Free Energy Surfaces and Transition State TheoryMany major chemistry textbooks contain diagrams exhibiting a function referred to vaguely as the "free energy" plotted against a quantity referred to by a phrase such as the "reaction co-ordinate." The diagrams usually have the general form indicated in Figure 1. The "free energy" curve goes through a maximum and this feature is interpreted as representing a "harrier" which the system must cross when reactants are converted to products.There are numerous variations-The term "free energy," (G), may sometimes be replaced by "free energy change," (AG); in some, rather rare, cases an attempt at qualifying the expression further to "standard free energy," (G°), or "standard free energy change," AG°is made and in some cases the same type of figure appears with the abscissa labelled "extent of reaction."As we shall see, the correct definition of such a free energy curve, or surface, requires highly sophisticated concepts (1-3) and involves the finest points of the definition of a transition state1 (4). It is the purpose of the present letter to try to establish that unless such diagrams are very precisely labelled and explained they are seriously misleading and often incorporate a major error of principle. Discussion of the points at issue can be found in some textbooks but rarely in those at undergraduate level (1,5,6). Extent of Reaction and Reaction Co-Ordinate The confusion probably begins with a tendency to regard the terms "extent of reaction" and "reaction co-ordinate" as interchangeable. A sharp distinction must be made between the two concepts involved in the definition of these terms. The usage in the present note is as follows.
Zvetkow twist viscosity measurements are reported for a series of cyanobiphenyl liquid crystals and their cutectic mixtures in the nematic phase. The values obtained exhibit a n odd-even variation. If the data are plotted against a reduced density, they divide into two groups, one corresponding to the odd and the other to the even series. This is attributed to effects of the configuration of the alkyl chain on the molecular packing density in the nematic phase. Using the Maier-Saupe theory, interaction energies are derived.
An expression for the intensity of light scattered by an isolated linear polymer chain with excluded volume has been evaluated for conditions corresponding to those met in experiment. The expression proves to be an excellent approximation to light scattering results for a very high molecular weight polystyrene fraction, thus providing an explanation for what appeared to be an anomalously large polydispersity when the data were treated according to the usual random flight model. Coil sizes were calculated and compared with the values obtained from the data by other methods.
The molecular weight of silk fibroin in several different solvent systems has been measured by light‐scattering techniques. The values obtained lie between 250,000 and 450,000, with a tendency to lie fairly close to 300,000. The difference between this value of the molecular weight and the values obtained by other workers is discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.