Hyperconjugation evidence against

Kinetic studies of mercuration have also been used as a test for hyperconjugation. Toluene and toluene-aaa-mercuric acetate (0.5 M) in acetic acid containing water (0.25 M) and perchloric acid (0.050 M) and an isotope effect, ArH/ArD = 1.00 0.03, obtained. This insignificant effect was considered as evidence against the participation of hyperconjugation in electron supply by a methyl group449. 

There is a large body of evidence against hyperconjugation in the ground states of neutral molecules.A recent study of the one-bond coupling constants for the aromatic system 141, however, appears to provide the first structural evi-... 

However, there is evidence against /-orbital overlap and the stabilizing effects have been attributed to other causes.In the case of a PhS substituent, carbanion stabilization is thought to be due to a combination of the inductive and polarizability effects of the group, and d-pn resonance and negative hyperconjugation play a minor role, if any. An ot silicon atom also stabilizes carbanions. ... 

According to Epiotis (279), the insensitivity of AHu for the isodesmic reaction involving methylenediamine (73, Eq. [43]) to a conformation about both C—N bonds in 73 should be regarded as an unambiguous evidence against the model of negative hyperconjugation ... 

The acidity of 1 -tridecafluoro [2,2,2 Joctane, like that of 1/f-undecafluoro-bicyclo [2,2,1 [heptane, can be quoted as evidence against fluorine hyper-copjugation as a significant stabilizing phenomenon in fluoroalkyl anions. The conclusion that inductive effects (- / and + /,) are fully adequate to rationalize the stabilities of fluorocarbanions is supported by the results of a recent molecular orbital examination of the importance of fluorine hyperconjugation in the 2-fluoroethyl and 2,2,2-trifluoroethyl anions. ... 

In conclusion, most of the evidence and molecular orbital calculations (see p. 27) support the view that cyclopropylcarbinyl cations have a bisected conformation that is stabilized because of the increased p-character of the bent C—C cyclopropane bonds which increases greatly their C—C hyperconjugative ability. The most important evidence that has been interpreted as arguing against this formulation is the NMR spectra of the cyclopropylmethyl cation itself, and hence there must be certain reservations about the < bove conclusion concerning this ion. It must also be... 

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