Trigonal geometry

The central four-membered ring of cation 3 is folded and the dihedral angle between the Sil-Si2-Si3 and Sil-Si4-Si3 planes is 46.6°. The bond angles at the silicon atoms in the four-membered ring suggest that the silicon atoms Sil, Si2 and Si3 (mean E°Si = 359.8, see Table XIII) have a completely planar trigonal geometry and Si4 has a distorted tetrahedral environment. The Si Si bonds of the cationic... 

The stereoselectivity of any particular reaction depends on the details of the structure of the transition state. The structures of several enone-Lewis acid complexes have been determined by X-ray crystallography.11 The site of complexation is the carbonyl oxygen, which maintains a trigonal geometry, but with somewhat expanded angles (130-140°). The Lewis acid is normally anti to the larger carbonyl substituent. Boron trifluoride... 

In principle, geometry optimization carried out in the absence of symmetry, i.e., in Ci symmetry, must result in a local minimum. On the other hand, imposition of symmetry may result in a geometry which is not a local minimum. For example, optimization of ammonia constrained to a planar trigonal geometry (Dbi, symmetry) will result in a geometry which is an energy maximum in one dimension. Indeed,... 

The particular values and power dependence for the d-s mixing term are also not too critical although a certain threshold must be achieved. Tetragonally elongated Jahn-Teller distortions of d9 CuNe species (36) and the trigonal geometry of the oxidized copper center in Type 1 metalloproteins (37) can be achieved with an inverse sixth order power dependence and an associated a6 parameter of at least 300,000 kcal mol-1 A6. However, since eds also depends on symmetry—e.g., it makes no contribution for octahedral complexes—there are many systems where d-s mixing has a minimal effect. 

Trigonal geometry with sp2 hybrid orbitals, ffybridization of an s orbital with two p orbitals gives a set of three sp2 hybrid orbitals. This trigonal structure has bond angles of about 120°. The remaining p orbital is perpendicular to the plane of the three hybrid orbitals. 

The electronic structure of dibromocarbene is shown next. The carbon atom has only six electrons in its valence shell. It is sp2 hybridized, with trigonal geometry. An unshared pair of electrons occupies one of the sp1 hybrid orbitals, and there is an empty p orbital extending above and below the plane of the atoms. A carbene has both a lone pair of electrons and an empty p orbital, so it can react as a nucleophile or as an electrophile. 

For the 2,2 -biallylene intermediate, with C(2)C(5) = 1.55 A, other CC bonds 1.45 A, and idealized trigonal geometry, we find the planar Dzn form 13.8 kcal/mole higher in energy than the perpendicular Dza intermediate. Hoffmann has calculated an energy difference of 4 kcal/mole 53> (note 7), in favor of the perpendicular form, and indicated that SCF n-electron calculations give nearly identical energies for the two species. 

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