Valence bond theory molecular rotation

Exercises 1. First,valence-bond theory clearly distinguishes between sigma and pi bonds. In valence-bond theory, it is clear that a sigma bond must be stronger than a pi bond, for the orbitals overlap more effectively in a sigma bond (end-to-end) than they do in a pi bond (side-to-side). Second, molecular geometries are more directly obtained in valence-bond theory than in Lewis theory. Third, Lewis theory does not explain hindered rotation about double bonds. 3a. Lewis theory does not describe the shape of the water molecule. 

The molecule sulfur hexafluoride (SFA has recently challenged both molecular spectroscopy with its unexpected rotational spectra 29) and electronic structure theories with novel correlation effects (30,31,5). The electronic structure must explain the molecule s high stability, octahedral symmetry, and, most importantly, provide a simple picture of the bonding. At first glance, the traditional chemical models do not appear to be appropriate because sulfur seemingly forms six bonds to fluorines, yet the sulfur s2pA valence configuration allows for at most two covalent bonds. 

EHT (extended Hiickel theory) was developed by Wolfsberg and Helmholz (1952) and used widely by Hoffmann (1963) [13] to provide qualitative insights into chemical bonding, particularly for inorganic compounds. In EHT, all valence orbitals (both rr and o) are included in the molecular orbitals it is not restricted to the 7T system. This method, however, still gives poor prediction of molecular properties such as dipole moments and rotational barriers. 

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