Editor's preface This volume is the first since Donald Bethell served as Editor of the series, and he set a very high standard both for the quality of the contributions presented and for the excellence of the editorial work. It is my hope as the new Editor to maintain this high level, and continue to provide the chemical community with authoritative and critical assessments of different aspects of the field of physical organic chemistry. The chapters in the previous volumes provide a lasting record that is widely cited and used, and will continue to serve for decades to come. Because this series has maintained such a high level of quality and utility there is little need for change, and one of the few innovations is the adoption of the numerical system of reference citation now used by almost all chemical journals. The four chapters in this volume are intimately related to the study of carbocations and of free radicals, which are two classes of intermediates that were both recognized as discrete reactive intermediates just at the beginning of the twentieth century. The first chapter, on excess acidities, is a lucid exposition of the current understanding of a field that has been relevant to many of the great triumphs of physical organic chemistry throughout the century. The second chapter, on the behavior of carbocations in solution, demonstrates the exquisite detail with which these processes may now be understood. Two chapters concern electron transfer, and thus involve not only free radicals but charged species as well. I wish to extend my thanks to the authors of the chapters in this volume for the uniformly high quality and timeliness of their contributions. The Advisory Board has been generous with their suggestions, and the success of this series is due in no small part to their efforts. Regretably the Board has suffered the loss of the services of Lennart Eberson, who died in February, 2000, and will be remembered as a distinguished chemist, a longtime contributor to this series, and a valuable member of the Board. The new century is a time of great opportunity for physical organic chemistry, which in recent decades has expanded far beyond its traditional boundaries. This now encompasses fields ranging from the purely theoretical to the largely applied, and includes chemistry in the gas, liquid, and solid phases, and many aspects of biological, medicinal, and environmental chemistry. It is our intention to cover as many of these areas as possible. It is also a time for reflection, for as I have discussed elsewhere (Pure and Applied Chemistry (1997), 69: 211-213), the history of the field of physical organic chemistry belongs almost completely in the twentieth century. Thus the seminal recognition of reactive intermediates including carbocations, free vii viii EDITOR'S PREFACE radicals, and carbenes came very early in the century, along with the mechanistic and theoretical tools needed for understandi