Application of the Valence Bond Mixing Configuration Diagrams to Hypervalency in Trihalide Anions: A Challenge to the Rundle-Pimentel Model

作者：Braida Benoit; Hiberty Philippe C^*

来源：Journal of Physical Chemistry A, 2008, 112(50): 13045-13052.

DOI：10.1021/jp803808e

The X(3)(-) hypercoordinated anions (H, F, Cl, Br, 1) are studied by means of the breathing-orbital valence bond ab initio method. The valence bond wave functions describe the different X(3)(-) complexes in terms of only six valence bond structures and yield energies relative to the two exit channels, X(2) + X(-) and X(2)(-) + X(center dot), in very good agreement with reference CCSD(T) calculations. Although H(3)(-) is unstable and dissociates to H(2) + H(-), all the trihalogen anions are stable intermediates, Br(3)(-) and I(3)(-) being more stable than F(3)(-) and Cl(3)(-). As a challenge to the traditional Rundle-Pimentel model, the different energies of the hypercoordinated species relative to the normal-valent dissociation products X(2) + X(-) are interpreted in terms of valence bond configuration mixing diagrams and found to correlate with a single parameter of the X(2) molecule, its singlet-triplet energy gap. Examination of the six-structure wave functions show that H(3)(-), Cl(3)(-), Br(3)(-), and I(3)(-) share the same bonding picture and can be mainly described in terms of the interplay of two Lewis structures. On the other hand, F(3)(-) is bonded in a different way and possesses a significant three-electron bonding character that is responsible for the dissociation of this complex to F(2)(-) + F(center dot) instead of the more stable products F(2) + F(-). This counterintuitive preference for the thermodynamically disfavored exit channel is found to be an experimental manifestation of the large charge-shift resonance energy that generally characterizes fluorine-containing bonds.

• 出版日期2008-12-18