Restricted bond direction

Restricted Rotation. A little reflection will reveal that maximum overlap in a 7r bond is produced when the p orbitals are parallel to each other, and any rotation of these orbitals about the bond direction would decrease overlap. Since the directions of the p orbitals are fixed at right angles with respect to the cr bond planes, the most stable configuration of ethylene with maximum tt overlap is achieved when the cr bond planes coincide in other words, all the atoms in the molecule should be in one plane. This is indeed true in ethylene, and experiments indicate that rotation of the two CH2 halves about the C=C bond results in an increase in potential energy. There is restricted rotation in ethylene. Rotation about pure cr bonds is not restricted in this way. 

Real chains are not freely hinged and have steric restrictions on bond rotations, of course. But these problems can be handled.16 Fixing the bond angle introduces a correlation between one bond direction and the next, such that is now given by Equation 8-8 ... 

Williams showed that an unrestricted bond dipole model gave erratic results for the bond dipole directions. To pursue the goals of chemical reasonableness and transferability, it seems appropriate to restrict the direction of the bond dipoles. The most natural direction is along the bond. Table 12 shows that restricted bond dipole models represent the electric potential about as well as monopole models. The two models also have nearly the same number of parameters. So the choice between these two models is a matter of convenience. If long distance interaction is considered, as in crystals, use may be made of the fact that dipole-dipole energy converges much faster than monopole-monopole energy. However, as mentioned above, if ions are considered, monopole interactions are still needed. Table 13 summarizes values of restricted bond dipole moments. 

The definition of a metal cluster proposed by Cotton (101, 102) requires only the presence of direct covalent bonds between metal atoms. As this includes the whole class of compounds containing metal-metal bonds, it has been considered desirable in this review to restrict the term cluster to those compounds containing at least three metal atoms linked directly by covalent bonds. Hence on the basis of geometry, clusters fall into two groups closed clusters, in which every metal atom is bonded directly to at least two others, and open clusters, in which some metal atoms are not bonded to two other metal atoms (222). 

There is another aspect to the question of the reactivity of the carbonyl group in r ck)hexanone. This has to do with the preference for approach of reactants from the axial ir equatorial direction. The chair conformation of cyclohexanone places the carbonyl coup in an unsynunetrical environment. It is observed that small nucleophiles prefer to roach the carbonyl group of cyclohexanone from the axial direction even though this is 1 more sterically restricted approach than from the equatorial side." How do the ctfcnaices in the C—C bonds (on the axial side) as opposed to the C—H bonds (on the equatorial side) influence the reactivity of cyclohexanone ... 

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