Bond orbitals, interactions

Another example which illustrates beautifully the mixing of a group orbitals to form delocalized molecular orbitals is benzene. First of all the six crcc bond orbitals interact to give six linear combinations which are delocalized over the entire carbon skeleton. The amplitudes of the various bond orbitals in each [Pg.23]

Since the CH bond orbitals are relatively distant, their interaction is very weak and the six crCH semilocalized orbitals are very close in energy. On the other hand the semilocalized orbitals are spread out, with the lower ones falling well below the group of crCH orbitals. 

The T—3 CC bond-orbital interactions between opposite CC bonds in a cyclobutane ring are an interesting exception. In this system there is significant mixing between the acc (and a c) orbitals on opposite bonds (Fig. 29). The two acc molecular orbitals are both occupied. The reader will recognize these orbitals, and the... 

Butadiene has two n bonds. The interaction between the two n bonds is one of the simplest models to derive molecular orbitals from bond orbitals. A n bond in butadiene is similar to that in ethylene. The n bond is represented by the bonding and antibonding orbitals. The interactions occur between the n bonds in butadiene. The bond interactions are represented by the bond orbital interactions. 

Fig. 12. Two through bond orbital interaction patterns. In (a) a dominant ng—ag interaction leads to an A, S order. In (b) a dominant ng— jru interaction leads to the opposite order...

CN dimers (from -140.5 kcal/mol to -71.4 kcal/mol) and in the linear CP dimers (from -154.7 kcal/mol down to only -5.8 kcal/mol) as we go from C—C to C—X to X—X coupling. In the CN dimers, the reduction in bond strength is caused by an enormous increase in AE° repulsion (from 144.3 kcal/mol to 371.3 kcal/mol) which is counteracted but not compensated for by a sizable increase in AEoi (from -288.2 kcal/mol to -446.5 kcal/mol). In contrast, in the CP dimers the calculated bond strength decreases because of a weakening of the bonding orbital interactions AEoi (from -320.9 kcal/mol to -104.1 kcal/mol), especially those associated with the a bond (A CT) in spite of an opposite trend of the repulsion AE°, which actually decreases (from 161.5 kcal/mol to 91.0 kcal/ mol). 

The sulfur-sulfur bond in H2S.. SH2 is mainly provided by the three-electron orbital interactions between the lb orbitals of the two fragments, that is, the 3px lone pair of H2S and the 3px SOMO of H2S+ (see Figure 14). Combining the repulsive and bonding orbital interactions together into one term, we obtain... 

This qualitative model, based on semiempirical MO theory, focuses entirely on the so-called electronic effects, as the A—H bonding orbital interactions are often called. However, steric repulsion (i.e., the destabilizing orbital interactions) between the hydrogen substituents in AH3 is just as important in the interplay of mechanisms that determine whether the molecule adopts a planar or a pyramidal shape. In fact, as will become clear from the following discussion, which is based on a Kohn-Sham DFT study at the BP86/TZ2P level,107 108 steric repulsion turns out to be the decisive factor in determining the pucker of our example.133... 

An analysis of the mechanism governing the couphng between the vicinal protons in ethane and fluoroethane has been studied by Contreras and co-workers by the developed by them method of natural bond orbital interactions between bonds and antibonds, cTin->a n. The authors have concluded that the main contribution to Jhh comes from the through space term while the introduction of a fluorine changes this term and yields a direct contribution to the coupling. 

Corrosion inhibition by it-bond orbital interaction is exhibited by compounds such as n-propanol, allyl alcohol, and 2-propyne-l-ol. As the structure goes from the single bond in -propanol to the double bond to the triple bond, the rt-bond interaction with the metal increases, decreasing the corrosion rate of carbon steel in 2.8N HCl from several thousand to three millimeters per year. However, steric interferences may decrease the inhibitor efficiencies. 

On the other hand, the rapid and extensive association facilitated by protonation of kraft lignin phenoxide moieties as the pH of the aqueous solution is lowered, or when they are present in non-aqueous solvents, is presumably mediated by hydrogen bonding and/or dipolar interactions, with non-bonded orbital interactions remaining the primary intermolecular forces. It has been shown that the absolute molecular weights of associated kraft lignin complexes encountered in /V,/V-dimethylformamide (DMF) could exceed values 1000 times larger than those observed in aqueous alkaline solutions. ... 

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