Showing papers in "Annual Review of Physical Chemistry in 1980"

Theory of the Main Lipid Bilayer Phase Transition

Abstract: Fortunately, the day when it was necessary to argue the biological relevance of fundamental physical and chemical studies on pure syn­ thetic lipid bilayers is past, and space will not be devoted to flogging that dead horse. The lipid bilayer phase transition (to be abbreviated LBPT), often called the gel to liquid crystal phase transition, appears to have direct biological relevance (1-5). Even if it did not, it is neverthe­ less a striking physical event, the study of which leads to a better understanding of the structure of biological membranes. In addition, the theoretical study of phase transitions in lipid bilayers is an interesting chemical physics problem in its own right and not just a problem for routine application of theoretical methods developed for simpler sys­ tems. The emphasis of this review is upon the lipid systems for which the most detailed and quantitative experimental and theoretical studies can be performed; the lecithin (phosphatidylcholine, abbreviated PC) bi­ layers with saturated and homogeneous hydrocarbon chains are the preeminent system, although a number of studies involving variations in the lipid molecules are mentioned. This review is not concerned with the proliferation of phases that occur at low water content (6a) or with the theory of self-assembly of the lipids into a bilayer in contrast to micellar forms (6b). Furthermore, this review is more concerned with equilibrium properties of the LBPT than with the dynamics of molecular motions. Even with these restrictions, the subject is a large one, and the author apologizes for omissions due to lack of space or oversight. Because this is an interdisciplinary area that attracts readers and authors from a variety of backgrounds with different perspectives, there has been confusion in evaluating the advances of different theories and

Laser Spectroscopy of Cold Gas-Phase Molecules

Abstract: This review describes the use of supersonic molecular beams or super­ sonic free jets to prepare gas phase samples for spectroscopic study under conditions where the internal degrees of freedom, the molecular vibration and rotations, have been cooled to very low temperatures. The dual objectives of preparing an isolated gas phase sample and of producing low internal temperatures have been traditionally desired by spectroscopists. However, the use of traditional techniques ultimately makes these two goals mutually exclusive. Almost arbitrarily low tem­ peratures can be achieved by the use of a proper refrigerant, but if temperatures much below the freezing point are desired, the reduced vapor pressure will require that spectroscopy be done on a solid sample. Frequently the goal of a spectroscopic study is the elucidation of those intermolecular interactions that occur in solids, and of course in these cases the low temperature solid is the sample of choice. In other cases the goal of the experiment is to study those intramolecular properties that are only slightly affected by the solid matrix surrounding the molecule of interest. In these cases the solid sample, although not ideal, is still adequate. However, there are cases where the desire is the study of properties of the isolated molecule that are completely overwhelmed in the matrix by intermolecular interactions, and in these cases the study must be carried out in the gas phase at the lowest temperature con­ sistent with adequate vapor pressure. The use of a supersonic expansion allows the preparation of internally cold, isolated, gas phase molecules and thereby retains the advantages of solid state or matrix isolation spectroscopy without the disadvantage

The Minimum Entropy Production Principle

Abstract: It seems intuitively reasonable that Gibbs' variational principle de­ termining the conditions of heterogeneous equilibrium can be gener­ alized to nonequilibrium conditions. That is, a nonequilibriurn steady state should be the one that makes some kind of generalized-entropy production stationary; and even in the presence of irreversible fluxes, the condition for migrational equilibrium should still be the equality of some generalized chemical potentials. We summarize progress to date toward this goal, reviewing (a) the early history, (b) work of Onsager and first attempts at generalization, (c) the new direction the field took after 1967 with the work of Tykodi and Mitchell, and (d) the present situation and prospects. Our conclu­ sion will be, briefly, that the outlook is good in that the basic principles are believed known; but we do not yet �now whether they can be reduced to simple rules immediately useful in practice, in the way that the Gibbs phase rule is useful. For this, we need more experience in the technique of applying them to particular cases, and more data to test some conjectures.

Light scattering studies of molecular liquids

Abstract: In this review we focus attention primarily upon the depolarized Rayleigh and Raman scattering from liquids composed of small, rigid molecules; however, we also refer to the results of other experiments (NMR, IR, and molecular dynamics) that provide complementary infor­ mation. Even within this limited framework we do not provide a comprehensive review of the recent literature but rather we select certain topics that are of particular interest to us and where important changes have recently occurred. We have attempted to integrate the information from different sources and to provide a critical (and clearly personalized) analysis. Several excellent reviews of various aspects of this subject have appeared since the last review in this series (1). In particular the text on light scattering by Berne & Pecora (2), reviews on vibrational dephasing and energy relaxation by Oxtoby (3,4), the articles on rotational relaxation by Steele (5) and Griffiths (6), and the review of collision-induced scattering by Tabisz (7) discuss the techniques and physical principles involved. The up-to-date treatment of many topics in these sources has helped us restrict our choice of subject matter. The Raman and depolarized Rayleigh spectra monitor the fluctua­ tions in the polarizability density that arise from the modulation of the intrinsic (gas phase) and the interaction-induced polarizabilities of the molecules by the molecular motion. If there were no interaction-induced polarizability the shape of the depolarized Rayleigh spectrum (DRS)

The Application of the Marcus Relation to Reactions in Solution

Abstract: In searching to understand the rate of a reaction in solution, the baby must not be separated from its bath water; the role of the solvent is all important. This first became apparent in the study of electron transfer reactions both at electrodes and in homogeneous redox reactions. No covalent bonds are formed or broken and so these reactions are the simplest elementary reactions we can have. Theoretical work by Marcus (1-5) and by the Russian school of Levich & Dogonadze (6-13) has built on the earlier work of Gurney (14) and Libby (15) to provide a description of the rates of these reactions that explains most of the experimental facts. The next simplest reaction, at least in water, is a proton transfer. Through the use of isotope effects and fast reaction techniques the understanding of proton transfers had been developed separately from the work of electrochernists on electron transfers. However, the application of the Marcus theory to proton transfers (16-19) has yielded new insights. The Br�nsted catalysis law is a particular case of the Marcus relation. Recent work by German & Dogonadze suggests again that the solvent plays a crucial role in mediating proton transfers. Up to a few years ago these developments had been largely ignored by physical organic chemists, who were to be found sheltering behind an elaborate construction of linear free energy relations and murmuring the Hammond postulate. Recently the Marcus relation has been successfully applied to methyl transfer reactions (20, 21), and the Swain-Scott relation (22), like that of Br�nsted, has been shown to be a special case. Time and theory are therefore ripe to explore and discuss

Theories of the Dynamics of Inelastic and Reactive Processes at Surfaces

Abstract: The blossoming in the last fifteen years of the field of gas-phase chemical dynamics has greatly enhanced our understanding of how simple chemical reactions occur. Experiments have been approaching more closely the ideal of bringing together reactants that have been prepared in particular internal and translational states and measuring the cross sections for formation of individual final elastic, inelastic, and reactive channels (1). Theory has achieved, for a few simple systems, the goal of quantitative a priori prediction of the outcome of a molecular collision event (2-5). For more complicated systems, theory has pro­ duced valuable qualitative insights, such as how the location of a saddle-point on a potential energy hypersurface can influence the en­ ergy requirements for reaction and the energy disposal in products (6). Theory has also provided graphic representations, via classical trajecto­ ries (6) and quantum wave packets (7), of the actual unfolding of individual chemical encounters. Over the same period of time there has been a growing effort, both experimental and theoretical, directed toward elucidation of the detailed dynamics of gas-surface interactions (8-19). Accomplishments have been more modest than those of gas-phase investigations, but ex­ citing and important nonetheless. The objectives of these inquiries parallel those of gas-phase chemical dynamics. Quantitative information is desired about elementary processes, e.g. the accommodation of trans­ lational, vibrational, rotational, and electronic energy, sticking probabil­ ities, residence times, surface diffusion constants, etc. Qualitative in­ sights are desired in more complicated situations, e.g. how is the collision dynamics influenced by particular features of the gas-surface

Two-Body, Spherical, Atom-Atom, and Atom-Molecule Interaction Energies

Abstract: It is a well-known fact that the body of literature on intennolecular forces is presently far too large to be reviewed critically in a complete way. Fortunately the number of reviews and books being written on the subject is also quite large (1-17) so that the different facets of this complex subject are being discussed one, or a few, at a time by different authors. For instance the reviews by Buck (14) and Pauly (15) are particularly useful to people who want to know about the quantity and the quality of infonnation on intennolecular forces contained in scatter­ ing data of different types (diffractive, rainbow, etc.) and also want to learn about the best ways to extract such information. On the other hand, if one wants to focus especially on nonspherical interaction potentials, the papers by Stolte & Reuss (13) and by Thuis et al (16) form an excellent starting point. Finally, if one is interested in the state of the art of a priori theoretical methods, Schaefer (17) has covered that part of the subject, although in a rather short and specialized way. In view of the above it is appropriate at this point that we illustrate the choices we have made to restrict the field to, what is for us, a manage­ able size. 1. The first point that we try to show is that for the usual benchmark systems (the noble gases and their mixtures) the precision of our knowledge about their two-body interatomic potentials is very high. By very high we mean that the internuclear distance at which the interac­ tion energy has (within reasonable bounds) an arbitrarily specified value is known with a precision of about 1%. 81

Simulation of Protein Dynamics

Abstract: Globular proteins exhibit a great variety of internal motions. One indication of this diversity is the range of characteristic times involved, which extends from 10-14 s for local vibrations to more than lcr s for certain structural transitions (1). Despite the obvious difficulties in dealing with such complicated systems, the dynamical richness of pro­ teins and the functional importance of their internal motions have attracted the interest of an increasing number of physical chemists in recent years (2-5). In particular, the experimental literature on protein dynamics has grown quite rapidly. Many different spectroscopic and kinetic techniques have been used to probe protein motions, in part because of the wide range of time scales involved. Several reviews of this work are now available (6-11). A small but increasing number of theoretical studies of protein dynamics have also appeared (11). Most of these studies are based on detailed computer models that can be used to simulate the classical dynamics of the atoms in a protein. As in the experimental work, it is necessary to use different theoretical techniques to study dynamical processes that occur on different time scales. The purposes of this review are to identify the different dynamical regimes of interest in proteins, to describe simulation techniques that may be useful for each regime, and to summarize the results of the various simulation studies. Before embarking on this program, it is useful to recall certain structural features of proteins.

Bimolecular Reactions of Vibrationally Excited Molecules

Abstract: ion reactions involving at least one species with an unpaired electron. As shown in Figure 8, the exoergic reaction between nitric oxide and ozone looks very interesting in this respect. The reaction proceeds by two parallel paths to yield ground-state vibrationally ex­ cited (A) and electronically excited (B) N02 molecules (178-180) NO + 03 � NOiCAI) + 02 £:=9.7 kJ mol-I E�= 17.5 kJ mol-I. The spacing of the three vibrational modes in 03 as well as the vibrational quantum in NO are comparable to the measured thermal activation energies. Both reactants can be vibrationally excited by molecular infrared lasers. For ozone a coincidence between the P(30) line of the CO2 laser in the 9.6 p.m band and the antisymmetric stretch vibration (001) exists. Nitric oxide has a near coincidence with the P(l3) line in the �v = 9-8 band at 5.3 /Lm of the CO laser. Because NO is a paramagnetic molecule, the NO absorption line can be "tuned" by a magnetic field (0.76 kG) in resonance with the laser line. Near-IR

Two-Photon Molecular Electronic Spectroscopy

Abstract: The excitation of rotational, vibrational, and electronic states of mole­ cules by the simultaneous absorption of two or more photons has developed into a significant branch of science in the last decade. Numerous compounds have now been studied by workers from several different disciplines: atomic spectroscopy, optical physics, solid state physics, and electrical engineering, as well as chemical physics and molecular spectroscopy. This rich background has given rise to an ingenuity of technique so astonishing that it seems at times almost to obscure the spectroscopic questions that originally motivated this field. It is our purpose in this article to return to the basic chemical questions. We shall review the advances in our knowledge of molecules that have been made using nonlinear spectroscopy, leaving aside all discussion of theory and technique. To begin this work, we had to collect a comprehensive bibliography of compounds that have been investigated by some form of two-photon spectroscopy. We made a computer assisted literature search of the IN SPEC data base (1) from 1973 to 1979 via the DIALOG terminal system. We specified a simple full-text search in the data base of the adjacent words "two" and "photon," appearing with the root "molec" anywhere in the INSPEC field (abstract, title, or special data-base descriptors). The specific DIALOG command was "TWO(W)PHO­ TON(F)MOLEC?". The question mark allowed for various possibilities such as "molecules," "molecular," etc. The search was run on October 2, 1979, netting 372 references of which some 220 were ultimately

Computer Simulations of Freezing and Supercooled Liquids

Abstract: The solid-liquid transition is one of the most common phase changes occurring in nature, yet our theoretical understanding of it is quite fragmented. Although there exist theories of freezing and of melting, these theories are often phrased in very different languages. In order to understand the thermodynamics and kinetics of the solid-liquid transi­ tion it is essential to have a good theoretical picture of the microscopic structure and dynamics of both the solid and the liquid phase, along the melting curve. The theoretical description of anharmonic, defect-rich solids and, in particular, of dense fluids is a formidable many-body problem. It is in tackling this problem that simulation techniques have proven to be an invaluable tool. In discussing the role of (computer) simulations in the study of the solid liquid transition, it is important to distinguish between the thermodynamics and the kinetics of the phase transition. Studies of the thermodynamics of the solid-liquid transition address themselves to the following question: Given the intermolecular forces for a particular system, what is the location of the melting curve and how do the thermodynamic properties of the system (e.g. density, entropy etc) change on melting? In contrast, studies of the kinetics of the solid-liquid transition focus on the question: By what microscopic mechanism does a system melt or freeze? Of course, as many fluids can be cooled well below their normal freezing point, related questions are: What factors favor supercooling and what is the nature of the glass transition? In general, the answers to these questions may be expected to depend on the dimensionality of the system and the nature of the intermolecular forces.

Saturation-Transfer Spectroscopy

Abstract: Appendix I. The Spin-Density Matrix and the Hamiltonian Appendix II. Motional and Distributional Models . Appendix III. Relaxation Rate Expressions Appropriate for the Fast Motion Region . . . . . . . . A. Electron Spin-Spin Relaxation Rate B. Spin-Lattice Transition Rates Appendix IV. Relaxation Matrix and Expressions for T{\"(mi) and Raoip, o) 149 151 165 170 181 202 203 207 212 213 217 227 236 237 238 245

Chemical Kinetics of High Temperature Combustion

Abstract: Combustion reactions have always interested chemists. The special attraction they have held for chemical kinetics has been enhanced, if not entirely caused, by unusual combustion phenomena such as explo­ sion limits, ignition delays, autocatalysis, and inhibition. These phenom­ ena reflect the interplay of a large and diverse group of elementary gas reactions, which have been gradually identified and characterized over many years of combustion kinetics research. This review is a critical survey of current experimental knowledge of the reaction mechanisms that are considered to describe the high­ temperature homogeneous reactions of 02 with H2, CO, small hydro­ carbons, CH30H, CH20, NH3, and H2S. Reactions of other fuels will be mentioned only peripherally. Although we understand the term "mechanism" primarily as the complete set of real elementary reactions, mention is occasionally made of abbreviated and partly fictionalized "semi-global" mechanisms which have proved to be useful for purposes other than trying to describe combustion chemistry in nature's terms. The restriction to high temperature means ignoring oxidation processes involving peroxides and ketones almost entirely; these appear to play only very minor roles under flame or explosion conditions. We also omit discussion of combustion gasdynamics and of the mechanisms of soot formation, ionization, and luminescence in flames, each of which deserves review in its own right. In general, we consider an elementary

Phase Transitions in Monomolecular Layer Films Physisorbed on Crystalline Surfaces

Abstract: The weakest form of interaction between two electrically neutral mole­ cules is the one due to induced dipoles. It has been known for a long time that rare gas atoms interact with each other via this mechanism, known as the van der Waals force. For all rare gases except He, the attraction among atoms is nevertheless sufficiently strong that at suit­ able low temperatures they will condense into liquid and solid phases. For 3He and 4He the quantum zero point motion repulsion is large compared to the weak van der Waals attraction, so their absolute zero temperature phase is a liquid rather than a solid. In physical adsorption (physisorption), individual atoms or molecules are attracted toward the surfaces of liquids or solids of a different chemical species by a weak force whose origin lies in the van der Waals attraction between the constituents of the solid (or liquid) and the individual molecule. The attractive force has to be weak enough for the adsorbed molecule (adsorbate) to retain its molecular identity and not to transform into a different chemical species. This consideration, not important for rare gas atoms, is relevant for molecular adsorbates (02' CH4, etc). Provided that physisorption occurs, it is possible to study how many molecules can be accommodated next to the adsorber surface (first layer) and what structural, thermal, and dynamic properties the first layer will have. For the past decade a great deal of effort has been

Optical Reflection Spectroscopy of Organic Solids

Abstract: There are singlet electronic transitions in most organic solids so intense that they must be studied by reflection spectroscopy. The interpretation and analysis of these spectra is equivalent to a study of the dispersion relations of polaritons- the elec tromagnetic modes of matter arising, in the case of organic solids, from the coupling of an exciton and a photon having identical wavevectors. A brief outline of the concept and theory of excitons and polaritons is given before any reflection spectra are discussed in detail. We also in later sections extend the theory and discussion to include surface excitons and surface polaritons. In this review we describe some unusual properties of molecular crystals that have been studied using reflection spectroscopy. No apol­ ogy is offered for the material selected for this review, the choice being strongly influenced by the author's own research interests and pre­ judices concerning the directions most likely to result in a greater understanding of some fundamental electronic processes in organic solids. Also no attempt is made to review the literature in any sys­ tematic or thorough way. However, we attempt to su mma rize the current status of the following fields: "metallic" reflection, "site shift" surface

Molecular vibrations of polymers

Abstract: Vibrational spectroscopy provides information on three important aspects of polymers, that is, their chemical composition, the geometric arrangement of atoms in space (physical structure), and the interatomic forces associated either with valence bonding or intermolecular interactions. The theory and experimental procedures developed so far for application of vibrational spectroscopy to small molecules and their crystals carry over to polymeric systems, with important differences that have to do with the unique structures exhibited by polymers. In this review, the theory of molecular vibrations of polymeric molecules is presented in a manner that emphasizes the basic structural differences between these molecules and their low-molecular-weight analogues. In particular, recent approaches to calculations of the frequencies of polymers with irregular conformational defects are reviewed. On the experimental side, the focus is on those experimental tech­ niques which, because of recent developments in instrumentation, per­ mit new or improved measurements. The most significant recent devel­ opments involve digital data acquisition and analysis, and the recent availability and use of Fourier transform infrared spectroscopy (FTIR). The improved specificity of infrared spectroscopy, achieved mainly through application of spectral subtraction techniques to high signal-to­ noise digital data, has led to a renewed interest in determinations of the chemical composition of macromolecular systems, including the nature of the bonding of chromophores in the polymer chain. Chemical com­ position descriptions rely on correlations between frequencies and specific chemical groups as, for example, the C = 0 stretching frequency of the carbonyl group. Although developing such correlations is a rather old topic, and one in which much progress and payoff have been