Showing papers in "Annual Review of Physical Chemistry in 1964"

Chemical and Electrochemical Electron-Transfer Theory

Abstract: One of the active areas in reaction kinetics during the post-war years has been that of electron-transfer reactions. These reactions constitute one type of oxidation-reduction process and include both chemical and electrochemical systems. Many rate constants have now been measured (1-8) and they have stimulated a variety of theoretical studies (9-37). The field has been characterized by a strong interplay of theory and experiment, which now includes the testing of theoretically predicted quantitative correlations (34). Because of a certain unique feature of the purely electron-transfer reactions--the absence of bond rupture in the reaction step--these correlations are unusual. They do not have the arbitrary parameters that occur in theoretical studies of most other reactions in chemical kinetics. This review will be limited to purely electron transfer reactions.

Quantum Processes in Photosynthesis

Abstract: The physical-chemical-biological process occurring in land plants and algae where water is oxidized to molecular oxygen and where CO2 is con­ comitantly reduced to carbohydrates in the presence of light is called photo­ synthesis (1). In bacterial photosynthesis, CO2 is reduced but the reductant is some inorganic or organic material other than water, and no oxygen is pro­ duced [see (1) chap. 5]. In the absence of light, the over-all chemical trans­ formation is energetically u p-hill. From the thermochemistry, about 112 kcal per mole of energy must be supplied by the light for the reaction to take place (2). This value represents a minimum energy requirement since there are many known energy-loss mechanisms which do not allow the reaction to be perfectly efficient. Therefore, for energetic reasons, at least three to four quanta of red light represent a \"rock bottom\" minimum requirement for the reduction of each CO2 molecule. Many workers now agree that 8 ± 1 quanta are probably necessary in steady state photosynthesis (3, 4, 5) even though somewhat lower values than this have been reported (6) . One part of the many-faceted puzzle of photosynthesis emerges when it is realized that in direct sunlight each chlorophyll molecule in the plant ab­ sorbs only about 12 quanta per second, and, what is still the more amazing, efficient photosynthesis is known to occur even when each molecule is ab­ sorbing only one quantum every 25-250 seconds (7)! The efficient utilization of energy from eight absorbed quanta at these low absorption levels therefore places a heavy burden on the photosynthetic apparatus with respect to energy funneling, energy transfer, energy storage, and final energy utiliza­ tion mechanisms. One or the other of these aspects of photosynthesis has re­ cently been the subject of excellent review articles (8-12) , symposia (13, 14, 15) , and a collection of papers (16) . The symposium in England (14) , and in particular the one at Warrenton, Virginia ( 15) , the complete proceedings of which became available in January 1964, are of keen interest because of their pertinence to the present state of research in photosynthesis. The line of thought in this review starts from a model for photosynthesis taken over from current ideas. Each part of the model is discussed in the light of available evidence. Much, but not all of this evidence has been col­ lected in the past two or three years, and in this sense, the paper is in accord with the philosophy of this kind of a review. Because of this approach, the

Contractile Processes in Fibrous Macromolecules

Abstract: Macromolecules possess as a characteristic property the ability to alter their dimensions when subjected to changes in their chemical or thermal environment. These molecular dimensional changes can, under suitable cir­ cumstances, be manifested in macroscopic changes. This phenomenon is observed in all synthetic fibers and i n many natural processes such as muscu­ lar contraction , cell motility and cell mitosis to cite but a few examples where macromolecules are obviously i nvolved. Though there are many i nstances where the dimensional changes incurred are not recoverable, reversible dimensional changes are also observed which, in principle and in practice, lead to processes wherein thermal or chemical energy is converted into mechanical work. An u nderstanding of the molecular basis of contrac­ tion is of obvious importance to many biological processes. However, it is not the intent of the present review to consider contractile processes in actually functioning physiological systems. Rather, we shall be concerned with the problem from the point of view of polymer principles. Discussion will therefore be limited to pertinent general principles common to all long­ chain molecules and to laboratory type experiments involving contractility. It is the hope that a satisfactory non-restrictive body of information will evolve which could then be applied to specific functioning systems. Long-chain molecules possess the unique property of being able to sustain large deformations without rupture and to return to the initial state when the applied stress is removed. The development of the thermodynamic laws of thermoelasticity by Lord Kelvin (1) and the experiments of Joule (2) and Gough (3) on natural rubber gave incontrovertible evidence that the deformation process was accompanied by a significant decrease in entropy. However, almost a century elapsed before the significance of these observations could be connected with the molecular nature of polymeric substances (4). It is now well recognized, however, that polymer molecules consist of hundreds to hundreds of thousands of atoms covalently linked together. Rotations about the individual chain bonds (though they may be hindered) permit a polymer molecule to assume an extraordinarily large

The Solid State: Intermetallic Compounds

Abstract: a substitutional solid solution, and, more particularly, the primary or terminal solid solution based on A or B. In addition.to the two terminal phases there may be one or more inter� mediate phases in the A-'-B system. These are phases which include the com­ position AnBm, with nand m usually small integers, and whose structures differ from those of the elemental components. A particular intermediate phase may exist over a considerable range of composition [e.g., the (3 phase (1) in the copper-zinc system] or it may be narrowly confined in composition [e.g., the phase KNa2 (2) in the sodium-potassium ]. On the basis of conven­ tional chemical'experie nce some writers reserve the term "intermetallic com­ pound" to those intermediate phases whose compositions are narrowly res tric ted, i.e. varying by not more than one atomic percent. However, there is no fundamental difference between this kind of intermediate phase and one .

Electron spin resonance

Abstract: We attempt to observe the energy level structure due to spin in several substances. We place the substance in a magnetic field to align the magnetic moments, then we expose it to an electromagnetic wave. At the right frequency, the oscillating magnetic field present in this wave will cause some of the electrons to switch to the other energy level. Initially, there are more electrons in the lower energy state than the upper, so the switch will result in a net absorption of energy. We can use the frequency and magnetic field strength that cause this absorption to figure out the Lande factor of the substance.