Introduction to Radicals Issue

The earliest applications of quantum chemical calculations were devoted to very small systems, which in large part served as tests of their accuracy and usefulness. Given the computational limitations of the day, the drive toward approximate methods, which could extend the reach of quantum chemistry to larger systems by a number of simplifying assumptions, was inevitable. As an early instance, our understanding of aromatic systems took a leap forward with the development of Hückel molecular orbital theory, the equations of which could be solved on the back of an envelope. As computers developed further, these simplifying approximations could be dispensed with, one by one, as ab‐initio calculations were able to extend their reach into ever‐enlarging systems. The later establishment of density functional theory (DFT) approaches renewed and reinvigorated the drive toward ever larger systems. In a parallel sense, the earliest ab‐initio studies of chemical systems focused primarily on closed‐shell systems, in part because solution of the associated equations was simplified by the spin‐pairing of all electrons. However, the very reactive nature of free radicals imbues them with great importance in chemistry and biology. As quantum chemical methods have been developed which can handle such systems with high accuracy, the topic of radicals and their associated reactions has undergone an accelerated level of study in recent years. The present issue of this journal is devoted to the task of presenting to the reader a sampling of some of the most current work that is taking place, dealing with radicals and their reactions. The first two papers examine some of the different specific methods that can be applied to these sorts of problems, and the level of accuracy that might be expected of each, using particular reactions as a test bed. Both works evaluate the accuracy of certain DFT methods. The succeeding three papers consider a variety of different radicals, generally small ones such as CH3SO. The next three papers are devoted to the very important role that radicals play in atmospheric chemistry, again considering small systems such as HOO. The size of the radicals is enlarged a bit in the next three papers, which focus on the aromatics as a class. And finally, as a number of biological processes are intimately connected with radicals, it is their role in biology that serves as the theme of the last four papers in this issue. These papers cover such diverse matters as enzyme activity and inhibition, nucleic acids, and selenopeptides. In summary, the purpose of this issue is not thorough and comprehensive coverage of the entire range of topics that fall under the rubric of quantum calculations of radicals. Such a goal could not be accomplished in a single issue. The aim is more limited in scope, providing a window into just a small sampling of the topics in radical chemistry that are currently being studied by modern quantum chemical approaches