Spectroscopy hyperconjugation

The Triduoroinethyl Group in Chemistry and Spectroscopy Carbon-Fluorine Hyperconjugation Stock L M Wasiliewski, M R Frog Fhys Org Chem 13 253-313 135... 

Photoelectron spectroscopy shows that the carbon-tin bond prefers that orientation in which it lies parallel to the pir orbitals of the double bond, to permit carbon-metal hyperconjugation (146). 

Recent results on the chemistry of persistent vinyl cations are summarized. / , / -Disilyl-substituted vinyl cations were synthesized by intramolecular addition of transient silylium ions to alkynes. The vinyl cations are stable at ambient temperature and were isolated in the form of their tetrakispentafluorophenylborate and hexabromocarboranate salts. The vinyl cations were characterized by IR and NMR spectroscopy and by X-ray crystallography. The experimental results for the a-alkyl- and a-aryl-substituted vinyl cations confirm their Y-shape structures, consisting of a linear dicoordinated, formally positively charged a-carbon atom and a trigonal planar coordinated /f-carbon atom. In addition, the spectroscopic data clearly indicate the consequences of, / -silyl hyperconjugation in these vinyl cations. Scope and limitations of the synthetic approach to vinyl cations via addition of silylium ions to C=C triple bonds are discussed. 

Photoelectron and electron transmission spectroscopy indicate that there is appreciable interaction between the acetylene units of [129] (Houk et al., 1985). Both homoconjugation and hyperconjugation are proposed. Dewar and Holloway (1984) suggest that the through-bond interactions dominate. Similar thermochemical studies to those performed with the triquinacene series were carried out on [129] and some acyclic homoconjugated acetylenes (Scott et al., 1988). From these data it was concluded that decamethyl[5]pericyclyne should be classed as a homoaromatic molecule. As already discussed for the triquinacene series, the species used as non-homoaromatic models (and the calculated compensations for strain energies) may be inappropriate and thus this conclusion should be treated with some caution. Using our probes for homoaromaticity we were not able to obtain any evidence in support of the homoaromaticity of [129] (Williams and Kurtz, unpublished results). 

Hehre and co-workers (DeFrees et al., 1977, 1979a) have published both experimental and theoretical evidence in support of negative ion (anionic) hyperconjugation. These workers determined the free energies for the gas-phase hydron3 transfer equilibria (31), (32) and (33) by pulsed ion cyclotron resonance spectroscopy (Wolf et al., 1976). These equilibria, which involve the gas-phase formation of a methylamino, a methoxy and a thiomethoxy anion, all lie to the right, i.e. the formation of the isotopically light anion is favoured. These results were rationalized in terms of the MO... 

The rate constants for the KIEs were measured using UV spectroscopy in separate kinetic runs using the undeuterated and deuterated substrates. Although this normal secondary /3-deuterium KIE could be due to hyperconjugation, the authors, like Streitwieser and Van Sickle (1962), preferred to attribute it to an inductive effect. 

The secondary /3-deuterium KIEs observed for the reaction of the same substrate with hydroxide ion and with tris(hydroxymethyl)methylamine in aqueous solution at 25°C were small, i.e. kH/kD = 1.09 0.01 and 1.10 0.01, respectively. While Kresge argued that the EIE was primarily due to hyperconjugation, the secondary /3-deuterium KIEs were attributed partly to hyperconjugation and partly to a polar (inductive) effect. The rate constants for the evaluation of both the EIE and the KIEs were determined in separate kinetic runs by following the increase in the absorbance due to the nitronate ion by UV spectroscopy. 

Delocalization of a free electron pair into an antibonding Hyperconjugation Resonance Absorption Spectroscopy... 

The Trifluoromethyl Group in Chemistry and Spectroscopy Carbon-Fluorine Hyperconjugation ... 

By UV spectroscopy in aqueous buffers, the secondary IE of six deuteriums on the acidity of 2-nitropropane, a((CH3)2CHN02)/Aia((CD3)2CHN02) was found to be 1.233 0.033, or 1.04 per D.60 The IE was attributed to a hyperconjugative interaction, whereby CH3 stabilizes the C=N+ double bond of the conjugate base, despite the anionic character. The secondary kinetic IEs of four deuteriums on the acid-catalyzed enolization (measured from the rate of bromination, but one that should parallel the equilibrium IE on acidity) of cyclopentyl-2,2,5,5-d4 and cyclohexyl-2,2,6,6-d4 phenyl ketones are 1.21 and 1.41, respectively.61... 

Studies on the distannylmethyleneborane 127109 using UB and 13C NMR spectroscopy provides evidence for strong C-Sn hyperconjugation with the boron... 

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