Showing papers in "Annual Review of Physical Chemistry in 1975"

Rayleigh and Raman Optical Activity

Abstract: The development of lasers and photon counting systems has made possible the observation of new polarization effects in Rayleigh and Raman scattering from optically active molecules. There is a difference between the Rayleigh and Raman intensities in right and left circularly polarized incident light, and there is a circularly polarized component in the Rayleigh and Raman light in linearly polarized or unpolarized incident light. These new polarization effects arise from interference in the electromagnetic waves whose amplitudes are proportional to the usual electric dipole polarizability tensor � and the higher polarizability tensors G and A (a tilde denotes complex quantities). The tensor � contains two electric dipole transition moments, G contains an electric dipole and a magnetic dipole transition moment, and A contains an electric dipole and an electric quadrupole transition moment. The contribution to the intensity from ii2 scattering may be about 103 times that from the interference terms iiG and ii,4 and about 106 times that from G2 and ,42. Furthermore, the iiG and ii,4 scattering contributions have a circular polarization dependence, which enables them to be distinguished from the ii2, G2, and ,42 contributions. It has been appreciated for some time that Ray leigh and Raman scattered light from optically active molecules could show new effects. Gans (1) considered the additional contribution from G2, but not iiG, to Rayleigh scattering, and he even claimed to have observed the G2 contribution through its influence on the depolarization ratio; but de Mallemann (2) pointed out that the depolarization ratio anomalies originated in optical rotation of the incident and scattered beams . (Notice that G2 scattering exists in a racemic mixture (3) since it is independent of the sign of G.) Bhagavantam & Venkateswaran (4) found differences in the relative intensities of some of the Raman lines of two enantiomers in unpolarized incident light, but these were subsequently attributed to impurities. Kastler (5) thought that a difference might exist in the Raman spectra of optically active molecules in right and left

The Architecture of Biological and Artificial Membranes as Visualized by Freeze Etching

Abstract: The purpose of this review is to analyze the contribution that the freeze-etch technique has made, and may give, in elucidating the architecture of biological membranes. The utility and limitations of the technique, especially where relevant to the interpretation of results, are emphasized. Recent results have demonstrated the unique character of this technique among the physical methods currently used in membrane research, in that a direct visualization of the distribution of membrane components can be made. Other physical techniques, such as X-ray diffraction, differential scanning calorimetry (DSC), electron spin resonance (ESR), and nuclear magnetic resonance (NMR) spectroscopy, give information originating from many membrane sites simul­ 'taneously, i.e. averaged information. The specific nature of the freeze-etch technique has allowed direct observations to be made as to whether there is a homogeneous or heterogeneous distribution of components in the plane of the membrane and also whether transmembrane asymmetry exists. In the first part of the review the physical and technical aspects are described as they relate to the explanation and interpretation of the freeze-etch morphology of membranes. For a general survey of the physical and technical aspects, the review of Zingsheim (1) is recommended. In the second part, a description of the freeze-etch ultrastructure of biological membranes is given in relation to membrane function. The location of the fracture plane, the nature of the particle, and the interparticle areas are discussed. The main and last part of this review focuses on the morphological consequences of lipid phase transitions in biological membranes. It is shown that the distribution of membrane components can be deduced from the micrographs of freeze-fractured membranes. For a better understanding of these phenomena, lipid phase transitions

Picosecond spectroscopy of molecular dynamics in liquids

Abstract: Intense laser pulses with a time duration of several 10-12 sec were first observed in 1966 (1). It was immediately recognized by investigators in the field that these picosecond light pulses permit the study of ultrashort processes in liquids and solids. In fact, tht; first paper on a dynamical process of dye molecules in liquids was published in 1967 by Shelton & Armstrong (2). During the past years successful investigations were conducted on a wide variety of problems, e.g. rapid relaxation processes, transfer rates, and reaction phenomena were studied on a picosecond time scale. Contributions to the field were made by several laboratories around the world. In this paper a review is given of 15 different topics of dynamical processes in liquids.

Double Resonance and the Properties of the Lowest Excited Triplet State of Organic Molecules

Abstract: This review, covering the period from 1966 to 1974, concentrates on the optical detection of zero-field transitions. It starts with a discussion of the relevant properties of the lowest triplet state, then proceeds to discuss the different double resonance techniques involving the triplet state, followed by a discussion of their applications to the study of the magnetic, nonradiative, radiative, and structural properties of the lowest triplet state. 181 references. (JSR)

Laser Studies of Gas Phase Chemical Reaction Dynamics

Abstract: This perspective is addressed primarily to active reaction dynamicists (i.e. chemical kineticists working on a reactant state to product state level of inquiry) to outline current capabilities of coherent light sources for the detailed study of chemical rate processes, to record past successes, and to identify future areas of important and likely progress. We assert that the following axioms, already well known to time­ independent spectroscopists, also apply to time-dependent measurements of evolving reactant-product systems: (a) any matter-radiation interaction can be studied better with a coherent, rather than an incoherent, light source, and (b) coherent light sources of nearly ultimate (i.e. uncertainty principle limited) frequency and time resolution are currently available or will soon be developed for the entire energy spectrum appropriate to chemical interactions. It is now timely and meaningful for every dynamicist to inquire whether existing experiments can be improved via laser devices and, indeed, whether totally new and highly refined experiments are feasible. We emphasize the state-to-state (i.e. microscopic) approach to chemical reaction dynamics; we wish to scrutinize chemical reactions on a global detailed level (1): at a given total system energy E, many reactant and product channels are open and many transition probabilities [i.e. the "rich person's" P-matrix (I)J must be con­ sidered. Of practical interest (2) to dynamicists (and others) are questions of energy requirements and energy disposal (1): e.g. (a) what is the optimal reactant internal state which promotes reaction at energy E? and (b) what is the distribution of product internal states for a particular reactant internal state at energy E?; as well as questions of energy redistribution: e.g. (c) are unimolecular reactants ergodic on the time scale of their fragmentation? and (d) what are the dominant collisional relaxation processes and time scales which degrade an initial nonequilibrium distribution? Laser methods aid the dynamicist in many ways: reactants can be selectively excited to particular internal states, product state distributions can be 1 Literature search completed in December 1974. 2 Camille and Henry Drcyfus Foundation Teacher-Scholar, Alfred P. Sloan Foundation Research Fellow. 3 Our own laser studies are supported by the Directorate of Chemical Sciences, US Air Force Office of Scientific Research under Grants AFOSR-73-2423 and AFOSR-74-2666.

Light Scattering by Gases

Abstract: In recent years, light scattering has developed into a full grown subject due to the availability of intense monochromatic laser sources and greatly improved spectro­ scopic techniques, allowing one to measure spectral shifts in a wide range, from 1 Hz or less to thousands of cm -1 (30 GHz = 1 cm -1 ). Several reviews and books (1-3) have dealt principally with various aspects of light scattering. In this series, Chu (4) in 1970 reviewed light scattering in its usual sense of polarized Rayleigh­ Brillouin scattering/ Berne & Pecora (5) discussed depolarized Rayleigh scattering in 1974; and Raman scattering was reviewed by Peticolas (6) in 1973. In this review we concentrate on the applications of light scattering techniques to the study of molecular dynamics and interactions in gases, omitting molecular structure determination from Raman measurements in low pressure gases (8). After recalling briefly the physical origin of light scattering in order to deduce what kind of information can be obtained, we discuss some results in gases in which we distinguish essentially two kinds of phenomena: 1. collective motions (sound and thermal waves) as determined by the Brillouin and polarized Rayleigh lines (9), and 2. collision processes as revealed by collision-induced light scattering or by the shapes of the depolarized Rayleigh and Raman lines.

Pollution of the stratosphere. [Role of supersonic transport]

Abstract: One vulnerable area of the earth is the stratosphere, where the air is thin and stable against vertical mixing by virtue of its temperature inversion. The Study of Critical Environmental Problems (SCEP) considered the role of supersonic transports (SST) and estimated the increase of stratospheric water, carbon monoxide, nitrogen oxides (NO/sub x/), and particulates as a result of regular operation of 500 SSTs. The possible role of artificially injected water in reducing stratospheric ozone was emphasized. The effect of nitrogen oxides on ozone was also considered. The Congress requested that the Department of Transportation (DOT) set up a program of interdisciplinary research and report their findings by the end of 1974. The Climatic Impact Assessment Program (CIAP) was set up in the fall of 1971. It has held four general conferences and the proceedings are published for three of them. It also prepared six monographs to be published in the summer of 1975. A condensed DOT Report of Findings was issued in January 1975. The findings of CIAP were reviewed by the Climatic Impact Committee of the National Research Council, National Academy of Sciences. This committee worked in parallel with CIAP for two years, conducted a summer study of themore » findings in 1974, and issued an independent report in April 1975. The CIAP embraced more than 50 projects, coordinated similar efforts in other agencies and other countries, and involved efforts of approximately 1000 scientists. During 1974, there was a flood of new results. This review includes the highlights of these results concerning stratospheric composition and the reduction of ozone by NO/sub x/. (CH)« less

Isotope Effects in Chemical Kinetics

Abstract: Recent studies on hydrogen isotope transfer reactions in the gas phase chemistry of thermal and excited species, ionic systems, and reactive scattering and theoretical attempts made to rationalize experimental observations are critically reviewed. Topics discussed are bimolecular reactions, termolecular reactions, unimolecular decomposition reactions, molecular beam scattering, chemiluminescence and laser reactions, ion-molecule reactions, and unimolecular ionic decomposition reactions. In the review of theory theoretical kinetic isotope effect calculations, trajectory calculations, and quantum chemical calculations are considered. 199 references. (JSR)

Transformation of solids to liquid fuels

Abstract: Potential conversion operations are discussed for coal, oil shale, tar sands, and organic wastes. 67 references.