Valence bond study of the SiH₃-F bond

The binding energy curves of SiH₃-F have been investigated using ab initia valence bond self-consistent-field (VBSCF) methods. The atomic core electrons are treated both all-electron and by using an effective core potential (ECP) representation; for comparison and testing purposes. The VB wave function is expressed in terms of the covalent (SiH₃:F) and ionic (SiH₃⁺F^-,SiH₃^-F ⁺) configurations, and the nonorthogonal orbitals are expanded in conventional atom-centered Gaussian basis sets. Several theory levels are applied, up to the use of different orbitals for different VB structures and allowing delocalization mixing among the passive SiHs and F fragment orbitals. Replacing the core electrons with an ECP is found to generally have a relatively small effect on the calculated ground state bond dissociation energy (BDE) curve, but a much larger effect on the individual covalent and ionic structure energy curves. Delocalization mixing is found to be important to achieving high accuracy for the equilibrium bond distance (R_e), BDE (D_e), and dipole moment of SiH₃F. The SiH₃⁺F^- ionic structure curve is found to lie below the covalent energy curve from at least R(C-F) = 1.3 Å out to ∼2.5 Å, but is stable relative to the dissociation asymptote by less than half of the ground state D_e. The magnitude of D_e in SiH₃-F is, therefore, determined by resonance coupling between the covalent and ionic structures (H₁₂), where the dominant VB structure at R_e is SiH₃⁺F^-. The SiH3:F covalent curve is found to be nearly as repulsive at its R_e value (1.59 Å) as previously found for CH₃:F at its R_e (1.38 Å). The proportionality constant K in the equation H₁₂ = KS₁₂[H₁₁ + H₂₂]/2, where H_ij and S_ij (i, j = 1, 2) are the Hamiltonian and overlap matrix elements, respectively, between the covalent and ionic configurations, has been evaluated using the results of these calculations. At the localized fragment theory level, K is found to be very close to 1 and remarkably constant over the range of R(Si-F) distances sampled here, independent of core representation and basis set.