Geminal interaction

In the previous sections, we focused our attention upon nonbonded interactions and their effect upon the relative stability of conformational and geometric isomers. We shall now use OEMO theory in order to examine how geminal lone pair-lone pair, bond pair-lone pair and bond pair-bond pair interactions can affect the shape of a molecule  

---

Keywords tr-Relaxation o-Relaxation Geminal interaction Inverted bond Lone pair effect Orbital phase continuity Ring strain... 

The geminal interactions support the relative strain energies. The geminal... 

It is inherently surprising that geminal interactions are typically weaker than vicinal interactions, because the former involve orbitals that are in closer spatial proximity. The reasons for this counterintuitive distance dependence can be seen by decomposing the geminal Fock-matrix element into individual atomic hybrid contributions. 

Figure 3.79 Geminal interactions in CH4 (a) and SiH4 (b), showing outline contours of three (hA, he, and hn ) of the four participating hybrids. Note the lesser nonbonded interaction in SiH4 (Fhh = —0.0105) compared with CH4 (Fhh = -0.0410), despite the greater hydridic character in the silane case.

Figure 3.81 Geminal interactions in cyclopropane and propane, showing bent cyclopropane (a) bond

These sense in which terms like conjugative interactions, nonbonded interactions, etc., are meant will become clear when we discuss each individual type of interaction or effect. Suffice to say that, in many instances, conjugative interactions as well as geminal interactions or bond ionicity effects contain implicitly the idea of nonbonded interactions. Thus, it should be emphasized that the labels of the basic types of interactions proposed here reflect the way in which the problem is formulated rather than different electronic principles. 

Theory of Lone Pair-Sigma Bond Geminal Interactions... 

We shall first illustrate how the OEMO approach can be used to elucidate the effect of geminal interactions on the stability and shape of molecules. We shall utilize the molecule H20 as the model system and compare the relative stabilization of the linear and bent geometries. The analysis involves the construction of the sigma MO s of H20 from the AO s of O and the group orbitals spanning the two hydrogens. 

Certain features of geminal interactions in AX3 molecules need emphasis ... 

The ideas discussed above have very general applicability. For example, they can be utilized in order to predict the shape of AYX3 molecules as a function of A, Y and X, the shape of planar AXYZ systems as a function of X,Y,Z, etc. In the space below we restrict our attention to only one further example of the applicability of the concept of geminal interactions to problems of molecular structure. 

It should be noted that the OCX angle variation seems to be accountable in terms of geminal interactions for X varying along a row of the Periodic Table. 

Table 47 shows typical results which illustrate that contrary to expectations based on consideration of geminal interactions alone, the XCX angle is smaller in CX2=0 than in CX2=CH2. 

---