract: Presented here is a review of computer simulations for the dynamics of polymeric systems. Even restricting the review to the more recent results, the subject is so large that some omissions are necessary. The review is concerned with the present theoretical understanding of irreversible processes in polymeric systems. Accordingly, I do not discuss the chain dynamics associated with conservative vibrational motions in either crystalline or liquid states. More detailed reviews on the static and dynamic properties of polymers are given in (1-3). Details concerning Monte Carlo simulations of polymers are given in (4). One of the most interesting and unusual features of linear macromole­ cules is their dynamical behavior. Due to its connectivity, the motion of a flexible chain is quite different from the motion of small molecules in ordinary liquids. Two fascinating phenomena result from this property: the internal Brownian motion of an isolated chain related to its elasticity, and the collective motion of many chains governed by the topological restriction on a given chain imposed by many other chains. I discuss the former aspect in the first section of the review, the latter in the following.

Characterization of Surfaces Through Electron and Photon Stimulated Desorption

Abstract: Electron stimulated desorption (ESD) and photon stimulated desorption (PSD) are the surface analogues of electronand photon-induced dissoci­ ation of gaseous molecules. In ESD and PSD, beams of energetic electrons or photons (typical energies range from'" 10 e V to > 1000 e V) incident on a surface containing terminal bulk atoms or an adsorbed monolayer of molecules or atoms will cause electronic excitations in the surface species; these excitations can cause desorption of ions, ground state neutrals, or metastables from the surface. A basic difference between surface and gas phase dissociation processes is that a solid surface provides extra pathways for deexcitation that are not possible in the gas phase. There has been a great deal of interest during the last few years in the experimental and theoretical aspects of ESD and PSD (1). Recent theoretical work (2a-e) has provided new understanding of processes involving ion desorption from the surfaces of ionic solids, as well as of mechanisms for ion formation and desorption from covalently-bonded species at surfaces. Angle-resolved ESD and PSD measurements have been established as valuable tools for characterizing the geometry of surface

Stiff-Chain Macromolecules

Abstract: Ordinary flexible-chain polymers such as polymethylene and polystyrene may be characterized by the proportionality ofthe mean-square end-to-end distance to the molecular weight (or the number of skeletal bonds) over a wide range in the unperturbed state with vanishing excluded volume effects (1). This property characteristic of the random-flight chain is violated by (static) chain stiffness arising from structural constraints and hindrances to internal rotation. This leads to the definition of semiflexibleor stiff-chain macromolecules in a broad sense (1 , 2); they include not only typical stiff chains such as DNA, a-helical polypeptides, and cellulose derivatives but also short chains of ordinary flexible polymers. The present review is intended to cover various aspects of equilibrium and nonequilibrium properties of such stiff chains without excluded volume in dilute solution. Emphasis is focused on molecular models, theoretical methods, and adaptation to real chains (or determination of model parameters). Among a number of models presented for chain molecules, the rotational isomeric state model (3) can best mimic the equilibrium conformational behavior of real chains of arbitrary length since it takes account of the details of the chain structure. However, for many equilibrium and steady-state transport problems on stiff chains, such details are not amenable to mathematical treatments, and moreover are often unnecessary to consider. Some coarse-graining may then be introduced to replace this discrete chain by continuous models, although the discreteness must be, to some extent, retained in a study of dynamic properties, especially local chain motions. The foremost of these models is the worm-like chain proposed by Kratky & Porod (4) in 1949 and its numerous, subsequent modifications. Those theoretical developments made before the early 1970s

State-Resolved Molecular Reaction Dynamics

Abstract: Gas phase molecular reaction dynamics is a mature field, but one that continues to offer exciting new perspectives on the fundamental nature of chemical transformations. Chemists have in their vocabulary such familiar concepts as "early" and "late" reaction barriers, the "harpoon mechanism," and "direct" and "complex" reaction dynamics. We also have a modicum of understanding about which forms of energy, i.e. vibrational, translational, electronic, or rotational, will successfully carry a reaction to completion. Much of this understanding comes from ingenious state-resolved experi­ mental measurements on elementary chemical reactions, coupled with the excellent insight provided by detailed theoretical calculations. To give the reader an appreciation for the magnitude of the field, a comprehensive review in 1979 contained 1 144 references (1). Numerous textbooks are also available on the subject (2-4). A number of other recent reviews are of special relevance. These include excellent chapters on the reactivity of selectively excited states (5, 6), energy disposal in simple reactions (7), and thorough introductions to the experimental methods (1 , 8). For theoretical discussions, the reader is referred to the recent articles in the book series edited by Henderson (9) and to earlier excellent reviews ( 10, 1 1). Because so much has already been discovered and said about reaction dynamics, it is important to try to identify significant new developments

Electronic Processes in Organic Solids

Abstract: In recent years, the number of studies of electronic processes in organic solids has assumed explosive proportions. In retrospect, it was inevitable. The increasing ability of organic chemists to design and synthesize molecules and structures almost to order has made it possible to provide an experimental testing ground for what were once figments of the imagi­ nations of theorists. Thus excellent approximations of ideal oneand two­ dimensional solids can be prepared with electrical properties varying from those of an insulator, to those of a superconductor. Disordered systems can be prepared with a large range of nearest-neighbor interaction energies, with and without additional trapping sites, making it possible to test almost any conceivable theory of energy or charge migration in a random or regular network. Increasingly sophisticated computer simulations are providing unusual insights into the microscopic details of dynamical processes, such as carrier or energy transport. These serve as a check on analytical theories and even a guide to indicate which assumptions are likely to be reasonable. The simulations can probe situations that are as yet not readily accessible to experiment, such as time domains in the femtosecond range. All of this ncw interest has been superimposed on what has been an orderly growth in a relatively isolated field, the study of the electrical and optical properties of polycyclic aromatic hydrocarbons

Interactions and kinetics in membrane mimetic systems

Abstract: Membrane mimetic chemistry has become a vitally important area of research (1-5). It is directed to the development of chemistries based on membrane-mediated processes in organized surfactant assemblies and molecular hosts. Aqueous and reversed micelles, microemulsions, mono­ layers, organized multilayers, bilayer or black lipid membranes (BLMs), and vesicles are considered to constitute the organized surfactant as­ semblies (1, 6). Molecular hosts include naturally occurring cyclodextrins (7) and synthetic crown ethers, cryptands, and spherands, collectively referred to as cavitands (8, 9). Organization and compartmentalization in membrane mimetic systems are exploited for reactivity control (2), transport (1 , 10), recognition (1 , 9, 1 1), drug delivery (12-14), and artificial photosynthesis (15-18). Reactivities' in the microheterogeneous environments of membrane mimetic systems cannot be described in terms of homogeneous kinetics. Attention has to be given to the partitioning of the reactants between the organized assembly and the bulk phase and to the structural and dynamic features of the system. Different kinetic treatments have been proposed for "fast" and "slow" reactions occurring in the different membrane mimetic systems. The important factor is, of course, the rate of reaction relative to the rates of reactant(s) and surfactant assembly reorganizations. A large variety of diverse approaches have been taken. Physical organic chemists . have focused their attention on reactions whose rates were appreciably slower than the time scale needed for the reorganization of the membrane

Electron Energy Loss Spectroscopy in the Study of Surfaces

Abstract: Inelastic electron scattering provides a powerful technique for the study of the excitations of free atoms, molecules (1, 2), and solids (3). However, its most recent and very significant application has been in surface science, where electron energy loss spectroscopy (EELS) is used extensively to study the excitations of clean surfaces and adsorbates. The first such study was reported by Propst & Piper in 1967 (4), who studied the vibrational spectra of W(lOO) surfaces exposed to various gases with a resolution of 50 meV. The resolution of EELS was significantly improved by Ibach, who in 1970 reported the study of the surface phonons of ZnO with a resolution < 20 meV (5). Due to the efforts of Ibach and others the resolution has bcen improving ever since (currently a 2-3 meV resolution can be achieved), and the scope and applications of EELS have been constantly widening (6). Electron excitation has several features that make it particularly appropriate for surface spectroscopy. First, it covers a very wide spectral range. The Fourier decomposition of the electric field at the surface produced by an electron at a distance d from the surface and moving with velocity v has frequency components up to a "cut off" frequency we "" vld. Thus the electron behaves like a source of continuum radiation-a "poor man's synchrotron." Therefore, low frequency phonons, high frequency intra-adsorbate vibrational modes, and electronic excitations can be monitored under the same scattering conditions, thus allowing a more complete understanding of the system under study. Second, the strong electron-matter interaction provides both a high surface selectivity and

Electronic Properties of Surfaces

Abstract: Surface physics and chemistry is one of the oldest branches of material science. Experimental studies have been broad and varied. Despite the age of the field and the large body of knowledge that has been accumulated, many researchers consider this area to be a young and vigorous one. One of the main reasons for this impression is that surface science had a renaissance of sorts in the 1970s. It was around this time that experimental studies of clean surfaces were judged to be adequately reproducible. In addition, new experimental and theoretical techniques evolved that became widely available and a concerted effort was undertaken to attempt to understand the properties of at least a few simple systems. This effort has grown and activity has increased, making this sub field one of the largest branches of condensed matter science. This review focuses on the theoretical accomplishments in surface physics. Experimental results are discussed only insofar as they bear on a theoretical result. In addition, because of space considerations, emphasis is placed on only a few theoretical techniques-primarily the pseudopotential approach. Some prototype systems are considered to illustrate the accomplishments of the theory for semiconductors, simple metals (i.e. sp metals), and transition metals. Some details of calculations' and results are given, but the reader should go to the original papers for most specifics.

DIFFERENTIAL ABSORPTION AND DIFFERENTIAL SCATTERING OF CIRCULARLY POLARIZED LIGHT: Applications to Biological Macromolecules

Abstract: A chiral object is defined as an object that is not superimposable on its mirror image. (The group theoretical definition of a chiral object is one that has no rotation-reflection symmetry axes, Sn). Chiral objects can interact differently with left arid right circularly polarized light. Therefore, it is of interest to study this interaction and to learn how this chiral interaction differs from the interaction with unpolarized, or linearly polarized, light. Our emphasis is on large chiral molecules, such as proteins, nucleic acids, and viruses. "Large" here means relative to the wavelength of light, but as circularly polarized X-radiation becomes available in synchrotron sources (1 , 2), "large" will include any molecule.

Human Effects on the Global Atmosphere

Abstract: This review considers whether human activities can significantly change important functions of the global atmosphere by altering the amount or distribution of certain trace species. It deals with three specific topics: stratopheric ozone, the role of species other than carbon dioxide on the greenhouse effect, and certain recently recognized atmospheric consequences of a large scale nuclear war. 64 references, 10 figures, 2 tables.