W0041

Modeling the Geometry of AX₃E and AX₂E₂ Molecules Through Non-Bonded Repulsions. Ronald F. See, Thomas A. Baker and Pamela J. Kahler, Dept. of Chemistry, Indiana Univ. of Pennsylvania, Indiana, PA 15705.

The empirically-observed structures of the AX₃E and AX₂E₂ molecules are successfully explained using an adaptation of the Ligand Close-Packing (LCP) theory of Bartell and Gillespie. The bond angles in these molecules vary spectacularly, ranging from 91º to 180º. This variation has been explained using many models (directed valence, VSEPR, perturbation theory, second-order Jahn-Teller effect, and more), but never satisfactorily or without exceptions. The LCP model suggests that molecular geometries are primarily a result of repulsive interactions between the terminal atoms or groups. In the case of the AX₃E and AX₂E₂ molecules, there are two sets of repulsive interactions: a) the repulsion between two terminal X atoms or groups; b) the repulsion between an X atom (or group) and the stereochemical nonbonding electrons (lone pairs) on the central atom A. We have quantified these repulsions using a simple, linear distance-interaction relationship for both sets of forces. For molecules with A = N, P, As, O, S or Se and X = H, F, Cl, Br, I, CH₃, CF₃, SiH₃ or t-butyl, a single linear regression describes all computational and empirically-observed structures to a high degree (R² = 0.99) of accuracy. The empirical data includes structures from X-ray diffraction, gas electron diffraction and microwave spectroscopy. This strong correlation, with no significant exceptions, is convincing evidence that the LCP model, quantified through a distance-interaction relationship, correctly models the physical forces that primarily determine molecular geometry. Potential applications to molecular mechanics calculations will also be discussed.