Molecular orbital theory of the electronic structure of organic compounds. I. Substituent effects and dipole moments.

Abstract

A recent approximate self-consistent molecular orbital theory (complete neglect of differential overlap or CNDO) is used to calculate charge distributions and electronic dipole moments of a series of simple organic molecules. The nuclear coordinates are chosen to correspond to a standard geometrical model. The calculated dipole moments are in reasonable agreement with experimental values in most cases and reproduce many of the observed trends. The associated charge distributions of dipolar molecules show widespread alternation of polarity in both saturated and unsaturated systems. These results suggest that charge alternation may be an intrinsic property of all inductive and mesomeric electronic displacements. ne of the long-term aims of quantum chemistry 0 is to provide a critical quantitative background for simple theories of electron distribution in large molecules. Most theoretical discussions of the role of electronic structure in organic chemistry are at present based either on qualitative arguments (such as the study of resonance structures) with no clear foundation in quantum mechanics, or on postulated relationships between charge distribution and various physical and chemical properties (reactivities, acidities, nmr chemical shifts, etc.), few of which can be subjected to direct test. If quantum mechanical calculations are to lead to independent methods of studying such phenomena, they ought to satisfy the following general conditions. (1) The methods must be simple enough to permit application to moderately large molecules without excessive computational effort. Quite accurate wave functions now exist for many diatomic and small polyatomic molecules, but it is unlikely that comparable functions will be readily available in the near future for the molecules of everyday interest to the organic chemist. To be accessible, a quantum mechanical theory has to be approximate. (2) Even though approximations have to be introduced, these should not be so severe that they eliminate any of the primary physical forces determining structure. For example, the relative stabilities of electrons in different energy levels, the directional character of the bonding capacity of atomic orbitals, and the electrostatic repulsion between electrons are all gross features with major chemical consequences and they should all be retained in a realistic treatment. (3) In order to be useful as an independent study, the approximate wave functions should be formulated in an unbiased manner, so that no preconceived ideas derived from conventional qualitative discussions are built in implicitly. For example, a critical theoretical study of the localization of a two-electron bond orbital ought to be based on a quantum mechanical theory which makes no reference to electron-pair bonds in its basis. Molecular orbital theories satisfy this type of condition insofar as each electron is treated as being free to move anywhere in the molecular framework. (4) The theory should be developed in such a way that the results can be interpreted in detail and used to support or discount qualitative hypotheses. For example, it is useful if the electronic charge distribution calculated from a wave function can be easily and realistically divided into contributions on individual atoms