Showing papers in "Annual Review of Physical Chemistry in 1990"

Block Copolymer Thermodynamics: Theory and Experiment

Abstract: Block copolymers are macromolecules composed of sequences, or blocks, of chemically distinct repeat units. The development of this field originated with the discovery of termination-free anionic polymerization, which made possible the sequential addition of monomers to various carbanion-ter­ minated ("living") linear polymer chains. Polymerization of just two dis­ tinct monomer types (e.g. styrene and isoprene) leads to a class of materials referred to as AB block copolymers. Within this class, a variety of molec­ ular architectures is possible. For example, the simplest combination, obtained by the two-step anionic polymerization of A and B monomers, is an (A-B) dioblock copolymer. A three-step reaction provides for the preparation of (ABA) or (BAB) triblock copolymer. Alternatively, "living" diblock copolymers can be reacted with an n-functional coupling agent to produce (A-B)n star-block copolymers, where n = 2 constitutes a triblock copolymer. Several representative (A-B)n block copolymer architectures

The Quantum Mechanics of Larger Semiconductor Clusters ("Quantum Dots")

Abstract: How can one understand the excited electronic states of a nanometer sized semiconductor crystallite, given that the crystallite structure is simply that of an excised fragment of the bulk lattice? This question is motivated by recent experiments on chemically synthesized "quantum crystallites," sometimes called "quantum dots," in which it is observed that the optical spectra are quite sensitive to size. For example, bulk crystalline CdSe is a semiconductor with an optical band gap at 690 nm, and continuous optical absorption at shorter wavelengths. However, 3540/~ diameter CdSe crystallites containing some 1500 atoms exhibit a series of discrete excited states with a lowest excited state at 530 nm (1-3). With increasing size, these states shift red and merge to form the optical absorption of the bulk crystal. Electron microscopy and Bragg X-ray scattering measurements show that these crystallites have the same structure and unit cell as the bulk semiconductor. Such changes have now been observed in the spectra of many different semiconductors. This phenomenon is a "quantum size effect" related to the development of the band structure with increasing crystallite size (4). Smaller crystallites behave like large molecules (e.g. polycyclic aromatic hydrocarbons) their spectroscopic and photophysical properties. They are true "clusters" that do not exhibit bulk semiconductor electronic properties. In this review

Photophysics and Molecular Electronic Applications of the Rhodopsins

Abstract: This review examines the photophysical properties of an interesting and biologically important class of proteins known as the rhodopsins. The visual rhodopsins are responsible for the conversion of light into nerve impulses in the image resolving eyes of mullusks, arthropods, and vertebrates. Despite independent evolutionary development, these systems have converged on proteinand chromophore-binding site designs that are remarkably similar. The bacterial rhodopsins represent a much broader class of proteins that serve both photosynthetic and phototactic functions. The best known of this latter group is bacteriorhodopsin, which converts light into energy via photon-activated transmembrane proton pumping. Regardless of function, all rhodopsins appear to share common features, which include a retinyl chromophore in a protein-binding site formed within the inner segments of seven transmembrane helices. Upon the absorption of light, the chromophore isomerizes to generate a bathochromically shifted photoproduct that stores a significant fraction of the photon energy in both electrostatic and conformational forms. Subsequent dark reactions carry out the relevant biological function. The nature of the primary photochemical events in the rhodopsins has

Bimetallic Surface Chemistry

Abstract: Bimetallic surface chemistry plays a crucial role in a number of tech­ nologically important areas, including catalysis, magneto-optical films, microelectronics fabrication, electrochemistry, corrosion passivation, and structural materials. With the advent of ultrahigh vacuum surface ana­ lytical techniques and the development of efficient methods for surface preparation, it has become possible since approximately 1970 to prepare clean single-crystal bimetallic surfaces that are well characterized with respect to the geometric locations of the two different metal elements and that are very homogeneous in this surface structure (i.e. having an ideal surface unit cell with few defects in repeating this unit across the surface). These well-defined bimetallic surfaces have offered for the first time an ability to correlate surface chemical reactivity with atomic-level surface structure. Such structure/function relationships must ultimately form the basis for any predictive ability in tailor-making bimetallic surfaces to have desirable reactive properties. This recognition has motivated a great deal of stud y over the past two decades concerning the structure of single-crystal bimetallic surfaces and their surface reactivity with respect to adsorption, desorption, decomposition reactions, and catalytic processes. Although this field is still in its infancy, a great deal has already been learned and trends arc beginning to emerge that give some predictive ability with respect to the surface structures assumed by bimetallic surfaces and their corresponding reactivity toward simple molecules. The aim of this chapter

Spectra and Dynamics of Coupled Vibrations in Polyatomic Molecules

Abstract: 1. During the last two decades Fourier transform and laser spectroscopy techniques have made high resolution infrared spectra close to the Doppler limit with the possibility of detailed rovibrational analysis much more easily accessible than before (4-7).. 2. The development of special techniques with greatly increased sensitivity has permitted the measurement of the weak spectra associated with high overtones of vibrations in the near infrared and visible (8-12). 3. The spectroscopy of molecules cooled to very low internal temperatures in supersonic free jet expansions has been used to obtain analyses of spectra for molecules and complexes, whose spectra at room temperature are much too complicated for analysis (13-17). 4. The understanding of the complex spectra of coupled vibrations has become important for newly developing fields of molecular reaction dynamics uch as infrared laser chemistry (18-22) or vibrational overtone photodissociation (23-26).

Chemical Kinetics and Combustion Modeling

Abstract: ion of H atoms from fuel or other species by R02 produces alkyl hydroperoxides ROOH that then decompose to produce RO and OH radicals. However, for hydrocarbon fuels more complicated than n-butane, a more rapid process for R02 is isomerization via internal abstraction of H atoms ( 1 94). The general features of this R02 isomerization theory provide the kinetic basis for global reaction schemes for engine knock in internal combustion engines ( 1 96-1 98). The major steps consist schematically of R02 +=t QOOH (internal H atom abstraction) QOOH � QO + OH (0-0 homolysis). At sufficiently low temperatures, molecular oxygen can add further to the QOOH radicals, leading eventually to an overall reaction QOOH + 02 = products + OH + OH. Both alternatives are important since they produce OH radicals through reaction sequences with relatively low energy barriers. ROz isomerization rates are determined primarily by the size of the ringlike intermediate transition state, by the bond energy of the H atom being abstracted internally, and by the equilibrium constant of the R02 addition reaction. For fuels of larger hydrocarbons many isomerizations are possible, and 0-0 homolysis of the QOOH product of the iso­ merization reaction yields a different stable oxygenated species for each isomerization reaction. Thus for n-alkanes, a 1 ,4-H -atom abstraction, followed by 0-0 bond fission, leads to a 3-membered oxygenated ring, an oxiran. Similarly, 1 ,5-processes lead to oxetans, 1 ,6-abstractions produce tetrahydrofurans, and l ,7-abstractions produce tetrahydropyrans. Cur­ rent models generally use activation energies tabulated by Baldwin et al ( 199), but with A factors slightly lower than the 1 0 1 2. 1 S 1 recommended by Baldwin et aI, closer to the value of 1 0 1 1 . 5 S 1 recommended by Benson ( 1 89) for unimolecular reactions involving a cyclic transition state. The isomerization reactions are reversible, and activation energies for the reverse isomerizations are easily computed from the activation energy of the forward (endothermic) reaction and the AH of the reactions ( 1 94). ROz isomerization through internal abstraction of an H atom from a site adjacent to the C-O bond, followed by breakage of the C-O bond, will lead to a conjugate olefin and H02_ Direct abstraction paths leading to the same products have been discussed by Gutman and co-workers (200, 20 1 ), favoring a path proceeding through R02 isomerization. The current work of Wagner et al (20 I) provides some insight into the diffi382 MILLER, KEE & WESTBROOK culties of this reaction, but this is one of the simplest of the R02 iso­ merizations, and there are many more such reactions for which complex analyses are needed to understand fully the detailed reaction rates and mechanisms. Reactions of the product epoxide and other oxygenated species must be included in kinetic models, but very few quantitative studies of H atom abstraction or other reactions for these species have been reported. Current models must estimate both the rates and products for reactions of the epoxides, primarily attributed to H atom abstraction by OH or H02•

Ultrafast Molecular Reaction Dynamics in Real-Time: Progress Over a Decade

Abstract: One of the goals of researchers in the field of reaction dynamics is to develop an understanding of the elementary steps involved in a chemical reaction on a molecular level (see e.g. Ref. 1). The century-old Arrhenius rate law, a phenomenological description of the temperature dependence of rates of reactions in bulk, has been used extensively to deduce activation energies and frequency factors. The activated complex theory (also referred to as absolute rate theory or transition-state theory, see e.g. Refs. 2, 3) postulated more than 50 years ago, provides a useful interpretation of the Arrhenius rate parameters in terms of molecular properties. These parameters contain practical information about rates, but they do not express the molecular details of a reaction. At this juncture, two types of questions can be raised--one concerning the effects of the environment on rates in condensed media, and the other, the purely molecular aspects of reactions in the absence of an environment, i.e. in an isolated molecular system. We restrict our attention to the latter case for the purposes of this review.

Electronic State-Specific Transition Metal ION Chemistry

Abstract: The importance of transition metal centers as catalysts for selective trans­ formations of small molecules into useful chemicals has prompted exten­ sive studies in the condensed phase. In the last decade or so , chemical analogues to these processes have also been investigated in the gas phase. The promise of the gas phase research is that more quantitative infor­ mation regarding the dynamics, kinetics, and thermochemistry of these processes can be obtained in a more controlled environment. A particularly active component of this gas phase research is the chemistry of atomic transition metal ions, which are conveniently studied by several types of mass spectrometric techniques (1-7). Although the relationship between this gas phase endeavor and condensed phase organometallic chemistry is still evolving , one area in which gas phase studies have contributed some insight into the reactivity at transition metal centers concerns the influence of the electronic state (8a). The abundance of low-lying electronic statcs is one of the features of transition metals that is integral to their ability to catalyze chemistry , since it allows a metal center to be a versatile reaction template in which many different types of species can bond and subsequently react. Unfortunately, this same feature makes it difficult to generate transition metals (atoms, ions , or complexes) in specific electronic states. Table I shows the extent of the problem for the first row transition metal ions. Since ion generation

Femtosecond Optical Spectroscopy: A Direct Look at Elementary Chemical Events

Abstract: Ultrafast optical spectroscopy using laser pulses as short as 6 femtoseconds provides a novel probe for molecular nuclear motions ( l , 2). Since the duration of such pulses is comparable to or shorter than typical molecular vibrational periods ( 1 0 fsec is the period of a 3300 cm -1 vibration), these pulses make it possible to probe molecular vibrations and elementary photophysical and photochemical events in real time. Femtosecond dynamics and relaxation studies have been carried out in solutions (39a,b), neat liquids ( 1 01 3), conjugated polymers ( 1 41 5), proteins and biological systems ( 1 6-1 7), crystals (I 8a-c) , surfaces ( 1 9a,b), semi­ conductors (20-21 ), molecular aggregates (22-23), the hydrated electron (24a-c), isolated molecules in supersonic beams (25-26), and the gas phase (27a,b). Femtosecond nonlinear optical measurements have numerous unique advantages. Consider, for example, the simplest ultrafast spectro­ scopic technique: pump-probe spectroscopy (4-6, 25a-c). Tn this technique the system is subjected to two short pulses separated by a time delay r. The first pulse (the pump pulse) has a frequency OJ 1 and the absorption of

FAST CHEMICAL REACTIONS: Theory Challenges Experiment

Abstract: where the activation energy, Ea, can be related approximately to the barrier height in the potential energy surface for the reaction. What is not generally realized is that there are many simple reactions of wide current importance that do not have rate constants of this form and, indeed, have rate con­ stants that decrease in magnitude when the temperature is increased (1-4). These reactions can have rate constants that are orders of magnitude larger than those obtained for reactions with activation energies, and it has not been easy to make accurate measurements of their rate constants, as highly reactive species such as free radicals and ions are usually involved. Furthermore, the rate constants can show interesting temperature depen­ dences, which can also be hard to measure, especially for low temperatures (1). Due to significant improvements in techniques, however, some of these difficulties are being overcome and a wealth of experimental data is now being produced on the rates of fast chemical reactions that appear to have zero or negative activation energies (1, 2). This advance has come about not only through the sophisticated use of laser and mass-spectrometric

Recent Advances in Quantum Mechanical Reactive Scattering Theory, Including Comparison of Recent Experiments with Rigorous Calculations of State-to-State Cross Sections for the H/D+H2→H2/HD+H Reactions

Abstract: It is well recognized that quantum mechanical reactive scattering theory provides the most complete description of an elementary bimolecular chemical reaction allowed by the basic laws of nature. Thus ever since the 1960s, when crossed molecular beam experiments opened the door to studying reactions at this most rigorous state-to-state level (la,b), there has been intense interest and effort devoted to developing the theory to the practical stage that reliable calculations can be carried out for real chemical reactions. The purpose of this review is to describe some rather dramatic progress that has been made in the theoretical methodology and its application in only the last few years. These theoretical developments have come at a very propitious time because of a variety of new experimental studies of state-to-state cross sections for the fundamental reactions

Berry's Phase

Abstract: Berry's phase (1, 2) is an example of holonomy, the extent to which some variables change when other variables or parameters characterizing a system return to their initial values (3, 4). A simple case of classical holonomy is shown in Figure 1; a particle (with a tangent vector indicated by an arrow) moves on the surface of a sphere, beginning and ending at the north pole, in such a way that locally it docs not rotate about an axis perpendicular to the surface. As a consequence of this parallel transport on the curved surface, however, a rotation can be accumulated when the particle returns to its original position.4 In a similar way, the state vector of a quantum system can \"rotate\" as it undergoes a cyclic evolution in state space, thereby accumulating a ho)onomy. The most general context for Berry's phase arises from the division of a system (perhaps the universe) into parts; the question is, what can we say about the full system, when a subsystem undergoes a cyclic evolution? Typically one might attempt a solution to the equations of motion, for example the Schrodinger Equation, for the full system; the fact that we can often do better in answering the question by recognizing the role of

Quantum Monte Carlo for the Electronic Structure of Atoms and Molecules

Abstract: Since the late 1960s molecular orbital (MO) theory has dominated the development of ab initio quantum chemistry. Specifically, the Hartree­ Fock-Roothaan (HFR) (1-3) formalism of the self-consistent field (SCF) method, which was greatly facilitated by the development of gaussian basis sets as suggested by Boys (4, 5), and the post-Hartree-Fock methods of configuration interaction (CI) (6-8), multiconfiguration SCF (MCSCF) (9, 10), many-body perturbation theory (MBPT) and coupled-cluster methods (CCM) (11), and Moller-Plesset perturbation theory (MPPT) (12). With the advent of high speed vector supercomputers the field has virtually exploded; larger molecules or very high accuracy for small molecules are now possible. In addition to such capability has come more accurate knowledge of the size of I-particle and N-particle basis sets required for high accuracy (13-16). Not only does the use of large basis sets require large memory and file-storage capability, but HFR and post-HFR algorithms contain significant bottlenecks to vectorization and parallelization. Although many new and efficient matrix methods have been developed

The Effect of Laser-Induced Vibrational Bond Stretching in Atom-Molecule Collisions

Abstract: In a recent review of statc-to-statc vibrational energy transfer processes Krajnovich et al (30) point out that "there remains the challenge of extend­ ing crossed beam studies to molecules prepared initially in excited vibrational states." Indeed, even for diatomic molecules most of the quan­ titative aspects of energy transfer from highly excited vibrational states are still poorly understood (3 1 ) . At present, promising new experimental techniques are becoming available to meet this challenge. These methods (see Table 3 below) rely on the exceptional tunability, power, and coherence afforded by laser radiation. In combination with molecular beams, which

Nonlinear dynamics and thermodynamics of chemical reactions far from equilibrium

Abstract: In a macroscopic, homogeneous reacting system, the rates of change of chemical species concentrations are governed by a set of coupled, nonlinear ordinary differential equations according to the law of mass action. Far from equilibrium, reactions in batch or in flow reactors exhibit the com­ plexity and phenomenological diversity typical of nonlinear dynamical systems: multiple steady states, relaxation oscillations, near-sinusoidal oscillations, complex oscillations with large and small amplitude peaks, bursts of high-frequency oscillations, birhythmicity, quasiperiodicity, and chaos have all been found in the study of chemical reaction networks ( 1-8). When a system is displaced from equilibrium by fluxes of matter or energy across its boundaries, multiple steady states may appear, if the reaction network exhibits autocatalysis, substrate inhibition of enzyme

Ordering in Liquid Alloys

Abstract: Liquid alloys, which are disordered systems in the sense of having no long­ range atomic or magnetic order, display a remarkable variety of local atomic structures. The richness of this variety has become apparent in recent years with advances in experimental techniques for structural deter­ mination and in methods for computer simulation of large disordered systems. At one extreme we encounter alloys where the constituents are chemically similar and undergo relatively small changes in electronic behavior on mixing; in these cases the local structure is not too different from that of the pure elements, and the changes in transport and thermo­ dynamic properties can be described in terms of small deviations from ideal behavior. At the other extreme are systems where complex ionic species are formed on alloying, characterized by remarkable geometric arrangements of the atoms, dramatic changes in the electrical behavior, and large anomalies in the temperature dependence of thermodynamic functions such as specific heat.

High resolution infrared spectroscopy of transient molecules

Abstract: In this review the topics of instrumentation (I), high temperature molecules (II D), free radicals (II E), and a few arbitrary selections from the semi­ stable molecule category (II B) are discussed. Furthermore, only gas-phase vibration-rotation transitions of ground electronic states are included. Such topics as infrared electronic transitions [e.g. SiC (1 )], pure rotational transitions [e.g. OR (2, 3)], and vibration-rotation transitions in excited electronic states [e.g. aId state of SO (4)] are excluded. The detection of

Molecular Intercalation Reactions in Lamellar Compounds

Abstract: Intercalation compounds are formed by the insertion of guest species into available sites in a host structure. Ideally, intercalation reactions are reversible, so that the original guest and host can be regenerated. Lamellar hosts provide the greatest structural flexibility for car rying out these reac­ tions because the covalently bonded host layers can be separated by large distances to accommodate the guest in thc so-called van der Waals gap of the host. A schematic diagram of the intercalation process for a layered host is illustrated in Figure I. A typical sequence of events for simple intercalation reactions involves (a) initial interaction of the guest with the host to facilitate layer opening, (b) initial penetration of the guest into the host, (c) diffusion of the guest species to interlayer sites to optimize guest­ guest and guest-host interactions, and (d) continued entry of the guest into the host until equilibrium is achieved.

Local Quantum Chemistry

Abstract: Quantum chemical methodology is at an important level. Highly detailed and reliable calculations of electronic structure can be carried out for most small molecule problems, where a small molecule means a species with up to about 50 electrons. Somewhat larger species can be studied just as well, though the sizable computing demands make such calculations less common. An overview of the development of ab initio electronic structure methods, and how the current level was reached, would point out certain areas of particularly intensive effort. Prior reviews in this series chronicle much of that effort ( 1-6). About two decades ago, a first hurdle was passed. It was the problem of basis set selection, generally for self-consistent field (SCF) level wavefunctions, and a number of standard basis sets were devised while computational procedures for evaluating the various types of integrals were developed. The next hurdle brought a major assault, beginning in the early or mid-1970s, on the problem of electron correlation. A variety of potent techniques emerged, and virtually exhaustive incor­ poration of electron correlation effects in small systems is tractable today. Most recently, another milestone has been nearly passed with the tremen­ dous developments for analytical evaluation of energy gradients. Molec­ ular properties and vibrational information are now tackled on the same footing as the molecular electronic energy.

Recent Advances In Quantum Mechanical Reactive Scattering Theory, Including Comparison Of Recent Experiments With Rigorous Calculations Of State-To-St

Abstract: It is well recognized that quantum echanical reactive scattering theory provides the most complete description of an elementary bimolecular chemical reaction allowed by the basic laws of nature. Thus ever since the 1960s, when crossed molecular beam experiments opened the door to studying reactions at this most rigorous state-to-state level (la,b), there has been intense interest and effort devoted to developing the theory to the practical stage that reliable calculations can be carried out for real chemical reactions. The purpose of this review is to describe some rather dramatic progress that has been made in the theoretical methodology and its application in only the last few years. These theoretical developments have come at a very propitious time because of a variety of new experimental studies of state-to-state cross sections for the fundamental reactions

Recent Advances in Electron Paramagnetic Resonance

Abstract: It has been known for some time that significant advances in the appli­ cation of electron paramagnetic resonance (EPR) spectroscopy could be realized if improvements in instrumentation could be achieved. Indeed, a comparison of EPR with nuclear magnetic resonance (NMR) readily suggests appropriate avenues for improved experimentation, e.g. use of sophisticated pulse techniques, more extensive utilization of high field and variable magnetic field capabilities, use of magnetic resonance imaging, more sophisticated use of multiple radiation field capabilities, etc. Obvious advances in these areas have been limited for EPR relative to NMR, however, because (a) unpaired or paramagnetic electrons occur fairly infrequently in nature, often only at concentrations low compared to the omnipresent and more abundant nuclear spins such as protons, with the consequence that signal-to-noise considerations are often much more severe in EPR and the range of applications is more limited, (b) electron paramagnetic relaxation times are typically much shorter than nuclear relaxation times, thus forcing pulse or time-domain experiments to be performed on nanosecond (nsec) to microsecond (flsec) timescales com­ pared to the microsecond to millisecond (msec) timescales typically used for NMR experiments, (c) EPR spectral widths are typically much greater