Showing papers in "Annual Review of Physical Chemistry in 1982"

Hydrocarbon Bond Dissociation Energies

Abstract: The best available values for homolytic bond dissociation energies (BDEs) of various classes of neutral compounds are considered in this review. (BDEs in ionic species is a legitimate subject that is touched on briefly and could easily be included in a longer review. The same can be said for heterolytic BDEs, which are not reviewed as such, although some of the ionic thermochemical data discussed yield values for these processes.) The major emphasis is on hydrocarbons and their nitrogen, oxygen, sulfur, halogen, and silicon-containing derivations, but limited data for inorganic molecules are included. The focus is particularly on prototypical radicals whose heats of formation, formerly thought to be well in hand, have recently been called into serious question. The intent is to include all the major types of sigma bonds, if not all specific cases where known or estimatable heats of formation allow bond dissociation energies to be generated. This review attempts to acknowledge all the standard techniques for measuring BDEs in polyatomic molecules and to offer critical analysis of selected portions of the literature. This leaves values that the authors recommend as the most likely to be correct at the time of this writing. 246 references, 9 tables.

Complex Coordinates in the Theory of Atomic and Molecular Structure and Dynamics

Abstract: Each of these processes is typified by the formation and decay of an intermediate state, or resonance, which is a nonstationary (or quasibound) state with a lifetime long enough to be well characterized, and long enough to make its explicit recognition of experimental and theoretical importance. The simplest, and most naive, mathematical description of such states is that they resemble bound stationary states in that they are "localized" in space (at t =0), and their time evolution

Photofragment Alignment and Orientation

Abstract: Early measurements demonstrated long ago the wealth of chemical information that can be extracted from photofragmentation experiments in which a target system is dissociated or ionized following the absorp­ tion of a sufficiently energetic photon (1-8). For many years experi­ mentalists measured primarily total photofragmentation cross sections or rate constants. More recently, a relatively few studies have determined some measure of the anisotropy characterizing the photofragmentation process (9-20). In contrast to isotropic rate constant measurements that ignore all directional information, measurements of fragment anisotropy give a much more detailed picture of the dynamics of the photoejection process. In the past it has been common to consider separately photodis­ sociation of isolated molecules and photoionization of free atoms or molecules. In what follows we shall call photodissociationjphotoioniza­ tion by the single name, photofragmentation, for, as we show, both processes can be treated together in a unified manner.

Theories of the Dynamics of Photodissociation

Abstract: This review is an account of soine theoretical methods in photo dis socia­ tion dynamics with which the authors are familiar, and a brief compari­ son of experiments with theory. For reasons of space, numerous topics have been omitted, such as a survey of experimental results ( 1 -3), infrared multiphoton dissociation (e.g. see 4), van der Waals complex dissociation (e.g. see 4), and complementary aspects of photo dissociation of polyatomics, reviewed by Gelbart (5). The fascinating aspect of photodissociation dynamics, particularly of a triatomic ABC --> A + BC, is the possibility of solution of the quantum equations of motion, on the ground and excited state potential surfaces, and eventual comparison with the experimental cross sections. This is much more difficult for a three body exchange reaction A + BC --> AB + C, mainly because of the different boundary conditions and because a wide range of impact parameters must be considered. The observables in photodissociation and reaction dynamics are very similar-the disposal

Dynamics of Molecules in Condensed Phases: Picosecond Holographic Grating Experiments

Abstract: The development of laser equipment that can operate routinely in the subnanosecond (and more recently in the subpicosecond) time regimes has made possible the investigation of a wide variety of fast chemical, physical, and biophysical processes. Most of the successful picosecond time scale experiments, although very sophisticated in technique, have utilized the basic approaches that have been applied on slower time scales. One method involves monitoring time-resolved fluorescence following picosecond excitation (1-3). Since the time scale of interest is very short, techniques such as single photon counting, streak cameras, and fluorescence mixing have been employed to provide the necessary time resolution of the fluorescence. In many other experiments, the picosecond probe pulse technique has been used to examine changes in a systems absorption following picosecond optical excitation (4, 5). In this article I discuss a different approach to the application of subnanosecond laser pulses to the investigation of molecular and excited state dynamics. This involves the optical generation of a transient holographic diffraction grating in a sample, and the observation of various time and frequency dependent phenomena via subsequent Bragg diffraction from the induced grating. The basic experiment works in the manner illustrated in Figure 1. Two time coincident picosecond laser pulses of the same wavelength are crossed inside of the sample to set up an optical

Dynamics of Entangled Polymer Chains

Abstract: Polymer melts flow like conventional (viscous) liquids if they are subjected to slowly varying perturbations; but, at somewhat higher frequencies w, they behave like an elastic rubber These mechanical properties depend strongly on the polymer structure (linear/branched, flexible/rigid, etc) In this review, we concentrate on the best understood case: linear, flexible chains, with a large number (N) of monomers per chain The crucial feature is that these chains are entangled The effects of entanglements on the mechanical properties of polymers have been recognized and analyzed in some detail More recently, entanglements have been probed at a more microscopic level: (a) self-diffusion, follow­ ing the motions of one labeled chain; (b) measurements involving spatial effects at distances smaller than the coil size: polymer/polymer welding; spinodal decomposition of blends; neutron measurements on stretched samples; chemical reactions in melts We discuss all these phenomena within a single model, based on the "reptation" idea (1) This model is not final, and may well require future adjustments, but it gives a comparatively simple framework with which to unify these very different types of experiments The viscoelastic properties of polymers have been measured with great care, beginning with the pioneering work of 1 Ferry (2) These properties display a certain characteristic time 7", which increases very rapidly with the molecular weight (or equivalently, with the number N of monomers per chain)

Dynamics of Proteins

Abstract: INTRODUCTION / It has long been recognized that proteins are dynamic systems. Although the three-dimensional structures, which have been determined for an increasing number of proteins, provide some basic insight into the architecture of these molecules, taken alone they cannot explain the observed properties and functions in any mechanistic sense. To quote an example, Perutz & Mathews first noted that the equilibrium structure of hemoglobin leaves no room for a ligand to reach the buried heme binding site (1); the same observation applies to the 02 storage protein myoglobin (Mb), as revealed by x-ray diffraction at 2 A resolution (2). There is ample evidence that the substrate gains access to the interior of Mb as a result of the thermal motion of the polypeptide chain (3). Chain mobility is obviously essential in protein folding (4, 5), but the question that concerns us here is the dynamics of the folded, native form. Conforma­ tional transitions (6-8), allostery (9), active transport (10), enzymatic activity (6, 11), and motility (12, 13) are processes that obviously depend on internal mobility. To delineate the driving forces and pathways of protein reactions is still a distant goal (14). Much progress has been made, however, in the description of the internal dynamics of proteins. While theory makes important predictions about the fluctuations in the thermodynamic parameters of macromolecules (15), a deeper under­ standing of protein dynamics has come from experiments and model calculations (16, 17) on several globular, water soluble proteins. There is hope that the results are representative for this class of proteins, but t�e methods used do not yet provide a comprehensive picture of protein dynamics. The problem is that the characteristic frequencies of the interatomic forces have an enormously wide spectrum, from 10-13 sec to

Time-Resolved Resonance Raman Studies of Hemoglobin

Abstract: The binding of small molecular ligands to the hemes in hemoglobin (Hb) is a highly localized perturbation. Nonetheless, this localized binding initiates a sequence of propagating structural events that culminates in a change in quaternary structure, proceeding from the low affinity deoxy T state to the high affinity liganded R state (1). Although these two equilibrium species have been well characterized, both the mechanism by which ligation triggers structural destabilization and the dynamic path­ ways relating ligation to change in quaternary structure remain as yet undetermined. Moreover, these questions are interdependent, i.e. the dynamics are influenced by the ligation-sensitive structures surrounding the heme. Understandably, it is of fundamental interest to probe the ligation-dependent interactions that affect the stability of the heme environment (hemepocket) in order to establish the relationship between the structural variations of this environment and kinetic phenomena. In the equilibrium structures of Hb the functionally relevant energies associated with the coupling of ligation and quaternary structure may be delocalized. However, in metastable species, generated immediately after ligation or deligation, these energies must be manifested, at least tran­ siently, at the interface between the binding site, i.e. the iron porphyrin, and the surrounding protein. Hence, appropriate time resolved studies of these metastable species would enhance the likelihood of detecting these hitherto unidentified, physiologically important heme-protein interac­ tions. Photolysis of liganded Hb's has been used extensively to prepare nonequilibrium populations of deoxy Hb. In turn, transient absorption

High Pressure Studies of Molecular Luminescence

Abstract: The studies of high pressure molecular luminescence reviewed, along with results for inorganic systems discussed elsewhere, provide evidence about the versatility and power of high pressure as a tool for characterizing electronic states, testing theories concerning electronic phenomena, and generally obtaining a better understanding of electronic behavior in condensed systems. 16 figures.

Ionization in Solution by Photoactivated Electron Transfer

Abstract: Electron transfer reactions have held a special place in chemistry because of their apparent simplicity. Both theoretical and experimental work have uncovered a world of detail beneath this plain exterior. The progress in understanding these reactions in photosynthetic systems and the rele­ vance of these processes to efficient utilization of solar energy have contributed to the momentum of advancement. In this article we restrict ourselves to a subtopic of this field: that of photochemical ion formation in liquids. Reviews in the general area of electron transfer reactions are available: for inorganic complexes, see (1); for photochemistry, see (2); for photosynthesis, see (3), and for radiation chemistry, see (4). We also set aside the topics of direct electron photoejection (5-7) and of multi­ photon events (8). The basic problem of photogeneration of ions in solution is how do the initially formed, highly reactive radical ions ever escape? It is argued that, aside from electron spin selection rules, a finite escape requires a finite jump to a distance greater than the contact or collapse radius of the initial ion pair. It is also argued that the escape and geminate recombina­ tion of the ions cannot be consistently described by the usual steady state kinetics but require an appropriate set of time dependent diffusion equations. We begin the review with a somewhat selective literature survey of photogenerated ions from charge transfer states and from bimolecular encounters. We choose work showing direct evidence for free ion formation from kinetic analysis of absorbance transients or from

Solid State NMR of Biological Systems

Abstract: NMR spectroscopy is widely used in the study of biological problems. A tremendous variety of biological systems have been studied: samples range in complexity from inert gases interacting with biomolecules to intact eukaryotic organisms. The field has been thoroughly described in monographs (1-4) and review volumes and series (5I I). There are also a large number of individual reviews on the subject, including in the Annual Review of Physical Chemistry ( 12-1 5). Even though essentially all of the components of cells have been studied by NMR, the amount, as well as the effectiveness, of the research have not always reflected the fundamental interest in the problems. This is because the studies are often closely linked with the available technology ( 16). Since the instru­ mentation and methods for high resolution NMR of liquids have been well developed for a long period of time, small and medium sized molecules in solution are well studied. In contrast, even though the properties of intact functional biological complexes are generally of greater interest than those of the isolated constituent subunits, much less is known about structures with mass greater than 106 daltons compared to those with mass of 1 04 daltons or less. This discrepancy results from the difficulties associated with chemical and physical studies of large and complex systems. The size, as reflected in the limited rates and ampli­ tudes of reorientation of sites, and complexity, resulting from many similar sites, combine to make NMR spectroscopy of very large mole­ cules and their aggregates particularly difficult. Investigations of crystals

Light and Radical Ions

Abstract: If an electron is removed from most neutral molecules, a radical cation is formed. Until recently, the gas phase, or isolated molecule, study of radical cations has largely been the domain of the mass spectroscopist. However, in the last three to five years, there has been great interest in the interaction of radiation and molecular ions. In the last issue (1981) of the Annual Review of Physical Chemistry, two articles dealt directly with the subject, "High Resolution Spectroscopy of Molecular Ions" (1) and "Fast Ion Beam Photofragment Spectroscopy," (2) with another two indirectly addressing it (3,4). It is certainly a measure of the vitality of the area that there exists anything left to review one year later. Limiting the scope of this review, however, rather than finding new material, turned out to be the larger problem. This review is confined to work involving the interaction of light (visible and near UV and IR radiation) with molecular ions. Because the spectroscopy of diatomic ions was treated in detail last year (1), only polyatomic ion work is covered, and its spectroscopic aspects emphasized. Furthermore, this review is restricted to positive ions and does not consider the large body of literature (5) on photodetachment and photoelectron spectroscopy of negative ions. When an isolated molecule, ion or not, absorbs a photon and under­ goes a transition to an excited electronic state, it may decay radiatively or nonradiatively. If the radiative decay pathway is negligible, then absorp­ tion spectroscopy is the usual means for probing the electronic transition. However, absorption spectroscopy of ions has been mostly ineffectual. Gas phase ion densities exceeding 108/cm3 (10lO/cm3 in plasmas) can only be obtained with great difficulty in the laboratory, and absorption spectroscopy is just not sensitive enough to be useful at these low

Molecular Features of Metal Cluster Reactions

Abstract: Molecular metal clusters now represent one of the largest families of metal complexes.l Until quite recently, the major thrust in metal cluster research had been synthesis and structural characterization. Presently, the emphasis is shifting slowly to a characterization of cluster chemistry. Essential to an understanding of clusters and their chemistry and to a logical exploitation of the chemistry is the delineation of the molecular features of important and typical cluster reactions. Inorganic and organometallic complexes with a single metal atom, an extensively studied area, display in their chemistry various classes of reactions of general and systematic importance (9). For example, a classic reaction is ligand substitution that has as the net reaction the replace­ ment of one ligand by another and that mechanistically may be associa­ tive, dissociative, or interchange in character ( 10).2 Electron transfer reactions, of inner or outer coordination sphere mechanistic form, com­ prise another important reaction class. The complementary reaction processes of oxidative-addition and reductive-elimination, as exemplified in hydrogen (H2 ) addition to and elimination from metal complexes, is

Recent Developments in Electron Paramagnetic Resonance: Transient Methods

Abstract: The most recent article in Annual Review of Physical Chemistry under the title "Electron Spin Resonance" is Freed's (1), which appeared in 1972. In the intervening years several thousand papers have been abstracted in Chemical Abstracts under the title " Electron Spin Resonance." In most of these papers "conventional electron paramagnetic resonance" (EPR) spectroscopy, i.e. recording of the absorption spectrum under slow pas­ sage steady state conditions, has been used. The principles and practice of such EPR spectroscopy are so well known that the results that it yields are not, I believe, appropriate for a review dealing with recent develop­ ments in EPR. Many of the findings are novel and of great interest to chemists, but would be more appropriately included in reviews of the particular chemical phenomena with which they deal. (The reader may note that Electron Spin Resonance has been metamorphosed to Electron Paramagnetic Resonance. I make the change to avoid irritating rigorous colleagues who point out that no pure electron spin resonance, com­ pletely devoid of orbital effects, exists. ) This review deals for the most part with the various methods in which time evolution of transient magnetization is observed. The book edited by Kevan & Schwartz on Time Domain Electron Spin Resonance Spec­ troscopy (2) contains much of the material of this review. Many im­ portant and elegant developments are not included herein.