How does the geometry of the tetrahedral XF4−nClnε (X = Al, Si, P) species depend on composition, or abinitiosicertumnescio

Abstract

The molecular parameters of the isoelectronic tetrahedral species NaF43−, MgF42−, AlF4−nCln−, SiF4−nCln PF4−nCln+, SF42+, and ClF43+ have been investigated by abinitio calculations at the 6-31G* level. The bond lengths and the critical radii of X in XF4ε are found to decrease monotonically with the formal oxidation state m of X up to and including X = S but show an upturn at X = Cl. The relative position of the critical point in the X—F bond, rc(X)/(X—F), remains relatively constant for m = 1–3; for m = 4–6 the critical point moves gradually closer to X, but for m = 7 it shifts appreciably away from X. The increase in the electron density at the bond critical point of XF4ε (and probably also of XCl4ε) is represented reasonably closely by a power function amb. The effect of progressive replacement of F in XF4ε (X = Al, Si, P) by Cl on the bond lengths and angles depends on the sign of ε. For ε = −1, X—F and X—Cl decrease and FXF increases; for ε = 0 and 1, X—F and X—Cl increase and FXF decreases. These changes increase in magnitude, in their respective directions, with |ε|. Relationships between the bond lengths and the bond angles are described. The excess total electronic energies [Formula: see text] are negative for all three X; they are largest (and approximately symmetric with respect to n) for Al, smaller (and asymmetric) for P, and very small for Si. The amount of charge transferred from F or Cl to X varies in a complicated manner with m and n, and the rates of transfer of charge with «in an XF4−nClnε molecule are not the same for F and Cl. More than one unit of charge is transferred from a halogen atom to X in ClF43+ and PCl4+.