bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic Studies of Liquid Structure and Solvation
Authors: Fawcett W. Ronald
Published: November 2020

Spectroscopy involves the study of the interactions of electromagnetic radiation with matter. In the case of liquids, radiation of a wide range of frequencies, and thus energies, has been used, all the way from radio-frequency waves to X-rays. Experiments involving neutrons, which are associated with very short wavelengths, are also important. In the spectroscopic experiment the incident radiation may be either absorbed or scattered and the experimental information is obtained by examining the intensity and direction of the radiation after it has passed through the sample. Several spectroscopic techniques will be considered in this chapter. X-ray and neutron diffraction techniques are powerful tools for studying the structure of liquids and have been introduced in chapter 2. They may also be used to study the structure of solutions and determine distribution functions for both the solute and solvent. The feasibility of these experiments depends on the number of different nuclei involved in the system. UV-visible spectroscopy is mainly used to study electronic transitions in polyatomic species. These species are often complex ions formed between the electrolyte and the solvent, or between the cation and one or more anions. Vibrational spectroscopy involves electromagnetic radiation of lower energy, usually in the infrared region. It is used to study intramolecular vibrational modes and how they are altered by the environment in solution. It can also be used to study the bonds formed between solute and solvent in the solvation process. Finally, nuclear magnetic resonance spectroscopy and its application to the study of solvation will be discussed. This is a particularly powerful technique because it provides information about the environment of a given nucleus, and experiments specific to a given nucleus can be carried out provided the nucleus has a non-zero magnetic moment. Several other spectroscopic techniques are commonly used [G1] but those considered here provide a representative picture of what can be learnt from those experiments. One should remember that the atoms and molecules in liquids are not motionless but in a state of flux determined by the intermolecular interactions and temperature. From the study of microwave spectroscopy discussed in chapter 4, it was found that rotational diffusion processes in liquids are characterized by relaxation times the order of a few picoseconds.
NMR Approaches to the Heat-, Cold-, and Pressure-Induced Unfolding of Proteins
Authors: Akasaka K. , Yamaguchi T.
Published: November 2020

Proteins are unique at least in two aspects. First, the atoms constituting a protein molecule act highly cooperatively in constructing its unique folded structure. As a result, their conformational transitions also occur in a highly cooperative fashion (often following a two-state transition). Secondly, the free energy balance between the folded (native) and unfolded (denatured) conformers are surprisingly marginal (usually less than 10 kcal/mole protein), despite the fact that interactions of thousands of atoms are involved in the folding. Such unique properties have been acquired by protein molecules through a countless number of random experiments and subsequent selections during the course of evolution of life, so that to our eyes, at present, they look as if they were carefully designed by Nature. It is important to recognize that such random experiments occurred in a dominantly aqueous environment. In order to understand the underlying principles of design as generally as possible, we need, at least, to characterize the factors that contribute to (1) the stability of protein structures, (2) the structural details of the folded and unfolded conformers, and (3) the kinetics of folding and unfolding reactions. In all these, the involvement, of water has crucial importance. NMR can provide unique information not only on aspect (2) above, but on all the above aspects when used under appropriate design. In this presentation, some examples will be shown from our current research. Our daily experience in the kitchen shows that proteins are easily deformed (denatured); a boiled egg can be prepared just by heating to not more than 100°C in water. The easy deformability (which is, in fact, a global conformational transition) in aqueous environment is not merely a matter of interest in a kitchen, but is a quality of design for proteins by Nature. A global conformational transition (unfolding) of a protein molecule occurs even under physiological conditions, although infrequently, as evidenced by hydrogen exchange of peptide NH protons that are completely buried in the folded structure.
Multidimensional NMR investigation of the Neurotoxic Peptide Mastoparan in the Absence and Presence of Calmodulin
Authors: Mari F. , Xie X.
Published: November 2020

Calmodulin (CaM) is the major Ca2+ receptor in eukaryotic cells (Means and Rasmussen, 1988). This paper begins an investigation into the structural requirements for neurotoxic peptide binding to CaM. In resting cells, CaM is deficient in Ca2+ (the protein has the potential for binding four Ca2+ ions with high affinity, pKd > 6 (Means and Rasmussen, 1988)). Following nerve cell excitation, intracellular levels of Ca2+ increase dramatically, from about 0.1 μM to about 10 μM, allowing CaM to become fully-loaded with Ca2+ . Ca2+ - loaded CaM has the ability to activate a number of neural enzymes, including cyclic nucleotide phosphodiesterase, adenylate cyclase, Ca2+ - CaM kinase and calcineurin (Kennedy, 1989). A tight-binding neurotoxic peptide would be expected to competitively inhibit activation of these enzymes. The high level of intercellular coordination required by higher organisms is attained, in part, by the complex interplay of the nervous and endocrine systems. Two important second messengers are involved in information transfer processes associated with the normal operation of these two systems: cyclic AMP (cAMP) and Ca2+ . Cyclic AMP is involved in trans-membrane information flow following the interaction of cell surface receptors with certain hormones (e.g., glucagon, epinephrine and ACTH), while Ca2+ is the principal information carrier in the nerve cell following stimulation of the system by membrane depolarization. CaM plays a pivotal role in second messenger function in both the nervous and endocrine systems. In the nervous system, calmodulin is the principal target for Ca2+. In the endocrine system, CaM (complexed with Ca2+) is responsible for activating the enzymes responsible for both cAMP synthesis (i.e., adenylate cyclase) and degradation (i.e., cyclic nucleotide phosphodiesterase). Additional linkage between the nervous and endocrine systems is evident from the fact that both systems are responsive to some of the same peptide messengers. For example, insulin, glucagon, angiotensin, and somatostatin have been found in the brain, and may function as neurotransmitters (Malencik and Anderson, 1982) perhaps through CaM mediation.
Ex Vivo MuEtinuciear NMR Spectroscopy of Perfused, Respiring Rat Brain Slices: Model Studies of Hypoxia, Ischemia, and Excitotoxscit
Authors: Litt L. , Espanol M.T.
Published: November 2020

We believe there are important roles for in vivo NMR spectroscopy techniques in studies of protection and treatment in stroke. Perhaps the primary utility of in vivo NMR spectroscopy is to establish the relevance of metabolic integrity, intracellular pH, and intracellular energy stores to concurrent changes occurring both at gross physiological levels (e.g., changes in cerebral blood flow, or blood oxygenation), and at microscopic or cellular levels. It has long been known that the brain is exquisitely sensitive to deprivations of oxygen, glucose, and cerebral blood flow. Routine human surgery on a limb takes place every day with tourniquets stopping all blood flow for up to two hours. In contrast, the deprivation of all blood flow to the brain (global ischemia) for approximately 5 minutes can result in severe, permanent brain damage. Research has gone on for more than 30 years to understand why the brain’s revival time is so much shorter, and to discover brain biochemical interventions that might dramatically extend the brain’s intolerance beyond 5 minutes, and therefore be relevant to protection and treatment of stroke. (Kogure and Hossmann, 1985; 1993) Stroke, defined as a permanent neurologic deficit arising from the death of brain cells, kills ∼ 150,000 people in the U.S.A. each year, and is the third leading cause of death (Feinleib et al., 1993). It is the next malady to escape, once one has dodged death from cardiovascular disease and cancer. Many, if not most, U.S.A. stroke victims will receive neurological clinical care not substantially different from what was provided 30 years ago. Most stroke patients will be put in intensive care units where blood pressure will be regulated and kept in a “safe” range, with the body given supportive care and the brain given an opportunity to heal itself. The problem of stroke is actually quite complex because there are several different kinds of stroke (ischemic, hemorrhagic, etc.), and because numerous systemic physiological factors are of relevance. Nevertheless, exciting advances in brain biochemistry suggest that stroke therapy and prophylaxis axe likely to improve dramatically in the near future (Zivin and Choi, 1991).
A Service-Based Model of a CASE Environment
Authors: Brown Alan W. , Carney David J. , Morris Edwin J. , Smith Dennis B. , Zarrella Paul F.
Published: November 2020

We have already described our three-level model of CASE tool integration in which services, mechanisms, and process all participate. In this chapter, we focus on the services aspect, and examine what we actually mean by “service.” We also consider the variety of services typically found in an environment, and the interrelationships between services and other aspects of an environment (i.e., interfaces between services). The vehicle for this examination is an abstract description called a “reference model.” We use the term reference model to characterize a model of a system taken from a purely conceptual standpoint. Such a conceptual model permits consideration of alternative implementations of a service, adding new implementations, and so on. We note the potential for confusion here, since the term reference model is often implicitly concerned with the notion of architecture; in many cases a reference model is understood to refer to a reference architecture. We do not use the term in this way. Our understanding of the term is closer to the concept of a “feature catalog”; it is explicitly not intended to identify an architecture, but rather is intended to provide a common reference point from which the functional capabilities of actual implementations can be described. This view is consistent with our separation of services (described in the reference model) both from the mechanisms that provide those services and also from the processes that they support. The reference model we use as our exemplar is the one developed by the U.S. Navy Next Generation Computer Resources (NGCR) Program. As part of the work of the NGCR, the Project Support Environments Standards Working Group (PSESWG) defined an environment reference model as its starting point for selecting interface standards. The working group examined many existing efforts (e.g., the Software Technology for Adaptable, Reliable Systems (STARS) Program, the National Institute of Standards and Technology (NIST) Integrated Software Engineering Environment (ISEE) working group), synthesized many aspects from them, and eventually created an entirely new document called A Reference Model for Project Support Environments [63].
Properties and Types of Integration Mechanisms
Authors: Brown Alan W. , Carney David J. , Morris Edwin J. , Smith Dennis B. , Zarrella Paul F.
Published: November 2020

A central theme of this book is the three-level approach to CASE environment design and construction that distinguishes conceptual issues (the services) from implementation issues (the mechanisms) and stresses the need for a design context (the process) that the CASE environment must support. Previous chapters have discussed this theme, as well as provided insight into a conceptual model that identifies and classifies the services that might be found in a CASE environment. However, regardless of the service model or conceptual approach selected, environment builders must eventually face the bottom-line decision of how to actually carry out tool integration. The choices they face in terms of potential mechanisms are numerous and include a full range of selections that provide varying degrees of effort and integrated capability. In this chapter, we first consider the properties of integration by continuing the discussion of integration as a relationship (see Section 2.2). In this discussion (Section 5.2), we highlight properties that are addressed by various integration mechanisms. Later sections of this chapter focus on the specific relationship between two particular classes of integrating mechanisms: those based on the sharing or transfer of data between tools (data integration), and those based on synchronization and communication between tools (control integration). In Section 2.2 we introduced the concept put forward by Thomas and Nejmeh that integration can be considered by defining the properties required of the relationships between different environment components. Their definition of integration is exclusively a conceptual view, and is independent of the particular technology being used to implement that integration.
Stress
Authors: Oertel Gerhard
Published: November 2020

Stress is a tensor quantity that describes the mechanical force density (force per unit area) on the complete surface of a domain inside a material body. A stress exists wherever one part of a body exerts a force on neighboring parts. Its orientation is not tied to any particular directions that are intrinsic to the material like, say, crystallographic axes. It is thus distinct from the matter tensors that were discussed in the preceding chapter, all of which have definitive orientations within a crystal or other anisotropic material; it is called a field tensor (and so is strain). The definition of stress depends on the concept of a continuum. Let f(xi) be a single-valued function defined for every point xi in a region. This function is said to be continuous at the point xi if the following holds for all paths of approach of xi to °xi:. . . f(xi) → f(°xi) as xi → °xi (4.1)· . . . Equivalently, for any number ∊, no matter how small, there exists a neighborhood of nonzero radius around the point xi in which: . . . . 〈f(xi) − f(°xi)〉2 < ∊,­ (4.2) . . . for all points xi in that neighborhood. A continuum is an idealized material whose physical attributes are continuous functions of position. Thus neighboring points remain neighbors, and a continuum cannot have gaps or jumps (discontinuities) in its properties. Surfaces bounding gaps or defining discontinuities must be specially treated in continuum mechanics. Examples are surfaces between two fluids of differing density or viscosity, or between solids with different thermal conductivity or elastic properties. Real materials are never continua; they are discontinuous at the atomic scale, and often at larger scales as well. The notion of a continuum is, therefore, only a macroscopic approximation, but it allows useful mathematical approaches to the treatment of real phenomena.
Plant-Animal Interactions
Authors: Greg Murray K. , Kinsman Sharon
Published: November 2020

The term “plant-animal interactions” includes a diverse array of biologically important relationships. Plant-herbivore relationships (in which an animal feeds on whole plants or parts of them) are examples of exploitation, because one species benefits from the interaction while the other suffers. Plant-pollinator and plant-seed disperser relationships (in which animals disperse pollen or seeds, usually in return for a food reward) are examples of mutualisms because they are beneficial to both parties. Another class of plant-animal mutualisms involves plants that provide nesting sites and/or food rewards to ants, which often protect the plant from herbivores or competing plants. Plantpollinator and plant-seed disperser mutualisms probably originated as cases of exploitation of plants by animals (Thompson 1982, Crepet 1983, Tiffney 1986). Many of the distinctive plant structures associated with animal-mediated pollen and seed dispersal (e.g., flowers, nectaries, attractive odors, fleshy fruit pulp, and thickened seed coats) presumably evolved to attract consumers of floral or seed resources while preventing them from digesting the pollen or seeds. mutualisms in structuring ecological communities. Competition and predator-prey interactions were more common subjects. Botanists had described the characteristics of the plant and animal players in pollination and seed dispersal mutualisms (Knuth 1906, 1908, 1909, Ridley 1930, van der Pijl 1969, Faegri and van der Pijl 1979), but these descriptive works did not fully examine plant-animal mutualisms in the context of communities. The opportunity to work in the neotropics, facilitated by the Organization for Tropical Studies (OTS), the Smithsonian Tropical Research Institute (STRI), and other institutions, attracted the attention of temperate-zone ecologists to the mutualisms that are much more conspicuous components of tropical systems than of temperate ones (Wheelwright 1988b). Plant-pollinator interactions have attracted more attention in Monteverde than plant-frugivore interactions, and plant-herbivore interactions remain conspicuously understudied. This imbalance probably reflects the interests of those who first worked at Monteverde and later returned with their own students, rather than differences in the significance of the interactions at Monteverde or elsewhere. Aside from a few studies of herbivory in particular species (e.g., Peck, “Agroecology of Prosapia,”), even basic surveys remain to be done.
Complex Oscillations and Chaos
Authors: Epstein Irving R. , Pojman John A.
Published: November 2020

After studying the first seven chapters of this book, the reader may have come to the conclusion that a chemical reaction that exhibits periodic oscillation with a single maximum and a single minimum must be at or near the apex of the pyramid of dynamical complexity. In the words of the song that is sung at the Jewish Passover celebration, the Seder, “Dayenu” (It would have been enough). But nature always has more to offer, and simple periodic oscillation is only the beginning of the story. In this chapter, we will investigate more complex modes of temporal oscillation, including both periodic behavior (in which each cycle can have several maxima and minima in the concentrations) and aperiodic behavior, or chaos (in which no set of concentrations is ever exactly repeated, but the system nonetheless behaves deterministically). Most people who study periodic behavior deal with linear oscillators and therefore tend to think of oscillations as sinusoidal. Chemical oscillators are, as we have seen, decidedly nonlinear, and their waveforms can depart quite drastically from being sinusoidal. Even after accepting that chemical oscillations can look as nonsinusoidal as the relaxation oscillations shown in Figure 4.4, our intuition may still resist the notion that a single period of oscillation might contain two, three, or perhaps twenty-three, maxima and minima. As an example, consider the behavior shown in Figure 8.1, where the potential of a bromide-selective electrode in the BZ reaction in a CSTR shows one large and two small extrema in each cycle of oscillation. The oscillations shown in Figure 8.1 are of the mixed-mode type, in which each period contains a mixture of large-amplitude and small-amplitude peaks. Mixedmode oscillations are perhaps the most commonly occurring form of complex oscillations in chemical systems. In order to develop some intuitive feel for how such behavior might arise, we employ a picture based on slow manifolds and utilized by a variety of authors (Boissonade, 1976; Rössler, 1976; Rinzel, 1987; Barkley, 1988) to analyze mixed-mode oscillations and other forms of complex dynamical behavior.
Delays and Differential Delay Equations
Authors: Epstein Irving R. , Pojman John A.
Published: November 2020

Mathematically speaking, the most important tools used by the chemical kineticist to study chemical reactions like the ones we have been considering are sets of coupled, first-order, ordinary differential equations that describe the changes in time of the concentrations of species in the system, that is, the rate laws derived from the Law of Mass Action. In order to obtain equations of this type, one must make a number of key assumptions, some of which are usually explicit, others more hidden. We have treated only isothermal systems, thereby obtaining polynomial rate laws instead of the transcendental expressions that would result if the temperature were taken as a variable, a step that would be necessary if we were to consider thermochemical oscillators (Gray and Scott, 1990), for example, combustion reactions at metal surfaces. What is perhaps less obvious is that our equations constitute an average over quantum mechanical microstates, allowing us to employ a relatively small number of bulk concentrations as our dependent variables, rather than having to keep track of the populations of different states that react at different rates. Our treatment ignores fluctuations, so that we may utilize deterministic equations rather than a stochastic or a master equation formulation (Gardiner, 1990). Whenever we employ ordinary differential equations, we are making the approximation that the medium is well mixed, with all species uniformly distributed; any spatial gradients (and we see in several other chapters that these can play a key role) require the inclusion of diffusion terms and the use of partial differential equations. All of these assumptions or approximations are well known, and in all cases chemists have more elaborate techniques at their disposal for treating these effects more exactly, should that be desirable. Another, less widely appreciated idealization in chemical kinetics is that phenomena take place instantaneously—that a change in [A] at time t generates a change in [B] time t and not at some later time t + τ. On a microscopic level, it is clear that this state of affairs cannot hold.
Polymer Systems
Authors: Epstein Irving R. , Pojman John A.
Published: November 2020

In the classic 1967 film “The Graduate” the protagonist, Benjamin (Dustin Hoffman), is attempting to plan his postcollege path. His neighbor provides one word of advice, “Plastics.” This counsel has become part of American culture and is often parodied. But, it is good advice, because not since the transformations from stone to bronze and then to iron have new materials so completely transformed a society. Plastics made from synthetic polymers are ubiquitous, from Tupperware to artificial hearts. About half the world’s chemists work in polymer-related industries. In this chapter, we will survey some of the work that has been done in applying nonlinear dynamics to polymerization processes. These systems differ from those we have considered so far because they do not involve redox reactions. We will consider polymerization reactions in a CSTR that exhibit oscillations through the coupling of temperature-dependent viscosity and viscosity-dependent rate constants. Emulsion polymerization, which produces small polymer particles dispersed in water, can also oscillate in a CSTR. Both types of systems are important industrially, and their stabilities have been studied by engineers with the goal of eliminating their time-dependent behavior. Our favorite oscillating system, the Belousov-Zhabotinsky reaction, can be used to create an isothermal periodic polymerization reaction in either a batch or continuous system. This, however, is not a practical system because of the cost of the reagents. In most industrial processes, nonlinear behavior is seen not as an advantage but as something to be avoided. However, we will look at several reaction-diffusion systems that have desirable properties precisely because of their nonlinear behavior. Replication of RNA is autocatalytic and can occur as a traveling front. Since not all RNA molecules replicate equally well, faster mutants gradually take over. At each mutation, the front propagates faster. Evolution can be directly observed in a test tube. Propagating polymerization fronts of synthetic polymers may be useful for making new materials, and they are interesting because of the rich array of nonlinear phenomena they show, with pulsations, convection, and spinning fronts. Finally, we will consider photopolymerization systems that exhibit spatial pattern formation on the micron scale, which can be used to control the macroscopic properties.
Biological Oscillators
Authors: Epstein Irving R. , Pojman John A.
Published: November 2020

Including a chapter on biological oscillators was not an easy decision. In one sense, no book on nonlinear chemical dynamics would be complete without such a chapter. Not only are the most important and most numerous examples of chemical oscillators to be found in living systems, but the lure of gaining some insight into the workings of biological oscillators and into the remarkable parallels between chemical and biological oscillators attracts many, perhaps most, new initiates to the study of “exotic” chemical systems. On the other hand, it is impossible for us to do even a minimal job of covering the ground that ought to be covered, either in breadth or in depth. To say that the subject demands a whole book is to understate the case badly. There are indeed whole books, many of them excellent, devoted to various aspects of biological oscillators. We mention here only four of our favorites, the volumes by Winfree (1980), Glass and Mackey (1988), Murray (1993) and Goldbeter (1996). Having abandoned the unreachable goal of surveying the field, even superficially, we have opted to present brief looks at a handful of oscillatory phenomena in biology. Even here, our treatment will only scratch the surface. We suspect that, for the expert, this chapter will be the least satisfying in the book. Nonetheless, we have included it because it may also prove to be the most inspiring chapter for the novice. The range of periods of biological oscillators is considerable, as shown in Table 13.1. In this chapter, we focus on three examples of biological oscillation: the activity of neurons; polymerization of microtubulcs; and certain pathological conditions, known as dynamical diseases, that arise from changes in natural biological rhythms. With the possible exception of the first topic, these are not among the best-known nor the most thoroughly studied biological oscillators; they have been chosen because we feel that they can be presented, in a few pages, at a level that will give the reader a sense of the fascinating range of problems offered by biological systems.
Rates and Equilibria in Organic Reactions : The Thermodynamic and Extrathermodynamic Approaches
Authors: Doraiswamy L. K.
Published: November 2020

In any reversible reaction such as . . . vA A + vB B ↔ vR R + vS S [2.1] . . . the system inevitably moves toward a state of equilibrium, or maximum probability. This equilibrium state is very important in analyzing chemical reactions because it defines the limit to which any reaction can proceed. Organic reactions, particularly those constituting a synthetic scheme for a fine chemical, usually involve molecules reacting in the liquid phase. The effects of reactant structure and of the solvent (medium) in which the reaction occurs (the solvation effects) are not included in the conventional macroscopic approach to thermodynamics. Therefore, the treatment of liquid-phase reactions tends to be less exact than that of gas-phase reactions involving simpler molecules without these influences. A convenient way of approaching this problem is to start with the conventional macroscopic or thermodynamic approach and add enough microscopic detail to allow for the effects of solute (reactant) structure and the medium. This approach is called the extrathermodynamic approach and may be regarded as bridging the gap between the two rather disparate fields of rates and equilibria represented by kinetics and thermodynamics, respectively. Such an approach is particularly useful in organic synthesis and forms the subject matter of this chapter. An important consideration in process calculations is the change that results in the basic thermodynamic properties, internal energy (U), enthalpy (H), Helmholtz work function (A), and Gibbs free energy (G) when a closed system of constant mass moves from one macroscopic state to another. For a homogeneous fluid, these change equations can be expressed in terms of four differential equations, which then can be written in difference form by employing the operator Δ to represent the change from state 1 to state 2: Of these, the enthalpy and free energy change equations are the most frequently used in the analysis of reactions.
Complex Reactions
Authors: Doraiswamy L. K.
Published: November 2020

When a reactant or a set of reactants undergoes several reactions (at least two) simultaneously, the reaction is said to be a complex reaction. The total conversion of the key reactant, which is used as a measure of reaction in simple reactions, has little meaning in complex reactions, and what is of primary interest is the fraction of reactant converted to the desired product. Thus the more pertinent quantity is product distribution from which the conversion to the desired product can be calculated. This is usually expressed in terms of the yield or selectivity of the reaction with respect to the desired product. From the design point of view, an equally important consideration is the analysis and quantitative treatment of complex reactions, a common example of which is the dehydration of alcohol represented by We call such a set of simultaneous reactions a complex multiple reaction. It is also important to note that many organic syntheses involve a number of steps, each carried out under different conditions (and sometimes in different reactors), leading to what we designate as multistep reactions (normally called a synthetic scheme by organic chemists). This could, for example, be a sequence of reactions like dehydration, oxidation, Diels-Alder, and hydrogenation. This chapter outlines simple procedures for the treatment of complex multiple and multistep reactions and explains the concepts of selectivity and yield. For a more detailed treatment of multiple reactions, the following books may be consulted: Aris (1969) and Nauman (1987). We conclude the chapter by considering a reaction with both catalytic and noncatalytic steps, which also constitutes a kind of complex reaction. Because both chemists and chemical engineers are involved in formulating a practical strategy for accomplishing an organic synthesis, it is important to appreciate the roles of each.
Catalysis by Solids, 2 : The Catalyst and Its Microenvironment
Authors: Doraiswamy L. K.
Published: November 2020

Solid catalysts by their very nature involve diffusion of reactant fluids within their matrix. These fluids react even as they diffuse. Thus the problem of internal diffusion accompanied by reaction becomes important. Another problem of equal importance is the transport of the reactant from the fluid bulk to the catalyst surface—often referred to as external diffusion. Together these constitute the microenvironment of the catalyst pellet and form the subject matter of this chapter. Consider the reaction between aniline and methanol on pellets of alumina to give dimethylaniline: Let the pellets be placed in a flowing stream of reactants inside a tubular reactor. Restricting our attention now to a single pellet and its immediate environment, the various steps involved in the overall process are shown in Figure 7.1. This physical-chemical circuit is built in analogy with the electrical circuit shown at the bottom of the figure. Clearly, the overall process is a complex combination of chemical and physical steps. Note, however, that the mathematical analysis of the parallel pathways (diffusion and reaction) is not always based on the addition of reciprocal resistances as in parallel electrical circuits but on the fact that the two occur simultaneously on a single pathway, that is, the molecule reacts even as it diffuses. The construction of a model based on one of the adsorption-reactiondesorption steps being the limiting step constitutes the core of the semiempirical approach considered in this chapter. In this approach, the microscopic origins of the observed macroscopic effects of catalysts (as described by many authors, e.g., Plath, 1989) are ignored. The models thus developed are commonly known as Langmuir-Hinshelwood models among chemists and as Hougen-Watson models among chemical engineers. We choose to call them Langmuir-Hinshelwood-Hougen-Watson (LHHW) models. For a more basic approach to kinetic modeling of catalytic reactions in general, reference may be made, among others, to Compton (1991) and Van Santen and Niemantsverdriet (1995). In the interest of generality, we consider hypothetical reactions and derive rate equations for a few typical LHHW models (Hougen and Watson, 1947; Yang and Hougen, 1950; Satterfield, 1980, Butt, 1980; Doraiswamy and Sharma, 1984; Boudart and Djéga-Mariadassou, 1984)..
Reactor Design for Solid-Catalyzed Fluid-Phase Reactions
Authors: Doraiswamy L. K.
Published: November 2020

Catalytic reactions are carried out in reactors with a fixed, fluidized, or moving bed of catalyst. Although the chemical kinetics of the reaction obviously remains the same for all these reactors, the hydrodynamic features vary considerably. Because no complete description of these features is possible, it is convenient to postulate different situations and develop mathematical models to represent these situations for each type of reactor. It is also important to note that wherever solid catalysts are used, the question of catalyst deactivation cannot be ignored. Several books and reviews covering a variety of situations have been written, including those marked with an asterisk in the list of references. They are recommended for general reading. Our intention in this chapter is limited, however: formulate approaches to the design of two main classes of catalytic reactors, fixed and fluidized bed; briefly describe selected procedures along with a few numerical (or methodological) examples to illustrate their use; and outline a procedure for incorporating the effects of catalyst deactivation in reactor design and operation. There are basically two types of fixed-bed reactors: (1) multitubular, in which tubes of approximately 1.5 to 4.0 cm in diameter are placed as a bundle within a shell through which a heat exchange fluid is circulated to control the temperature profile within the reactor; and (2) adiabatic, in which the catalyst is placed directly inside a reactor (with no a priori limitation to the diameter), and heat removal is accomplished by multistaging the bed and removing the heat of reaction by heat exchange between stages. Four major models have been proposed for describing the behavior of a packed tubular reactor (see Doraiswamy and Sharma, 1984). Of these, the most extensively used is the quasi-continuum model in which the fluid-solid system is assumed to act as a single pseudohomogeneous phase with effective properties of its own (as for any true single phase). Thus the procedures developed in Chapters 4 and 10 for the homogeneous model can be used to determine the axial profiles of concentration and temperature. One can also allow for radial transport gradients within each tube [two-dimensional (2-D) models], as opposed to the simpler models in which these gradients are neglected—the one-dimensional (1-D) models.
Gas-Liquid Reactions
Authors: Doraiswamy L. K.
Published: November 2020

The general physical picture characterizing fluid-fluid reactions is sketched in Figure 14.1. A is the solute in phase 1 (gas or liquid) and is slightly soluble in phase 2, which is always a liquid. Upon entering phase 2, A reacts with B present in that phase. When phase 1 is a gas, the reaction is almost always restricted to phase 2 (except in the rare case of a desorbing product reacting in the gas phase), but when phase 1 is a liquid or a solid, reaction can occur in both phases. In this chapter we consider the case where reaction is confined to phase 2, gas-liquid reactions. Because two phases are present, mass transfer across the interface is clearly an important consideration. Therefore, the basis of the analysis is interaction between mass transfer and reaction, leading to the formulation of conditions and rate expressions for reactions with varying roles in the two processes (i.e., with different controlling regimes). Consider a reaction of the general form . . . vAA(g) + vBB(I) → R [14.1] . . .where vA = 1. Our objective is to examine the effect of chemical reaction on mass transfer.
Phase-Transfer Reaction Engineering
Authors: Doraiswamy L. K.
Published: November 2020

There are many situations in organic synthesis where it is desirable to bring about reaction between reactants present in two (or more) immiscible phases. Agents known as phase-transfer catalysts are used for this purpose. Their role in initiating or accelerating such reactions has been proven extensively since the early seventies, and the principles of their operation are being increasingly understood [see Weber and Gokel, 1977; Reuben and Sjoberg, 1981; Frechet, 1984; Freedman, 1986; Goldberg, 1992 (English translation); Dehmlow and Dehmlow, 1993; Starks et al., 1994; Yufit, 1995; Sasson and Neumann, 1997; Naik and Doraiswamy, 1998]. To date, an estimated 500 different commercial chemical processes (mostly for small volume chemicals) using about 5-25 million pounds per annum of phase-transfer catalysts have been reported (Starks et al., 1994), and well over 6,500 compounds have been synthesized in the laboratory using PTC (Keller, 1979, 1986). A large number of industrial applications of phase-transfer catalysis are found in the pharmaceutical, agrochemical, and fine chemicals industries. Additionally, it is now being increasingly used in processes related to the environment, in process modifications for eliminating the use of solvents, and in reactions related to the treatment of poisonous effluents. Not surprisingly, then, there has been a constant stream of publications and patents every year. Phase-transfer catalysis (PTC) is an area that has largely been the province of the preparatory organic chemist (defined broadly to include organometallic and polymer chemists). It is only since the early eighties that the engineering aspects of phase-transfer catalysis are being explored, including such traditional features as mass and heat transfer and reactor design. Our main objective is to present a brief but coherent engineering analysis of PTC, following an introduction to its basic principles. When two reactants are present in two different, immiscible liquid phases (usually one aqueous and the other organic), they can often be brought together by addition of a solvent that is both water-like and organic-like (e.g., ethanol, which derives its hydrophilic nature from its hydroxyl group and its lipophilicity from the ethyl group). However, the rate enhancement tends to be limited due to excessive solvation of the nucleophile.
Electroorganic Synthesis Engineering
Authors: Doraiswamy L. K.
Published: November 2020

Historically, electrochemical processes have been limited to the production of inorganic compounds, and commercial processes based on electroorganic synthesis have found only limited application. It appeared to be an “odious truth” (Fry, 1972) that electrochemical techniques were ignored in organic synthesis. But the past 25 years have witnessed the introduction of a fairly large number of new electroorganic processes with attendant advances in electrochemical process analysis. The most remarkable has been Monsanto’s highly successful electrochemical route for the production of adiponitrile. A particularly notable advance is the electrosynthesis of fine chemicals and natural products. Combinations of electrosynthesis with other strategies of rate or selectivity enhancement such as catalysis by PTC and by enzymes (Chapters 19 and 20) are also adding exciting possibilities to organic synthesis. Simultaneously, fundamental understanding of the principles of organic electrochemistry, electrode kinetics, and transport processes in electrochemical systems has grown rapidly in the last decade. A number of books and reviews have appeared on electroorganic chemistry during this period, for example, Eberson and Schafer (1971), Fry (1972), Beck (1974), Perry and Chilton (1976), Rifi and Covitz (1975, 1980), Weinberg (1974, 1990), Swann and Alkire (1980), Kyriacou (1981), Fletcher (1982), Baizer and Lund (1983), Baizer (1973, 1984), Shono (1984), Fletcher and Walsh (1990), Little and Weinberg (1991), Bowden (1997), Bockris (1998), Hamann (1998). This period also saw the emergence of electrochemical reaction engineering as a distinct discipline of chemical reaction engineering, as evidenced by a number of books and reviews on the subject, for example, Picket (1979), Udupa (1979), Danly (1980, 1984), Alkire and Beck (1981), Weinberg et al. (1982), Alkire and Chin (1983), Fahidy (1985), Mine (1985), Goodridge et al. (1986), Rousar et al. (1986), Heitz and Krysa (1986), Ismail (1989), Scott (1991), Prentice (1991), Goodridge and Scott (1995). Electroorganic synthesis offers opportunities for performing many of the conventional organic reactions at controlled rates and greater product selectivities without the addition of any catalyst. The processes almost always employ milder conditions and are characterized by greatly reduced air and water pollution. Further, there are a number of syntheses that can only be carried out electrochemically, such as the Kolbe synthesis and electrochemical perfluorination.
Multifunctional Reactor Engineering
Authors: Doraiswamy L. K.
Published: November 2020

Looking back to the progress achieved in the areas of conventional separation and reactor design, it seems that major advances have now been made. This has led to increasing research into methods in which reaction and separation are combined in a single unit. The equipment in which this dual function is carried out is sometimes referred to as the combo reactor. Combo reactors can be of two types, separation oriented and reaction oriented. In the first, reaction is used to achieve efficient separation, such as in the separation of p-cresol from its mixture with m-cresol. This method need not necessarily be restricted to separation, for it can also be attractive from the reaction point of view in the following scenario: the required product from a process comes out with a byproduct of low value in a very difficultly separable mixture of the two. If the byproduct can be converted to a useful coproduct in an easily separable mixture with the primary product, then we would have an attractive process. Alternatively, there can be two unimportant by-products in a difficultly separable mixture which can be converted to an easily separable mixture of useful products. An interesting example of this is illustrated later in this chapter. In the second, separation is used to enhance the reaction. An example of this is the intentional biphasing of a homogeneous liquid-phase reaction by addition of a second liquid phase. The second phase can act in several ways to enhance the productivity of the system. Another example is the large-scale version of the chemist’s apparatus in which a reflux column condenser is connected to a batch reactor (usually a round-bottomed flask) as shown in Figure 25.1. The product and the heat of reaction are continuously removed, and the reactant is returned to the reactor. Modeling of type 1 systems leads to equations for the separation factor of a mixture enhanced by reaction. For type 2 systems, on the other hand, equations are obtained for the conversion or yield enhanced by separation. If the component separated is the desired product, it is really immaterial which definition is used.
Pressure-jump Relaxation Kinetics of Unfolding and Refolding Transitions of Staphylococcal Nuclease and Proline Isomerization Mutants
Authors: Vidugiris Gedlminas J. A. , Thomas Raj
Published: November 2020

We present here the first report of the pressure dependence of pressure-jump relaxation kinetics for protein folding transitions. We have studied the relaxation kinetics for the unfolding/refolding of wild-type Staphylococcal nuclease and have found that the relaxation kinetics observed at high pressure are much slower than those observed by pH or denaturant jumps at atmospheric pressure. This indicates that these processes have large, positive values for the activation volumes, most likely stemming from exclusion of solvent from a transition state that is less well packed than the native state. We examined the pressure-jump relaxation kinetics of three single-site mutations in nuclease that lead to alterations in the interactions between the two domains of the protein and changes in the equilibrium constant for isomerization of the lysine-116 to proline 117 peptide bond away from the cis form that predominates in the wild-type enzyme. At comparable pressures, the relaxation times for these mutants were significantly shorter than those observed for the wild type, indicating lower values of the activation volumes. We propose that these mutations cause a decrease in the cooperativity of the unfolding of the two domains, leading to a decrease in the degree of solvent exclusion at the rate-limiting step. The mechanism by which a particular amino acid sequence determines the fold and stability of globular proteins remains one of the most interesting and important unresolved issues in biophysical chemistry. The approaches to increasing our understanding of this phenomenon typically have involved perturbation of the proteins by chemical means or by temperature extremes. The equilibrium or time-dependent responses to these perturbations are then monitored (using a spectroscopic signal, activity, or some other observable) to extract the energetic or kinetic aspects of the unfolding or refolding transitions. Another means of perturbing the system is to modify the protein itself, either chemically or by site-directed mutagenesis, and to assess the effects of modification on the equilibrium or kinetic folding or refolding profiles. This approach has generated a great deal of information about small globular proteins that denature reversibly.