## Abstract

The binding energy curves of SiH_{3}-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 (SiH_{3}:F) and ionic (SiH_{3}^{+}F^{-},SiH_{3}^{-}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 SiH_{3}F. The SiH_{3}^{+}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 SiH_{3}-F is, therefore, determined by resonance coupling between the covalent and ionic structures (H_{12}), where the dominant VB structure at R_{e} is SiH_{3}^{+}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 CH_{3}:F at its R_{e} (1.38 Å). The proportionality constant K in the equation H_{12} = KS_{12}[H_{11} + H_{22}]/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.

Original language | English |
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Pages (from-to) | 4996-5003 |

Number of pages | 8 |

Journal | Journal of Physical Chemistry A |

Volume | 101 |

Issue number | 27 |

DOIs | |

State | Published - 3 Jul 1997 |

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