hyperconjugationn.

1939 R. S. Mulliken in Jrnl. Chem. Physics VII. 340/2 In the cyclic dienes..there is interaction between this [pair of electrons] and the unsaturation orbitals, resulting in what may be called ‘cyclic conjugation’. This phenomenon has been..discussed by Hückel, except for the case where A is CH₂. For this case we may introduce the term ‘hyperconjugation’, meaning an additional conjugation beyond that ordinarily recognized. [Note] The term..was suggested by Professor W. G. Brown, as an improvement on the term ‘superconjugation’ at first used by the writer.

1949 Q. Rev. 3 229 Delocalisation of this type is known as σ-hyperconjugation, and since σ electrons are relatively tightly bound, the resulting energy of delocalisation is likely to be very small. Consequently, σ-hyperconjugation is generally ignored.

1952 Chem. & Engin. News 17 Nov. 4881/2 Hyperconjugation denotes resonance interaction by alkyl, and particularly methyl, substituents in systems exemplified by H₃ ≡ C⃕—C⃕ = C ← → H⁺H₂C = C—C⁻.

1962 M. J. S. Dewar Hyperconjugation i. 13 The evidence..for the importance of hyperconjugation is much less conclusive than has been commonly supposed.

1965 C. S. G. Phillips & R. J. P. Williams Inorg. Chem. I. xi. 400 Hydrogen is also unable to act as a π-donor, except possibly to a small extent in groups of hydrogen atoms as in CH₃, the so-called hyperconjugation effect.

1949 Q. Rev. 3 233 The carbon–carbon distance in acetaldehyde is approximately 0·04 a. less than the normal value, and..the shortening has been attributed to hyperconjugated structures.

1959 E. S. Gould Mechanism & Struct. Org. Chem. ii. 49 Hyperconjugated structures in which a shift of electron density..has left the CH bonds with partial no-bond character.