Whiffen effect

In the butadiene system of a cyclopentadienylstannane, the LUMO has the same phase at the 1- and 4-positions 25, and it is argued that this results in enhanced hyperconjugation through the Whiffen effect,307 which was first... 

This ability of ESR to quantify hyperconjugation showed that the magnitude of the effect is subject to symmetry control, and can be enhanced or forbidden when the CH group is bonded to both ends of a conjugated system which carries the unpaired electron, in what is known as the Whiffen effect (3-5).10-11 The hyperfine coupling is now given by the equation 3-29, where ca and ca are the coefficients at the two flanking ends of the n system. 

The Wheland intermediates that are formed in reaction 3-36 are important as their pentadienyl cation LUMO (3-10) has the appropriate symmetry for (Whiffen) enhanced hyperconjugation, which is in large part responsible for the high ipso reactivity, and the relative reactivities of compounds C6H5MEt3, compared with benzene, towards electrophiles, as M varies, are approximately (benzene 1), Si, 5 x 104, Ge 106, Sn 5 x 1011, Pb 1020.25 The 4-stannylbenzyltin compounds should further benefit from both the Whiffen effect at the ipso position and the simple C-Sn hyperconjugation at the 4-position (3-11). 

Figure 8,28 The Whiffen effect describes MO symmetry effects on hyperconjugation. Symmetry of the cyclic system favors interaction of a C-H bond with the LUMO of pentadienyl but not with the LUMO of allyI.

Davies, A. G. (1999). The Whiffen effect of symmetry-enhanced and symmetry-forbidden hyperconjugation in spin-paired molecules. Journal of the Chemical Society, Perkins Transactions 2, 11, 2461-2467. 

Comparison to experimental results is more conveniently carried out in the fiill symmetry-coordinate representation, as this is the representation in which the experimental force constants are directly determined. Symmetry coordinates are also preferable to a valen< -coordinate representation, as any error in the experimental symmetry force constants due to unresolved effects will spread over the entire force field when transformed into the valence-coordinate representation. We have used the full set of symmetry coordinates introduced by Whiffen [42]. 

Powder ENDOR lines are usually broadened by the anisotropy of the hyperfine couplings. The parameters of well resolved spectra can be extracted by a visual analysis analogous to that applied in ESR. The principle is indicated in Fig. 3.25 for an 5 = V2 species with anisotropic H hyperfine structure, where the hyperfine coupling tensor of axial symmetry is analysed under the assumption that 0 < A < Aj. < 2 vh- The lines for electronic quantum numbers ms = V2 and -Vi, centered at the nuclear frequency vh 14.4 MHz at X-band, are separated by distances equal to the principal values of the hyperfine coupling tensor as indicated in the figure. Absorption-like peaks separated by A in the 1st derivative spectrum occur due to the step-wise increase of the amplitude in the absorption spectrum, like in powder ESR spectra (Section 3.4.1). The difference in amplitude commonly observed between the ms = /2 branches is caused by the hyperfine enhancement effect on the ENDOR intensities first explained by Whiffen [45a]. The effect of hyperfine enhancement is apparent in Figs. 3.25 and 3.26. 

Apart from NMR and die mefliods suitable for characterization of overall chain dimensions (which we will touch on below), fliere are not many simple experimental techniques that can be used to study sugar conformation and that are directly correlated to theoretical results based on calculated potential energy surfaces or force fields. An exception is given by chiro-optical techniques, which provide important (although empirical) structural information as optical rotation experiments have been shown to be very useful and informative in flic study of saccharides in solution. Literature gives die experimental evidence for the effects of the external conditions (solvent and temperature) on the optical rotation. The additivity mediods, proposed in the Fifties by Whiffen and Brewster, " were... 

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