Hyperconjugation^*

Abstract

method, free radicals.(2) According to a private communication (see also reference 1, footnote 20), E. Hückel has also used the MO method in similar ways.

Toward an experimental bond order

“…The trend in bond order in the methyl halides, for which the bond order increases in going from F to I is just the opposite of the decrease observed in many series of diatomic halides in Table 1. Also, the rather larger values of the C -C bond orders for the cyanide (1.20) and isocyanide (1.21) are attributable to hyperconjugation effects [49].…”

“…However, it is well known that in ethane the bond length is shorter and the force constant larger than expected for a single bond. In order to explain this observation, Mulliken introduced the concept of hyperconjugation [49], in which ionic valence-bond structures from the C -H bonds can add electron density to the p-orbital of the C -C bond. Applying a simple LCAO-molecular orbital calculation, he obtained a value of 1.12 for the bond order.…”

The Role and the Status of Thermodynamics in Quantum Chemistry Calculations

“…I think it is important not only to check if his holistic molecular conception changed the way thermodynamics became involved in chemical quantum works; but also to compare it to Pauling's own use of thermal data. Mulliken's studies of hyperconjugation are a relevant case study to grasp the role and the status of thermodynamics in such a chemical quantum background (Mulliken et al, 1941). Mulliken's calculations taken in connection with thermal and bond distance data indicated the conjugating power of chemical groups such as the landmark methyl group.…”

“…However, we shall work with heats of combustion.' (Mulliken et al, 1941). Heats of combustion enabled Mulliken to put forward formula to calculate conjugation energies from heats of combustion that fitted the available consistent data for gaseous saturated hydrocarbons -except methane -with considerable accuracy -mostly better than 1 kcal.…”

“…Nevertheless, their self-consistency gives a distinct support to our numerical results, since we have found that such self-consistency is not easy to attain.' (Mulliken et al, 1941). The authors called for more accurate thermal and bond distances data, those researches got into an endless and open circle of refinements that linked calculations with empirical data.…”