Sigma interactions

The appropriate interaction diagrams are similar to that of Fig. 28. Reasoning as before, we conclude that pi nonbonded interactions favor the staggered conformation. Sigma nonbonded interactions, on the other hand, favor the eclipsed conformation. We expect pi nonbonded attractive interactions to dominate sigma interactions and the resulting order of stability is predicted to be > Cee. 

We can extend the above theoretical approach to isoconjugate molecules such as acrolein and glyoxal. Arguing as before, we conclude that the relative order of conformational stability will be gauche > tram > cis if pi nonbonded interactions dictate the preferred conformation, cis > gauche > trans if sigma interactions are dominant, and trans > gauche > cis if steric effects are the most important factor. 

An important point to be stressed before we proceed further with our analysis concerns the formal correspondence, of hybrid orbitals and delocalized group orbitals. For example, consider the model system HN=NH which can exist in a cis or a tram geometry. We can understand whether sigma interactions favor one or the other geometry be means of one of the following two approaches ... 

Ri R2C=NR3 molecules represent another important class of Ri R2 AAR3 systems. In the parent system shown below the strength of the various sigma interactions varies in the order ... 

As in Chapter 5, we will first consider group orbitals on ligands based on Oh symmetry, and then consider how these group orbitals can interact with orbitals of matching symmetry on the central atom, in this case a transition metal. We will consider sigma interactions first. The character table for Oh symmetry is provided in Table 10.4. 

These are octahedral ions with only sigma interactions. The ammonia ligands have no 77 orbitals available for significant bonding with the metal ion. The donor orbital of NH3 is mostly nitrogen orbital in composition, and the other p orbitals are used in bonding to the hydrogens. ... 

TABLE 10.10 Angular Overlap Parameters Sigma Interactions... 

Using the angular overlap model, determine the relative energies of d orbitals in a metal complex of formula ML4 having tetrahedral geometry. Assume that the ligands are capable of sigma interactions only. How does this result for A compare with the value... 

Halide ions donate electron density to a metal via orbitals, a sigma interaction the ions also have p and p orbitals that can interact with metal orbitals and donate additional electron density by pi interactions. We will use [MXs]", where X is a hahde ion or other ligand that is simultaneously a a and a tt donor. 

For Ni(CO>2[H4B2 2P(CH3)3] the CO asymmetric stretching frequency is found at 1909 cm , indicating clearly that a distorted sigma interaction is being observed. 

Edwards S, Li CM, Levy DL, Brown J, Snow PM, Campbell JL (2003)Saccharvmyces cere-visiae DNA polymerase epsilon and polymerase sigma interact physically and functionally, suggesting a role for polymerase epsilon in sister ehromatid cohesion. Mol Cell Biol 23(8) 2733-2748... 

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