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Contents Empirical Force Field Calculations. A Tool in Structural Organic Chemistry C. Altona and D. H. Faber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Removal of Orbital Symmetry Restrictions to Organic Reactions F. D. Mango . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 The Relationship between Mass Spectrometric, Thermolytic and Photolytic Reactivity R. C. Dougherty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Chair-Chair Interconversion of Six-Membered Rings J. E. Anderson 139 Dynamics of Eight=Membered Rings in the Cyclooctane Class F. A. L. Anet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Errata T. Nakajima . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 Empirical Force Field Calculations A T o o l in Structural Organic Chemistry Dr. Cornelis Altona and Dr. Dirk H. Faber Gorlaeus Laboratory, The University, Leiden, The BYetherlands Contents I. Introduction II. General Considerations III. Functions ...................................................... ............................................. and Farametrization IV. Minimization Methods V. Comparison of Force Fields VI. References ...................................... .............................................. ......................................... ........................................................ 2 3 13 24 29 36 C. Altona and D. H. F a b e r I. I n t r o d u c t i o n Every student and researcher working in organic and structural chemistry nowadays feels free to use mechanical molecular structure models in order to gain insight into the configurational or conformational properties of the molecules under study, to explain the results of physical measurements, to discuss even fine points on chemical reactivity at different sites in a molecule. This practice is currently accepted in the chemical literature, but it should be remembered that "thinking in three dimensions" emerged only recently in chemical history. Mohr's discussion 1) on structural properties of cyclohexanoid systems (including bicycloE3.3.1] nonane and adamantane!), illustrated by ball-and-stick drawings, unfortunately does not seem to have made a favourable impression on his contemporaries. For example, the first three-dimensional representation of the skeleton of norbornane appeared as late as 1935 2), the first distinction between axial and equatorial positions on a cyclohexane ring was made in 1931 3). Seen in historical perspective, important advances in chemical thinking have always been accompanied by a consistent and daring use of certain models as a basis for further experiments and for building up our theoretical insight. For example, Barton a) opened new ways in chemical thinking by showing the existence of meaningful correlations between chemical reactivity on the one hand and the position of the substituent (axial vs equatorial) on the six-membered rings of the steroid skeleton assuming the "all-chair" model. In the decade following Barton's publication numerous papers have appeared in which the course and rates of reactions involving certain fundamental groups in saturated non-aromatic systems were related, albeit often more qualitatively than quantitatively, to structural features at the reaction site. Several specialized textbooks have treated these subjects exhaustively. Two more or less interrelated concepts were (and are) especially useful tools for the organic chemist, one concerning the purely stereochemical requirement