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The first part of this chapter presents experimental data on vibrational relaxation times and vibrational populations obtained in shock tubes and shock tunnels. The corresponding methods of measurement such as interferometry, Raman diffusion spectroscopy, infra-red absorption, and emission are described. The TV and VV relaxation times and non-equilibrium vibrational populations deduced from these measurements are then analysed, and measurements of accommodation and exchange coefficients are described. In the second part, values of dissociation rate constants for various species are proposed. Time-resolved spectra of radiating species (CN, C₂) behind strong shock waves are presented; they allow for the analysis of the kinetics of mixtures characteristic of planetary atmospheres. Experimental results of shock stand-off distances over blunt bodies obtained in shock and gun tunnels are compared to computed results. Experimental data on chemical catalycity are discussed. Finally, in the appendices, kinetic models for relaxing and/or reactive mixtures, simulation methods of emission spectra, and precursor radiation measurement in shock tube are discussed.

Many reactions in solution involve acids and bases, and so this chapter examines these important reactions in detail. Topics covered include the ionisation of water, pH, pOH, acids and bases, conjugate acids and conjugate bases, acid and base dissociation constants, the Henderson-Hasselbalch equation, the Henderson-Hasselbalch approximation, buffer solutions and buffer capacity. A unique feature of this chapter is a ‘first principles’ analysis of how a reaction buffered at a particular pH achieves an equilibrium composition different from that of the same reaction taking place in an unbuffered solution. This introduces some concepts which are important in understanding the biochemical standard state, as required for Chapter 23.

Quantitative treatment of acid–base behaviour is presented, including the definition of acids, bases, and dissociation constants, and the important relationships between solution pH, acid strength, acid–base ratios, and species distribution. Molecular structural features of acids, which influence both the acid strengths and their dependence upon solvent, are summarized. They include the bond strength, the ability to stabilize anions and cations by charge dispersion, and the nature of the atom to which the proton is bonded. The acidity of carbon acids, which are widely used in synthetic procedures, is reviewed. Structural rearrangements on ionization of ketones, esters, and nitroalkanes, which allow the negative charge generated on ionization of the C-H bond to reside on oxygen, leads to greatly enhanced acidity. The inductive influence of strongly electron-withdrawing groups ? to the ionizing C-H bond is important for nitriles and sulphones.

Tables of dissociation constants are given for a wide range of substrates in the most extensively studied non-aqueous solvents, including methanol, dimethylsulphoxide, dimethylformamide, acetonitrile, and tetrahydrofuran. Equations representing correlations among the solvents and with water are also included.

Most biological processes occur in an environment that is predominantly water: a typical cell contains 70-85% water and the extracellular space of most tissues is 99%. Even the brain, with its complex arrangement of cells and myelinated processes, is ≈ 80% water. Drug molecules can be introduced into the body in a variety of ways; the effectiveness of drug therapy depends on the rate and extent to which drug molecules can move through tissue structures to reach their site of action. Since water serves as the primary milieu for life processes, it is essential to understand the factors that determine rates of molecular movement in aqueous environments. As we will see, rates of diffusive transport of molecules vary among biological tissues within an organism, even though the bulk composition of the tissues (i.e., their water content) may be similar. The section begins with the random walk, a useful model from statistical physics that provides insight into the kinetics of molecular diffusion. From this starting point, the fundamental relationship between diffusive flux and solute concentration, Fick’s law, is described and used to develop general mass-conservation equations. These conservation equations are essential for analysis of rates of solute transport in tissues. Molecules that are initially localized within an unstirred vessel will spread throughout the vessel, eventually becoming uniformly dispersed. This process, called diffusion, occurs by the random movement of individual molecules; molecular motion is generated by thermal energy.

In multicellular organisms, thin lipid membranes serve as semipermeable barriers between aqueous compartments. The plasma membrane of the cell separates the cytoplasm from the extracellular space; endothelial cell membranes separate the blood within the vascular space from the rest of the tissue. Properties of the lipid membrane are critically important in regulating the movement of molecules between these aqueous spaces. While certain barrier properties of membranes can be attributed to the lipid components, accessory molecules within the cell membrane—particularly transport proteins and ion channels—control the rate of permeation of many solutes. Transport proteins permit the cell to regulate the composition of its intracellular environment in response to extracellular conditions. The relationship between membrane structure, membrane function, and cell physiology is an area of active, ongoing study. Our interest here is practical: what are the basic mechanisms of drug movement through membranes and how can one best predict the rate of permeation of an agent through a membrane barrier? To answer that question, this section presents rates of permeation measured in some common experimental systems and models of membrane permeation that can be used for prediction. The external surface of the plasma membrane carries a carbohydrate-rich coat called the glycocalyx; charged groups in the glycocalyx, which are provided principally by carbohydrates containing sialic acid, cause the surface to be negatively charged. On average, the plasma membrane of human cells contains, by mass, 50% protein, 45% lipid, and 5% carbohydrate. Given the mass ratio of protein to lipid is ~ 1 : 1, and assuming reasonable values for the average molecular weight and cross-sectional area for each type of molecule (50 × Mw,lipid = Mw,protein; Alipid = 50 Å2 and Aprotein = 1,000 Å2), the area fraction of protein on a typical membrane is ~ 33%. The lipid composition varies in membranes from different cells depending on the type of cell and its function. In addition, the outermost monolayer of lipids, called the outer leaflet, has a different lipid composition from the inner leaflet.