XXIIIrd International Congress of Pure and Applied Chemistry, Volume 1 compiles lectures presented in Boston, USA on July 26-30, 1971. This book is organized into three main topics: application of quantum mechanics to organic reaction paths; intramolecular rearrangements, valence isomerization, and cyclo-addition; and photochemistry. This publication specifically discusses the quantitative SCF MO studies of reaction mechanisms, interaction of particular orbitals in chemical reactions, and potential surfaces for the addition reactions of ?-systems. The ring opening reactions of aziridines and oxiranes, mechanism in the system of dimers of butadiene, and thermal cyclisation of unsaturated carbonyl compounds are also elaborated. This text likewise covers the low temperature photochemistry of organic compounds, photochemical modification of biologically significant compounds, and photochemistry of thioketones. This compilation is useful to chemists and specialists working in the field of pure and applied chemistry.