Orbital rehybridization

It would usually be assumed that abstraction/recombination reactions such as those illustrated in this section would proceed with racemization at the reacting centers. It has been reported22, however, that photocyclization of amide 1 proceeds with complete retention of absolute configuration. Racemization at the site of abstraction requires orbital rehybridization, passing through a planar intermediate. In this case rehybridization appears to be markedly slowed. This may be an electronic effect due to the heteroatom substituent on the intermediate radical, or simply a steric effect. The structure of product 2 was established by X-ray crystallographic analysis. 

E2 elimination requires partial formation of a new pi bond, with its parallel p orbitals, in the transition state. The electrons that once formed a C—H bond must begin to overlap with the orbital that the leaving group is vacating. Formation of this new pi bond implies that these two sp3 orbitals must be parallel so that pi overlap is possible as the hydrogen and halogen leave and the orbitals rehybridize to the p orbitals of the new pi bond. 

Ostrom H, Triguero L, Weiss K, Ogasawara H, Gamier MG, Nordlund D, Nyberg M, Pettersson LGM, Nilsson A (2003) Orbital rehybridization in n-octane adsorbed on Cu(llO). J Chem Phys 118 3782... 

Fladdon R C, Brus L E and Raghavaohari K 1986 Rehybridization and n-orbital alignment the key to the existenoe of spheroidal oarbon olusters Chem. Rhys. Lett. 131 165... 

The phenyl cation is an extremely unstable cation, as is reflected by the high hydride affinity shown in Table 5.2. In this case, the ring geometry opposes rehybridization so the vacant orbital retains sp character. Because the empty orbital is in the nodal plane of the ring, it receives no stabilization firom the n electrons. 

The deviation from planarity that is present in a structure such as 1 raises the question of how severely a conjugated system can be distorted from the ideal coplanar alignment of p orbitals and still retain aromaticity. This problem has been analyzed by determining the degree of rehybridization necessary to maximize p orbital overlap in 1. It is found that rehybridization to incorporate fractional amounts of s character can improve orbital alignment substantially. Orbitals with about 6% s character are suggested to be involved... 

A nucleophilic addition reaction to an aldehyde or ketone. The nucleophile approaches the carbonyl group from an angle of approximately 75° to the plane of the sp2 orbitals, the carbonyl carbon rehybridizes from sp2 to sp3, and an alkoxide ion is formed. 

The K conjugate molecules usually have planar geometries and no difference between the two faces above and below the molecular plane. When substitutions break the symmetry with respect to the plane, n orbitals mix a orbitals orthogonal prior to the substitution. Rehybridization occurs and the unsaturated bonds have... 

We discount the likelihood of reaction R5 because two metal-bonded methyl groups have no molecular orbitals readily available for reaction. These groups are sp3 hybridized with all 4 orbitals fully occupied. The lowest lying unoccupied orbital is so much higher in energy that it is inaccessible, and there arc no obvious interactions that might enable a rehybridization to lower this energy. 

As already mentioned, the chemical reactivity of the nitrosyls is strongly related to the type of bonding, because it controls the rehybridization of NO-based orbitals and associated significant redistribution of the electron and spin densities within the M—NO moiety. 

The major contribution to the difference in BDEs comes rather from the second factor, i.e. the difference between the changes in hybridization that occur upon removing a hydrogen atom from NH2 and CH3 77 Loss of H from both NH2 and CH3 allows the 2s character in the remaining doubly occupied a orbitals to increase. However, the electrons in the lone pair orbital of triplet NH benefit from rehybridization more than those in the C-H bonding orbitals of triplet CH2 or, for that matter, more than the electrons in the N-H bonding orbital of triplet NH. The reason is that the lone pair of electrons in NH is localized almost entirely on N whereas, the pair of electrons in a C-H or N-H bond is shared between hydrogen and the atom to which it is bonded. 

The R(0)HF/6-311G(2d,p) orbital energies for NH3, NH2, and triplet NH are depicted in Fig. 13,77 which shows graphically the 38 kcal/mol decrease in the energy of the lone pair orbital on going from NH2 to triplet NH. Fig. 13 also shows that an increase in 2s character, due to rehybridization, also stabilizes the... 

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