Notes on Molecular Orbital Calculations

Abstract

From the Preface: For practicing organic chemists the simple, linear-combination-of-atomic-orbitals (LCAO), molecular-orbital method permits useful calculations of semi-empirical electronic energies of unsaturated molecules with no more than high school algebra. Anyone who can find the roots of x^4 - 5x^2 + 4x = 0 graphically, analytically, or by successive substitutions can obtain the energy levels and calculate the [Greek pi]-electron energy of bicyclo[1.1.0]butadiene. [figure of bicyclobutadiene] If in addition he can solve x^4 - 4x^2 = 0, then he can compare bicyclobutadiene with cyclobutadiene and predict what changes the 1, 3 bond would make in the a-electron energies. With no more advanced mathematics, one can compute the bond orders, charge distributions, and reactivity parameters for both free-radical and polar processes. The results may be crude, but they are often highly suggestive; there is no excuse for a modern organic chemist not to be able to use the LCAO method. The notes that make up this book have been used for many years at the California Institute of Technology to introduce seniors and graduate students to the elements of the simple LCAO method. A fairly large number of exercises are interspersed in the text to illustrate important points. It is recommended that these be solved as encountered. Some of the problems are hoped to be suggestive of possible research problems in the field. These Notes are not intended as a complete course of study and should be supplemented by the reference works listed in the Bibliography. No attempt has been made to survey the recent literature. The purpose has been to provide a practical introduction. As a result no appropriate acknowledgement to either the priority of ideas or to their development has been given. This set of notes would never have been written without the generous contributions of Professor W. G. McMillan and Dr. V. Schomaker to the author's education in the subject matter. Camera copy was prepared by Mrs. Allene Luke with the aid of Miss Joy Matsumoto. JOHN D. ROBERTS