Adiabatic Exchange Research Articles

PMR studies on the structures of water-ethyl alcohol mixtures

The PMR signals of each individual proton species present in water-ethanol mixtures were analyzed throughout the entire range of composition. Chemical shifts and line shapes were studied as a function of ethanol mole fraction ( f), temperature, and working frequency, from −50 to +80°C, at 60, 100 and 220 MHz. The experimental results are discussed in terms of possible mechanisms of molecular aggregation in the various regions of ethanol concentration. There is evidence that addition of small quantities of ethanol to water promotes H-bonding association among water molecules (at t = 20°C, f ≲ 0.08). At intermediate ethanol concentrations (at t = 20°C, 0.25 ≲ f ≲ 0.75), the linear dependence of f exhibited the chemical shift of the water signal evidences that the water structures are progressively disrupted by increasing alcohol concentration. At the higher ethanol concentration (at 20°C, f ≳ 0.8) the substantial independence of both ethanol and water O H chemical shifts from the concentration suggests that water molecules either coordinate or/and are incorporated into the linear aggregates peculiar to pure ethanol. The width of the f range in which the hydroxyl signals coalesce is progressively reduced with decreasing temperature. We qualitatively tested the applicability of an adiabatic two-site exchange model to the hydroxyl protons of these mixtures. This model is correctly applicable to solutions at both extremes of the concentration range, but it cannot describe the linewidth behaviour of the mixtures at intermediate concentrations.

Differential cross sections for chemically reactive systems

A quantum mechanical theory is developed for the differential and total elastic and reactive scattering cross sections for an electronically adiabatic bimolecular exchange reaction, with the restriction that the three atoms are constrained to move on a straight line, but with the whole system free to rotate in three dimensions. The introduction of a set of natural collision coordinates, together with a vibrationally adiabatic approximation, is used to reduce the scattering problem to the solution of one-dimensional Schrodinger equations. Semi-classical expressions for the elastic and reactive phase shifts are derived. The partial wave summations that occur in the theory are evaluated by semi-classical techniques and elastic and reactive differential cross sections are calculated for three different kinds of potential surface. A feature of the calculations is the appearance of a new kind of rainbow, which is named a ‘cubic’ rainbow since it arises when the deflection function varies cubically with impact p...

An analysis of an adiabatic adsorption column: Part 1. Theoretical development

A theoretical analysis of multicomponent adsorption in adiabatic exchange columns is presented. The system considered here is one-dimensional with negligible effects of diffusion and in which local equilibrium (of a nonlinear nature) obtains between the phases in contact. The theory of simple waves and that of discontinuities play key roles in the analysis. Countercurrent operations give rise to a free boundary analogue and introduce the concept of boundary discontinuities. Dependence of the system behavior upon the flow rate ratio is discussed. A finite column operation usually reaches a steady state and it is shown that the number of steady states attainable in the contacting region by varying the flow rate ratio is equal to the number of solute species present plus two. Formulations and discussions are carried out for countercurrent contact but these are equally well applicable to fixed bed operations by letting the flow rate ratio vanish. Applications will be illustrated in the following parts.

Homogeneous and heterogeneous optical and thermal electron transfer

A unified theory of optical and thermal outer-sphere electron-transfer processes is outlined, in which the equations are obtained as special cases of general expressions for radiative and radiationless transition probabilities. For a particular donor—acceptor ion system, this leads to correlations between the rate of homogeneous thermal electron exchange and the frequency and bandwidth of the corresponding optical intervalence transfer absorption. These in turn can be correlated with thermal and optical transfer at a metal/solution interface. The relationship between the transmission coefficient in adiabatic thermal exchange and the transition probability in the corresponding optical transfer is discussed, and a number of predictions are made for the optical transfer of electrons between a molecule or ion and an electrode, which has so far not been observed.

Quantum-Mechanical Treatment of Inelastic Collisions. II. Exchange Reactions

The theory for the quantum-mechanical treatment of inelastic collisions developed in the first paper of this series is extended to treat collinear, electronically adiabatic exchange reactions. The formalism is applied to three model potential energy surfaces for the exchange of identical particles. The calculated reaction probabilities are reasonable and two independent checks indicate that they are reliable.

Chemical and genetic relationships among meteorites

Optical and X-ray studies have been made on the olivines, pyroxenes and metal phases from thirty-four chondrites for which chemical analyses exist. These data enable a selection of reliable analyses to be made, and confirm a conclusion first expressed by Prior—namely, that chondritos are samples of a homogeneous parental material which varied widely in its state of oxidation. There is an almost continuous range in oxidation states from carbonaceous chondrites, in which all the iron and nickel is oxidized, through to enstatite chondrites, in which all the iron is reduced, and which contain, in addition, reduced Si, Cr, Ca and P. The mineralogy and chemistry of silicate and metal phases in chondrites are dominated by this oxidation-reduction equilibrium. The original state of the parental chondritic material probably resembled that displayed by carbonaceous chondrites. This material was cold and oxidized, containing up to 5 per cent carbon, and 20 per cent water, together with obscure compounds of carbon, hydrogen, nitrogen, sulphur and other volatiles. Subsequently it was subjected to higher temperatures under such conditions that the carbon (and hydrogen) reacted with oxidized iron and nickel to form a metal phase in situ, whilst volatiles (principally water and CO 2) were lost. Mineralogical and textural evidence shows that the silicates in most chondrites crystallized from the liquid state at a temperature below 1000°C. A high pressure of volatiles (H 2O and CO 2) must have been present to produce the observed melting point depression of the silicates. Primary chondritic textures are tuffaceous in nature and therefore of volcanic origin. They have formed by the rapid liberation of H 2O and CO 2 from the chondritic magma. Sudden loss of volatiles has caused rapid crystallization and chondrule formation. The principal difference between formation of chondrites and terrestrial tuffs is due to the relatively low viscosity of chondritic magma, which has facilitated chondrule formation. Chondrites have subsequently been exposed to varying degrees of metamorphism, which has caused compaction and recrystallization. The range of pressures indicated is such that at least one of the parent bodies of chrondrites was of lunar size. The genesis of other groups of meteorites can be understood in terms of melting and differentiation of a small amount of parental chondritic material. Several lines of evidence indicate that the irons have crystallized under high pressures,—probably exceeding 30,000 atm. It therefore seems probable that melting and differentiation occurred near the centre of a parent meteoritic planet, and that the metal partially segregated to form a small core. Melting and differentiation of the parent meteoritic planet (or planets) occurred about 4.5 × 10 9 years ago. Subsequent cooling below 450°C occurred within approximately 10 8 years. Cooling of the small molten core occurred by adiabatic heat exchange with the outer chondritic mantle, which had been rapidly cooled to about 300°C, during the endothermic volcanic phase of evolution. The meteoritic planet(s) broke up less than 10 9 years after melting and differentiation, and the fragments have since been colliding and becoming further reduced in size, thus forming the asteroids. Some suggestions regarding the cause of the initial break-up are hazarded.

Nuclear Photoprocesses at High Energy

In view of recent interest in the influence of pion production on the high energy photonucleon cross sections, an attempt is made to give a systematic discussion of the background effects, i.e., those high energy effects which are not directly concerned with either the production of pions or relativistic corrections to the nucleon motion. An appropriate definition of each high energy (irreducible) multipole moment is given. It is shown that the Siegert theorem does not apply, so even the electric dipole transition may be affected by (adiabatic) exchange currents. These and other high energy effects are found to contribute about 5 percent to the electric dipole photodisintegration cross section of the deuteron. Larger corrections are anticipated for heavier nuclei. It is shown that the corrections are calculated most readily by using the usual form of the multipole moment operators, rather than the formally correct irreducible operators.