Hyperconjugative orbital interaction

The vibrational spectrum of 1,4-dioxin was studied at the MP2 and B3-LYP levels in combination with the 6-3IG basis set [98JST265]. The DPT results tend to be more accurate than those obtained by the perturbational approach. The half-chair conformation of 4//-1,3-dioxin 164 was found to be more stable than the corresponding conformations of 3,4-dihydro-1,2-dioxin 165,3,6-dihydro-1,2-dioxin 166, and of 2,3-dihydro-1,4-dioxin 167 (Scheme 114) [98JCC1064, 00JST145]. The calculations indicate that hyperconjugative orbital interactions contribute to its stability. 

Some other mechanisms will be discussed in later sections The Grant-Cheney formulation (88) of steric compression effects will appear in the context of y-gauche substituent effects (Section III-C), and hyperconjugative orbital interactions will be cited in interpretations of y-anti substituent (Section III-D) and intramolecular interaction effects (Section IV). 

Calculations and Experiments on the Stereomutation of Cyclopropane. In 1965, Hoffmann published a seminal paper on trimethylene, another name for propane-1,3-diyl (8). He used extended hiickel (EH) calculations and an orbital interaction diagram to show that hyperconjugative electron donation from the central methylene group destabilizes the symmetric combination of 2p-n AOs on the terminal carbons in the (0,0) conformation of this diradical. Hoffmann s calculations predicted that the resulting occupancy of the antisymmetric combination of 2p-n AOs in 8 should favor conrotatory opening of cyclopropane (7), as depicted in Figure 22.8. 

Scheme2.1. Orbital interaction and canonical forms for hyperconjugation between a bonds.

The overlap of a C-H a orbital with the n (or p) orbital on a directly bonded carbon atom is termed hyperconjugation. This interaction has a shortening effect on the C-C bond length, a good example being the structure of the tert-butyl cation [C(CH3)3]+. 

The reason for the higher basicity and nucleophilicity toward silicon ( silicophilicity ) of D2 compared to D3 and D4 is a different orbital interaction pattern involving the lone electron pair of oxygen, due to the difference in structures of their ring skeletons. Contribution from the hyperconjugation effect no—Kj sio in polysiloxane significantly reduces the electron density on oxygen. This effect is less important in D2 because the no— o siSi interaction is much weaker [8]. 

This leads to a stabilizing effect called hyperconjugation. Hyperconjugation is what happens when there is an unfilled (antibonding or vacant) C-C ti orbital and a filled C-H a bond orbital next to each other. The result is that the filled C-H o orbital interacts with the unfilled C-C ti orbital and stabilizes the molecule. The more highly substituted the molecule, the more chances there are for hyperconjugation and thus the more stable the molecule is. 

Hyperconjugation of C H Bonds with C H Bonds. Using hybridised orbitals for C H bonds, and mixing them in the usual way to show conjugation, creates the molecular orbitals of Fig. 2.13, which is set up for the anti-periplanar interaction. There is an equivalent set of orbitals interacting in a syn-coplanar arrangement, the relative merits of which are discussed on pp. 98-100. 

The stereochemistry of exo closure in a case like the radical 7.92, giving the cis product 7.94 (cis. trans 72 28), is controlled by the usual preference for the resident substituent to adopt an equatorial orientation 7.93 and for the chain of atoms to adopt a chair-like conformation.1031 In the case of a radical like 7.95, however, there is a clearly contrathermodynamic preference for the formation of the cis product 7.97.1032 It has been suggested that the endo-like transition structure 7.96 might have an attractive secondary orbital interaction from the filled hyperconjugative orbitals with the n orbital, rather like that used to explain the endo rule for Diels-Alder reactions. The preference for cis products like 7.97 does not extend to all substituents on the radical centre, and Z-substituents often lead to trans products, possibly because they lack the same set of orbitals, but probably also because they are larger than a methyl group. 

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