STUDY OF BOND ANGLES AND BOND LENGTHS IN DISILOXANE AND RELATED MOLECULES IN TERMS OF THE TOPOLOGY OF THE ELECTRON-DENSITY AND ITS LAPLACIAN

We have calculated the electron density distributions for the series o f molecules HnXOXHn, X = Li to F and Na to Cl, and some related molecu les. We have analyzed these distributions and their Laplacian to obtai n atomic charges, electron densities at the bond critical point, and t he charge concentrations revealed by the Laplacian. On the basis of th is information and an analysis of the X-O bond lengths and angles, we have examined the factors that determine the lengths of the X-O bonds and the XOX bond angles. The XO bond length reaches a minimum value at boron in period 2 and at silicon in period 3 when the product of the charges on X and O reaches a maximum value, consistent with a predomin ately ionic model for the molecules X = Li, Be, B, Na, Mg, Al, and Si. In the remaining molecules of both series, the XO bonds have an incre asing covalent character. The bond length and the bond angle in disilo xane are consistent with the ionic character of the molecule, and ther e is no evidence for the frequently quoted back-bonding model. In disi loxane and related molecules in which the ligand is considerably less electronegative than oxygen the electrons in the valence shell of oxyg en are not well localized into pairs, so the bond angle is intermediat e between the tetrahedral angle expected when the valence shell electr ons of oxygen are strongly localized into four tetrahedral pairs and t he 180 degrees bond angle expected on the basis of the electrostatic a nd/or steric repulsion between the positively charged X atoms. The eff ects on the bond lengths and angles of substituting oxygen by sulfur a nd hydrogen by fluorine are discussed.