Stabilizing interaction between the

The stability order of alkenes is due to a combination of two factors. One is a stabilizing interaction between the C=C tr bond and adjacent C-H a bonds on substituents. In valence-bond language, the interaction is called hyperconjugation. In a molecular orbital description, there is a bonding MO that extends over the four-atom C=C—< -H grouping, as shown in Figure 6.6. The more substituents that are present on the double bond, the more hyperconjugation there is and the more stable the alkene. 

In the corresponding -alkene, where this factor is not present, the cyclization is much less stereoselective. A stabilizing interaction between the siloxy oxygen and the Hg2+ center has also been suggested.108... 

Although the stabilizing interactions between the amino acid side chains of PLC/j, and the choline headgroup are readily apparent in the PLC fc-phosphonate inhibitor complex, it is more difficult to identify contacts between the protein and the acyl chains of the inhibitor [45]. In part this is because thermal motion in the acyl side chains, especially the sn-1 chain, renders them somewhat disordered. Consequently, the measured distances between the side chain carbons... 

Clearly, in the syn conformation there is a stabilizing interaction between the fluorine 2px lone pair and the LUMO of the methyl group. This type of interaction... 

The endo selectivity in many Diels-Alder reactions has been attributed to attractive secondary orbital interactions. In addition to the primary stabilizing HOMO-LUMO interactions, additional stabilizing interactions between the remaining parts of the diene and the dienophile are possible in the endo transition state (Figure 3). This secondary orbital interaction was originally proposed for substituents having jr orbitals, e.g. CN and CHO, but was later extended to substituents with tt(CH2) type of orbitals, as encountered in cyclopropene57. 

Berson and colleagues119 re-examined the Diels-Alder reaction between 1,3-diphenyli-sobenzofuran and cyclopropenone. They selectively obtained the exo adduct, as was confirmed by X-ray analysis. Ab initio calculations indicated a kinetic preference for the exo isomer due to stabilizing interactions between the ether oxygen and the carbonyl carbon in the exo transition state120. 

The 3D structure of 11P-HSD-2 shows that NAD+ has stabilizing interactions between the ribose hydroxyl and aspartic acid-91, serine-92, and threonine-112. Replacement of NAD+ with NADP+ reveals a coulombic repulsion between the 2 -phosphate group and aspartic acid-91. However, 11P-HSD type 2 lacks a nearby amino acid with a positively charged side chain that could compensate for the negative charge on aspartic acid-91. This explains the preference of 11 P-HSD-2 for NAD+. 

FIGURE 6. Stabilizing interaction between the HOMO of the nucleophile and the LUMO of the jt system and the destabilizing interaction between the HOMO of the nucleophile and the HOMO of the jt system. Reprinted with permission from Reference 42. Copyright (1986) American Association for the Advancement of Science... 

Fig. 7. The ionic lock stabilizes interactions between the cytoplasmic ends of TM3 and TM6 in the inactive state. Agonist binding disrupts these interactions.

C. at higher temperatures, the increase in polymerization rate is more rapid. These results indicate that high temperature activation is important not only for removing water from the support but also for inducing the stabilizing interaction between the support and chromium trioxide. 

There are two more important reasons why the staggered conformation of ethane is lower in energy than the eclipsed conformation. The first is that the electrons in the bonds repel each other and this repulsion is at a maximum in the eclipsed conformation. The second is that there may be some stabilizing interaction between the C-H G bonding orbital on one carbon and the C-H C antibonding orbital on the other carbon, which is greatest when the two orbitals are exactly parallel this only happens in the staggered conformation. 

Exciplex phosphorescence can be studied with complexes which are present in the ground state [118]. Equation (43) and its corollaries with respect to variations in A are applicable also to exciplex phosphorescence. For example, positive deviations (A > 0.18 eV) which have been found in the phosphorescence of complexes with (E — E d) > 2.75 eV are ascribed to the stabilizing interaction between the triplet CT state and energetically higher locally excited triplet states [108,119],... 

---