Showing papers in "Annual Review of Physical Chemistry in 1993"

Pressure stability of proteins

Abstract: The stability of proteins toward temperature has been explored in much detail since the time that proteins were characterized as chemicals of constant composition, but the study of the effects of pressure upon pro­ teins is much more recent and has been much less frequent than that of temperature. However, there is every reason to expect that the effects of pressure would be more amenable to interpretation than those of tem­ perature: An increase in temperature changes both the energy content and the volume of the system, and because proteins are flexible polymers that maintain secondary, tertiary, and quatenary structure by bonds of strengths not much larger than the thermal energy, the internal interactions of the protein are changed by temperature in ways that cannot be easily foreseen. On the other hand, application of pressure affects internal inter­ actions exclusively by the changes in the distances (volumes) of the com­ ponents, whereas the total energy of the system remains almost constant. Hence, observations of pressure effects ought to precede those of tem­ perature in order to facilitate the interpretation of the more involved effects of the latter variable.

Aspects of Structure and Energy Transport in Artificial Molecular Assemblies

Abstract: The last decade has seen an explosion of interest in the special properties that may be imparted to molecular assemblies prepared on planar sub­ strates by the self-assembly (SA) and Langmuir-Blodgett (LB) techniques. This interest is fueled by the concept of molecular engineering, or the purposeful design of structures that are supermolecular in scale and in which molecular units are arranged in specific spatial (and possibly tem­ poral) arrangements to realize a coordinated functional goal. This rather broad concept encompasses a wide variety of issues of keen current tech­ nical interest. Nonlinear optical devices play a central role in prototype photonic signal processing schemes, and researchers quickly appreciated that organic: materials, in which the nonlinear polarization is almost entirely electronic in origin, could provide large, fast nonlinearities for optical switching. However, organic molecules far too often crystallize in centro symmetric space groups, thus negating, on a macroscopic scale [X(2) = 0], the advantages associated with a large molecular hyper­ polarizability, /3. To circumvent difficulties with centrosymmetric arrange­ ments of molecules, noncentrosymmetric layered structures can be built by utilizing the LB (I, 2) and SA (3, 4) fabrication techniques to maintain the directionality of the molecular hyperpolarizability tensor. Surface

In-Situ Electrochemical Surface Science

Abstract: : The study of electrochemical interfaces is a pursuit almost as diverse as it is venerable. The characterization of such systems, especially for metals in contact with electrolyte solutions, was originally undertaken as a central part of classical physical chemistry during the early decades of this century. More recently, surface electrochemistry has evolved in several different (and somewhat disparate) directions, triggered in part by the wide utility of electrodes in analytical chemistry; indeed, research work along such electroanalytical lines has been emphasized, notably in the USA. The importance of electrochemical interfaces in a variety of technologically important circumstances, including energy conversion and metal corrosion, fuels further our scientific desire to develop a more fundamental physical understanding of the structural and dynamical properties of metal-solution interfaces. The broadbased impact of surface electrochemistry is founded in the unique and controllable interplay between electrical and chemical phenomena afforded at metal-solution interfaces. Thus the continuous alterations in electron energy that can be induced externally at electrochemical interfaces allow subtle (as well as substantial) modifications to the electronic state of interfacial components. One can therefore be justified in asserting that interfacial electrochemistry should form a centrally important part of contemporary surface physical chemistry.

Ordering in Metal Halide Melts

Abstract: The crystal structures formed by compounds of strongly electropositive and strongly electronegative elements are described crystallographically in terms of interpenetrating Bravais lattices occupied by the various chemical species. The coexistence of crystalline long-range order and chemical order up to melting can be explained by electronic charge transfer and local compensation of positive and negative ionic charges. An examination of the local structures found in crystalline metal halides shows that charge compensation occurs in two qualitatively distinct ways. The first involves halogen sharing and high coordination numbers for the metal ions; the structure of alkali, alkaline-earth, and lanthanide metal halides are ex­ amples of this type of order. In the second, charge compensation takes place within well-defined molecular units, either monomeric ones, as for example in HgC12 and SbCI3, or dimerie ones, as in AlBr3' Studies of molten salts have generally been based on the assumption that the melting transition, which involves the loss of crystalline long­ range order" largely preserves the type of local order found in the crystal. Determinations of the structure of metal halide meits carried out over the past two decades with neutron diffraction have largely substantiated this

The Adiabatic Theory of Heavy-Light-Heavy Chemical Reactions*

Abstract: Many important chemical reactions consist of a light atom exchange between two heavy atoms or groups of atoms. Using the convention of Polanyi et al (1), we refer to such reactions as Heavy-Light-Heavy (HLH). Usually, but not always, HLH systems are hydrogen-atom transfer reac­ tions. Because HLH reactions are of great practical importance as well as chemically interesting, the dynamics of these systems have been the object of much research (1-48). Some of the issues that have attracted the most attention include: (a) the existence of reactive resonances and, possibly, vibrationally bound states; (b) transition state recrossing effects and oscil­ lating collinear reaction probabilities; (c) the role of light atom tunneling; and (d) the nature of transition state spectroscopy involving electron photodetachment from bound negative ions. We can single out the exper­ imental photo detachment work of Neumark (29, 31, 39) and the theoretical quantum scattering calculations of Schatz (35, 36) for generating new excitement about this class of reactions. From the standpoint of fundamental dynamical theory, what makes this class of reactions so special are the intrinsic timescales. Under normal circumstances, the light atom will tend to move much faster than the

Scattering-State Spectroscopy as a Probe of Molecular Dynamics

Abstract: Traditional bound-state molecular spectroscopy has long served as a powerful probe of molecular structure and is increasingly being applied to the study of molecular dynamics. For example, the study of spectroscopic perturbations ( l ) can yield precise measurements of the strength of the nonadiabatic interactions that often determine the evolution of molecular processes. The spectroscopy of the continuum, or "scattering" states, of a transient coillision complex allows a more direct probe of bimolecular dynamics and is the subject of this review. Free-state absorption or emis­ sion spectroscopy yields direct information on the continuum structure, i.e. on the shape of the quasimolecular potential energy surfaces in the regions accessible by the Franck-Condon principle (2--4). Unlike bound­ state molecular spectroscopy, here the spectra are often unstructured, thus making interpretation difficult and somewhat perilous. In some cases, however, pronounced structure is observed in the spectra that may be clearly identified with features in the potential energy surfaces, such as local extrema or saddle points (2-4). Although it is rarely possible to invert the data directly (4), the spectra can provide a stringent test for model potential energy surfaces. Final scattering-state resolved measurements of the continuum "action" spectra are particularly sensitive to the dynamical evolution from the

Scanning tunneling microscopy studies of low-dimensional materials: charge density wave pinning and melting in two dimensions.

Abstract: A major goal of current efforts in condensed matter research is to under­ stand the factors that determine the structure, electronic properties, and phase transitions in materials, as such knowledge will lead the way to the rational design of new solids with predictable properties (I, 2). Essential to the achievement of this goal is a detailed understanding of how material properties vary and can be controlled by atomic level modifications, such as substitutional doping. In principle, doping is a straightforward process with readily predictable effects (e.g. doping semiconductors). However, in many materials, especially those that exhibit cooperative phenomena, such as charge density waves (CDW) and superconductivity, the role of dopants and impurities is not well understood (3, 4). Past difficulties in elucidating the microscopic effects of dopants and impurities on material properties can be traced to the fact that conventional diffraction and spectroscopic techniques provide only an averaged view of a solid (5). These techniques have not been able to assess unambiguously structural disorder and elec­ tronic inhomogeneity in doped solids (3, 4, 6).

Theory of the Equation of State at High Pressure

Abstract: of quantum and statistical mechanics. The application of pressure offers a means by which the lattice constant or density may be varied, thus resulting in changes in properties, including transitions to new structures or phases and modif ications in electronic configurations. This review focuses on how the application of theor etical techniques has provided a deeper under­ standing of these changes on an atomistic level. Recent theoretical developments have resulted from rapid advances in computational capabilit ies and high-pr essure experi mental techniques. Accurate measurements are now carried out by static and dynamic methods to pressures of several Mbar and several thousand Kelvins, and these have stimulated new theoretical efforts.