Organic reactivity. New light on old concepts

For over 50 years the theoretical concept of the transition state has been an invaluable aid for rationalising empirical data concerned with the rates and mechanisms of organic reactions. Modern quantum chemical studies of transition state structures and properties are now providing new insights into the nature of organic reactivity. The primary role of theoretical organic chemistry is to establish and refine concepts and models for interpreting and predicting organic molecular behaviour. There are two facets to the theoretical investigation of organic reactivity. The use of computational quantum chemistry as an accurate 'numerical spectrometer' for the determination of transition state structures. The development of reliable models, firmly based upon theory, for the origin of chemical reaction barriers and the nature of reactivity itself. Molecular orbital (MO) theory has provided a practical and effective tool for the former task, while for the latter purpose modern valence bond (VB) theory is emerging as a useful source of chemical insight. Theoretical modelling provides a valuable tool for the investigation of organic reactivity to complement experimental methods. Modern theory is shedding new light on old concepts concerning transition state structure and the origin of energy barriers to chemical reactions.