Molecular-orbital calculations carbanions

Many such examples are known. In most cases where the stereochemistry has been investigated, retention of configuration is observed,225 but stereoconvergence (the same product mixture from an E or Z substrate) has also been observed,226 especially where the carbanionic carbon bears two electron-withdrawing groups. It is not immediately apparent why the tetrahedral mechanism should lead to retention, but this behavior has been ascribed, on the basis of molecular orbital calculations, to hyperconjugation involving the carbanionic electron pair and the substituents on the adjacent carbon.227... 

Ab initio and semiempirical molecular orbital calculations have been used, together with charge-transfer theories, to investigate the structures of organodioxide anions and related charge-transfer complexes between carbanions and molecular oxygen.219... 

On the other hand, there is the well-known fact that sulfur is an excellent carbanion stabilizer. But not for those reasons popularly ascribed to dir-pir backbonding of the carbanion lone pair into the vacant 3d orbital of sulfur, as suggested earlier. Recent molecular orbital calculations appear to challenge the validity of this model. These calculations, for example, predict the order of gas phase (no solvent effects to worry about) carbanion stabilization as sulfur > oxygen > carbon whether or not the 3d orbitals of sulfur participate in the calculations. Besides, these theoretical considerations surprisingly show that the C-S bond in the thiomethylene carbanion is longer than in the neutral thio-methane parent. Until recently, the prevailing mechanism of stabilization was centered around the classical concept of polarizability of the electron cloud around the sulfur nucleus. 

In this section we will discuss various reactive intermediates that contain silicon. In particular, the silicon analogues of the classic organic reactive intermediates such as carbenes (silylenes), carbenium (silicenium) ions and carbanions (silyl anions) have attracted the interest of experimentalists and theoreticians. In light of the elusive nature of these species it is not surprising that much of what we know about their properties comes from theory, in particular ab initio molecular orbital calculations. 

Photoelectron studies of (CH3)3P=CH2 have provided data on the energy of the highest occupied molecular orbital and some of the lower-lying states (4-9). The frontier orbital energy of 6.81 eV is very low and seems to illustrate the carbanionic nature of the carbon, where this orbital is largely localized. This is borne out by semiquantitative CNDO/2 calculations on this molecule. The orbital sequence obtained is satisfactory according to ab initio results. 

The INDO molecular orbital coefficients for the HOMO of the benzyl carbanion are given in Table X. As indicated, the symmetry of this orbital is appropriate for overlap between the CL> and C7 pz orbitals and a combination of lithium px and py orbitals. In addition, the contribution of the Ci pz orbital to the HOMO is relatively small. The HOMO of the complex Li(NH3)2C7H7 (Table X) is made up of the HOMO of the carbanion and the Li px and pv orbitals with a substantially smaller proportion of lithium pz. The values of the overlap integrals, Li (px>py)— C(p ) and, for comparison, the value of the C(pz)—C(pz) overlap integral for adjacent carbon atoms in the benzyl carbanion are also given. Therefore, both symmetry and INDO calculations are consistent with a substantial degree of three-center carbanion-metal bonding of the type described. 

Calculate DE py, 9 and for the allyl radical, carbonium ion, and carbanion. Sketch out the molecular orbitals for the allyl system. 

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