Showing papers in "Annual Review of Physical Chemistry in 1989"

Theoretical Methods for Rovibrational States of Floppy Molecules

Abstract: Theoretical calculation of rotation-vibration energy levels of polyatomic molecules is a topic with a long history, characterized by a close, symbiotic relationship with molecular spectroscopy on one side and quantum chem­ istry on the other. What brings them together is the notion of the potential energy surface, which plays a central role in our understanding of the molecular structure and dynamics. In the case of polyatomic molecules, the experimental spectra cannot be inverted directly to yield potential surfaces [see Ref. (1) for some recent efforts within the semiclassical SCF approach], but they do provide a stringent test for the theoretically obtained potential surfaces and observables derived from them. These surfaces, usually from ab initio calculations, seldom meet the standards of spectroscopic accuracy, especially if more extended, high-energy regions are of interest. The only practical way available to test and improve them is by comparing the calculated and the experimental spectra, and minimizing the difference between the two. The subject of the theoretical treatment of coupled molecular vibrations has undergone a real renaissance in the past decade. Significant conceptual advances have been made, particularly concerning highly vibrationally and

Spectroscopy of the Diatomic 3d Transition Metal Oxides

Abstract: The diatomic oxides of the 3d transition metals have astonishingly com­ plicated spectra which even now are by no means fully understood. The interest in them stems from their importance in astrophysics, high tem­ perature chemistry, and in theoretical understanding of the chemical bond­ ing in simple metal systems. The 3d transition metals have a special place in astrophysics because of the great stability of the nuclei. 56Fe, for example, has the lowest mass per nucleon of any nucleus, so that it is the final product in the thermonuclear processes that fuel stars. The nuclei surrounding iron in the 3d transition series are almost as stable, which makes for a local maximum in the cosmic abundances of these elements ( 1 ). Because of the high cosmic abundance of oxygen and the large dissociation energies of the diatomic oxides of the earlier 3d metals, the band systems of compounds such as TiO and VO completely dominate the spectra of cooler (M-type) stars if they contain metal-rich recycled supernova material (2). Low-temperature astrophysics is high-temperature terrestrial chemistry and, not surprisingly, it is found that the thermodynamically stable high temperature forms of the metal oxides are diatomic vapors. However, by the standards of room tem­ perature chemistry the 3d monoxides are highly refractory materials, so that the difficulty of getting them into the gas phase under nonequilibrium low temperature conditions has held back progress with their spectra. The real problems with their spectra come from the many unpaired electrons, which produce huge numbers of low-lying electronic states,

Dynamics of Solvation and Charge Transfer Reactions in Dipolar Liquids

Abstract: The effects of solvent and solvent dynamics on chemical reactions, especially on charge transfer processes, have long been a subject of great importance in physical chemistry. In the past, attention was focused pri­ marily on equilibrium solvent effects, such as the effect of solvent polarity on the reaction potential surface. Tn recent years it has become clear that in many fast reactions solvent dynamics can play a direct role and can affect both the rate and the outcome of a reaction profoundly. Thus, an understanding of the time-dependent response of a polar solvent to a changing charge distribution in a polar solute molecule is essential to understand the role of solvent in many important chemical and biological processes in liquids. Such understanding can be achieved by studying the dynamics of solvation of a newly created ion or of an instantaneously changed dipole in a polar liquid. This subject has undergone a renaissance in recent years because of the availability of ultra-short laser pulses that make it possible to study solvation dynamics directly with a time resolution hitherto impossible. An understanding of the details of solvent response to a sudden change in the charge distribution of a polar solute "probe" molecule is beginning to emerge. Experimental studies on the dynamics of solvation are usually carried out by instantaneously creating a charged species inside a polar solvent and subsequently monitoring the emission/absorption spectrum of this

Oriented Molecule Beams Via the Electrostatic Hexapole: Preparation, Characterization, and Reactive Scattering

Abstract: The steric effect is one of the oldest and most intuitive concepts in chemical kinetics, yet our quantitative understanding of it has been quite limited. Chemists recognized very early that the need for "proper" mutual orien­ tation of reactants is second only to energetic requirements for a "suc­ cessful" collision. "Steric factors" were included long ago in the pre­ exponential term of simple rate expressions for elementary reactions; these factors reflected the probability of achieving proper orientation given the random nature of molecular encounters. However, measuring the reactiv­ ity for specific collision geometries, i.e. selected impact parameters and reagents' mutual orientation in an elementary reaction, appeared to be impossible. Control of the impact parameter is still beyond reach for bimolecular reactions of isolated molecules in the gas phase, but control of the reactant

Measurement of Forces Between Surfaces in Polymer Fluids

Abstract: The physical origins of the forces exerted among polymer molecules include those arising from sources familiar to small molecules (van der Waals, electrostatic, hydrogen bonding, and others) as well as those that can be traced directly to the chain-like character of the polymer. Macromolecules multiply the effects of interactions among their constituent monomers so that a small intersegmental interaction can produce a large intermolecular effect. This is a key factor in polymer adsorption on solids, interaction of polymer molecules with solvents, and interface formation in polymers via thermodynamic phase separation. Chain connectivity and rotational isomerism give a random (or self-avoiding) walk character to macro­ molecular configurations ( 1 ). Distortion of this "natural" population of conformations by the application of perturbations, such as surface poten­ tials, mechanical strain, or electromagnetic fields, produce "elastic" or confinement forces arising from the tendency to maximize the randomness of the conformational distribution. Macromolecular fluids, as they are viscous and frequently highly entangled, can also exhibit nonequilibrium forces, which arise from incomplete adaptation to changes in their environ­ ment. Such forces can be very slow to relax (2).

Hole-burning spectroscopy

Abstract: The study of optical relaxation processes in solids at low temperature has progressed tremendously in the last 20 years due to the development of coherent optical techniques. The latter would not have been possible without the advent of narrow-band tunable lasers, on the one hand, and lasers with very short pulses (picoseconds, femtosecoods) on the other hand. Such coherent techniques are necessary because spectral line shapes of solids doped with guest molecules are seldom determined by dynamical interactions but principally by strain or structural disorder. This gives rise to inhomogeneous broadening, rlOh, which in crystalline hosts at low temperature varies between '" 0.1 and 10 cm -I, whereas in glasses it may amount to � 100-500 em-I. One of the techniques to achieve narrowing of spectral lines is site­ selection spectroscopy (1-3). A sub-ensemble of molecules within thc inhomogeneously broadened absorption band is selectively excited by mcans of a narrow-band laser, and the fluorescence or phosphoresccnce signal is detected with a high-resolution monochromator and/or a Fabry­ Perot interferometer. Fluorescence line-narrowing ( I) is a special case of this technique (for a review see 3). The intrinsic or homogeneous spectral linewidth, r hom, which yields information on the relaxation processes of the excited state, is given by the effective optical dephasing time, T2:

Picosecond Vibrational Energy Transfer Studies of Surface Adsorbates

Abstract: Energy transport between molecules and surfaces plays an extremely important role in many chemical and physical processes. At the micro­ scopic level, molecules may be chemisorbed or dissociate on a substrate during a chemical reaction, they may undergo diffusion and desorption, or when adsorbed may change the physical or optical properties of the surface. Over the last few decades, breakthroughs in the development of sophisticated surface-sensitive techniques have occurred, making it pos­ sible to identify and characterize very detailed properties of these types of adsorbate-surface interactions ( 117). On the whole, these methods probe the static nature of surfaces and reveal information about the system on a relatively long (> I sec) timescale. When chemical transformations occur at interfaces, however, vibrational energy transfer can play a key role in determining the outcome of processes, whether thermal or caused by interactions with energetic laser (2-6), atomic (7), or molecular beams (810). To obtain a better understanding of these types of interactions, it is advantageous to perform direct measurements of adsorbate vibrational energy transfer. This chapter reviews advances made over the last few years to directly measure mode-specific adsorbate vibrational population or energy relaxation lifetimes (T1) with ultrashort pulsed (ca. 10-12 sec) laser techniques. We also refer to TJ studies for several molecules in solution

Atomic-Resolution Surface Spectroscopy with the Scanning Tunneling Microscope

Abstract: The study of surfaces has enjoyed an explosive growth during the last 2 5 years, due largely to the development of new techniques for probing the symmetry, chemical composition, and the electronic and vibrational states of surfaces and of adsorbed atomic and molecular species. Although a veritable arsenal of surface science tools is available, the study of surfaces is often so complex that even when several tools are applied simultaneously, unambiguous results may not be obtained. The study of surfaces has been greatly advanced during the last five years by the newly developed technique of scanning tunneling microscopy (STM) (1-6). In this technique, a very sharp tip (usually of tungsten) is brought to within a few atomic diameters of the surface under investigation without actual physical contact, so that there is a very small overlap of the wavefunctions of the surface with the nearest atom of the tip. When a small bias voltage (10 mV--4V) is applied between the sample and tip, electrons tunnel across this gap with a probability that increases exponentially as the tip approaches the sample. This exponential dependence of the tunneling current on the sample-tip separation provides an extremely sensitive way of detecting the small changes in the surface height due to the individual atoms, thus providing the basis for the scanning tunneling microscope. The images obtained in STM are often strongly dependent on the sample-tip bias voltage in a nontrivial manner. Although early STM stud-

Transport of electrons in nonpolar fluids

Abstract: Electron transport in nonpolar fluids has been investigated for several decades. At first the emphasis was on liquified rare gases, and later on excess electrons were also studied in molecular liquids such as alkanes. Now mobility data are available for approximately 75 nonpolar liquids ( 1). Progress in this field was hampered initially by impurities like O2, CO2, and various electrophilic compounds in the liquids. The availability of adequate methods of purification has stimulated experimental studies. Some new compounds have been studied recently, and new experimental techniques are being applied. Innovative theoretical approaches are now being tried. These efforts will eventually provide a better understanding of the basic problems of electron scattering and localization in liquids. Excess electrons can be introduced in liquids either by ionization or by injection. Ionization may utilize single or multiphoton absorption or high energy radiation. Injection can be from a cathode immersed in the liquid (photoelectric effect) or by field emission. Most studies of electron trans­ port require a short excitation pulse. The electrons produced thereby can be monitored by a number of techniques (2). One is that of DC conduc­ tivity. Even though ions may be concomitantly formed, electrons move much faster than molecular ions, therefore the current at short times is due to the motion of the electrons. Another technique is that of microwave

Computer Simulations of Globular Protein Folding and Tertiary Structure

Abstract: In summary, although a large number of disparate techniques have been applied to predict the tertiary structure of globular proteins from their amino acid sequence, the solution is not yet at hand. Methodologies for predicting the conformation of constrained, small protein fragments appear to be successful. As the size of the system increases, the level of detail of the treatment decreases; approaches that employ very detailed potentials appear to be limited to about 30-40 residues. Although this is a major advance, methods that reduce the effective number of degrees of freedom are clearly required. Lattice representations coupled to highly efficient Monte Carlo procedures appear to be one such approach. Thus, although a number of theoretical advances in the computer simulation of globular protein structure have been made, much work remains to be done before the globular protein folding problem is solved.

The metal-insulator transition in expanded fluid metals

Abstract: Over the past two decades, a considerable amount of effort has been centered on the experimental and theoretical investigation of liquid metals expanded by heating toward the liquid-vapor critical point. Much of the activity is motivated by the large number of current and potential applications of fluid metals as high temperature working fluids for advanced energy technologies. From the scientific point of view, the main object of this effort is to find out how the properties of metals vary with large changes in density, large enough to change the liquid metal into a nonmetal at large enough expansion. Figure I shows why such a metal to nonmetal transition must occur with decreasing density. The discontinuous liquid-vapor phase change of a metal at low temperatures near the triple point is obviously accompanied by a discontinuous metal(M)-insulator(I) transition. In such a situation the liquid metal is reasonably well described by the nearly-free-electron model, whereas in the dilute insulating vapor the great majority of the electrons are attached to their parent atoms occupying spatially localized atomic orbitals. It follows that the precise nature of the electronic interactions between atoms must change on going by a suitable combination of temperatures and pressures continuously along the dashed path round the critical point from the liquid-like (M) to vapor-like (I) densities. Somewhere along this line a transition range must exist where metallic properties evolve into those characteristic of non­ metals.

Theory of Adsorbate Interactions

Abstract: For at least 20 years, surface scien tis ts have been promising that their work would lead to a materials science of surface chemis try. Bu t i t is only recen tly, as a result of the developmen t of a varie ty of ne w experimen tal tools , tha t the focus of surface science has turned from "the surface s tructure pro blem," i.e. what a toms are on a s ta tic surface and where they are relative to o ne ano ther, to the mechanis tics of the fundamental processes of surface ch emistr y , such as energy transfer between reactants a nd surfaces, s ticking , dissociation , diffusion , reaction , and desorp tion. The solution of the s tructure pro blem means that i t is no w possi ble to s tudy these processes on a variety of interesting, well-defined, single-crys tal surfaces. In parallel, i t implies that surface theorists need to turn their a tten tion from what have become "conventional" chemisorp tion pro blems to si tuations relevan t to dynamical phenomena on single crystals. The mos t significan t missing ingredi en t in our a bili ty to simulate surface processes , at present, is a predictive u nderstanding of the energy of an adsorp tion sys tem as a func­ tion of the locations of the surface a toms that are involved. This ar ticle is a discussion of the s ta tus of chemisorp tion theory, aimed at assessing available and developing techniques tha t might provide such an under­ s tanding. Reflecting the nature of mos t of the relevan t work in this area, the focus is almost exclusively on chemisorp tion on metals. In teractions among adsorba tes and between adsorbates and surfaces are o bvious ly a t the heart of surface physics and chemis try . Surface micro­ scopies (1-2) as well as o ther surface-sens itive e xperimen ta l techn iques ( 3-

Proton Translocation in Proteins

Abstract: The active transport of protons across the low dielectric barrier imposed by biological membranes is accomplished by a plethora of proteins that span the ca. 40 A of the phospholipid bilayer. The free energy derived from the proton electrochemical potential established by the translocation of these protons can subsequently be used to drive vital chemical reactions of the cell, such as ATP synthesis and cell locomotion. Membrane-bound proton translocating proteins have now been found for a variety of organisms and tissues (1). The driving force for proton pumping in these proteins is supplied by numerous mechanisms, including light absorption (e.g. bacteriorhodopsin) (2a,b), ligand binding (e.g. ATPase) (3), and electrochemistry (e.g. electron transfer through cytochrome c oxidase) (4). Thus nature has devised a variety of methods for supplying the energy required for proton pumping by these proteins. Such diversity notwithstanding, the proteins most likely share some common elements of structure and mechanism that allow them to function as proton pumps. A number of theoretical mechanisms have been put forth for both general proton translocation (5-7) and for energy coupling in specific proton pumps. However, despite almost three decades of intensive research, the details of the mechanism(s) and structural requirements for proton pumping remain largely unresolved. To some extent this is the result of the paucity of structural information available for integral membrane proteins. This situation may soon improve as a result of advances in protein methodologies that have allowed several integral membrane proteins to be successfully crystalized (8), and the increased use of genetic engineering to obtain recombinant proton translocating proteins that will offer an opportunity to assess the importance of specific amino acids for the proton translocation process (9).

Vacuum UV Photophysics and Photoionization Spectroscopy

Abstract: Vacuum ultraviolet frequencies of the electromagnetic spectrum lie in the region in which air absorbs radiation, so experiments must be carried out in a vacuum. In practice, this region extends from about 180 nm to about 30 nm. Below 110 nm is the windowless region in which a LiF window no longer transmits light. The field of vacuum UV (VUV) photophysics is difficult to review because it is so broad. It encompasses experiments in the gas, liquid, and s<1lid phases. It covers, dynamics, structure, and spectroscopy. To review the field so broadly would clearly be inappropriate. Instead, this review is limited to the major recent advances made in the areas of gas-phase spectroscopy and dynamics. The experiments of interest are those carried out with conventional UV light sources, synchrotron radiation, VUV lasers, and multiphoton ionization. The latter is not technically a VUV source, since the photon energy of interest is achieved by exciting the molecule or atom with two or more photons of lower energy. However, as long as the final states investigated lie in the VUV region, it seems appropriate to include a discussion of these studies. On the other hand, the experiments on ion spectroscopy carried out with IR lasers, are not discussed even though the information obtained by this method is very similar to that obtained by high resolution VUV spectroscopy (e.g. threshold photoelectron spectroscopy).

Electron Spin Resonance

Abstract: In vivo detection of free radical metabolites by spin trapping, R.P.Mason et al theoretical aspects of ESR, A.Hudson transition metal ions, J.F.Gibson recent developments of ENDOR spectroscopy in the study of defects in solids, J.-M. Spaeth inorganic and organometallic radicals and clusters prepared in a rotating cryostat by metal vapour techniques, J.A.Howard and B.Mile inorganic and organometallic radicals, Martyn C.R.Symons metalloproteins, G.R.Hanson and G.L.Wilson complexes of paramagnetic metals with paramagnetic ligands, Sandra S.Eaton and Gareth R.Eaton.