ABINITIO SCF-MO AND NATURAL BOND ORBITAL STUDIES OF 7-SILANORBORNADIENE AND 7,7-DIMETHYL-7-SILANORBORNADIENE - 2 MOLECULES POSSESSING AN INVERTED SEQUENCE OF PI-ORBITALS

The geometries of norbornadiene, 1, 7-silanorbornadiene, 2, and 7,7-dimethyl-7-silanorbornadiene, 3, have been optimized at the HF/STO-3G and HF/3-21G levels (within C2v symmetry constraint). In addition, 2 has also been optimized by using the 3-21G(⋆) basis set which contains a set of polarization functions on Si. It was found that, although the π canonical MOs (CMOs) of 1 followed the natural sequence, i.e., π+ (a1) below π_ (b1), the inverted sequence is observed (i.e., π+ above π_) for both 2 and 3. Through-bond (TB) orbital interactions are therefore more important than through-space (TS) orbital interactions in 2 and 3. The π⋆ CMOs, on the other hand, followed the natural sequence in all three molecules. Orbital interactions in 1 and 2 were dissected into TS and TB components by using the Weinhold natural bond orbital (NBO) localization procedure. It is found that TS interactions are weaker in 2 than in 1, owing to the former molecule having a larger dihedral angle (120°) for the six-membered ring compared to 1 (114°). In addition, TB coupling to the π+ orbital (which raises that level) is substantially larger in 2 than in 1, an effect that is attributed to the more electropositive nature of silicon compared to carbon. The combination of weakened TS interactions and strengthened TB interactions is sufficient to cause the inverted sequence of π levels in 2 (and in 3). TB and TS effects are found to reinforce each other in the π⋆ manifolds of both 1 and 2; consequently, an inverted sequence of ⋆ levels is unlikely to be found in any type of simple norbornadiene molecule. The NBO analysis of interactions in the π manifold showed that σ/π interactions dominate over σ⋆/π interactions. However, both σ⋆/π⋆ and σ/π⋆ interactions should be considered in the analysis of TB coupling in the π⋆ manifold, with the former being slightly larger than the latter. © 1990, American Chemical Society. All rights reserved.