Hyperconjugation mechanism

In this case, the dominant interactions will be crcH-aco and the tendency for the gauche structure will be diminished relative to the case of CH2F—CH2F since this interaction is weaker than the nCH—ctcf interaction. The trend will continue along the series F, OR, NR2 and CR3. At this point, it should be mentioned that Pople etal, have considered a hyperconjugative mechanism in CH2F—CH2F68. ... 

Fig. 9 The HF/3-21G HOMO and LUMO of the rigid benzoquinone-6-aniline donor acceptor system. The HOMO is associated with the aniline donor and the LUMO with the benzoquinone acceptor. These MOs are the active orbitals involved in optical ET in this molecule (see Fig. lb). Note that the benzoquinone LUMO is not entirely localised within this group, but extends into the bridge, by a hyperconjugation mechanism, the LUMO amplitude decaying exponentially with increasing penetration into the bridge. This type of orbital extension is also observed for the aniline HOMO.

The dependence of the hyperfine splittings of alkyl groups upon their conformation in aliphatic radicals has been described.66 The transmission of spin to /1-protons is by a mainly hyperconjugative mechanism such that the /J-H splitting is given by Eq. (11) where pt is the 7t-spin population at the a-atom i to which the alkyl group is attached, 9 is the dihedral angle between the pz(7t) orbital of the a-atom / which contains the unpaired electron and the /J-C-H bond, and B and B0 are constants.15... 

In most cases, carbon-centered radicals carry the unpaired electron in p(7t)-type orbitals. Since C isotopes are present at a very low percentage (natural abundance, 1.11%), the spin distribution is monitored by adjacent H nuclei. The spin from the p-type orbital of the C atom is transferred to the adjacent H atom (H ) via n-spin polarization (Fig. 7.2a) however, spin transfer to more distant protons follows the model of a hyperconjugation mechanism illustrated in Fig. 7.2b. The closer the Cp-Hp is oriented toward the z-axis of the C p orbital, (0 = 0°) the bigger becomes the Hp hfc. When 0 is equal to 90°, the Hp hfc reaches its minimum value. This behavior is described by an empirical formula ... 

This stabilization has been shown to be primarily a delocalization effect by demonstrating a strong dependence on the dihedral angle between the Si—C bond and the empty p orbital of the carbocation. Kinetic rate enhancements of about 1012, 0, and 104 were found for compounds with angles of 180°, 90°, and 0°, respectively. This is consistent with a hyperconjugation mechanism and with analyses of orbital overlap that show considerably stronger overlap for the anti (180°) than syn (0°) orientations. ... 

In closing, we should mention that the importance of a hyperconjugative mechanism in enforcing gswche preference in molecules containing adjacent lone pairs, such as H2O2, N2H4, etc., has been discussed by Fbple at However, these... 

A balanced description of relative donor ability of o-bonds can be accomplished if the donor and acceptor sites are not directly connected, e.g. by comparing the relative total energies and geometries of two conform-ers ( hyperconjomers ) of 5-substituted cyclohexyl cations. In these systems, interaction between the bond and the empty p-orbital is transmitted via the double hyperconjugation mechanism (see Chapter 8). Such description partially cancels the non-hyperconjugative contributions such as the inductive and field effects (Figure 5.26). ... 

For discussing the results obtained, use can be made of a simple model (Sagdeev et al, 1969), based on the assumption that the spin density propagates successively via chemical bonds in accordance with spin-polarization and hyperconjugation mechanisms. Such consideration leads (for constant on y protons) to equation (2), which in principle is analogous to the equation derived by Barfield (1964) for the long-range spin-spin interaction constant via four a bonds ... 

The first term in this equation corresponds to two pathways of the spin density transfer to the orbital of the H atom. One of the pathways is the spin density transfer to orbital G2 by the hyperconjugation mechanism. The second pathway is the p-cr3 interaction by the hyperconjugation mechanism with subsequent transfer of the spin density to the H orbital by the spin polarization mechanism. Since in the given case spin polarization must lead to the reversal of the spin density sign, while hyper-... 

The second term in equation (2) corresponds to the spin density transfer from orbital p to H through orbitals C2 and C 2 by the hyperconjugation mechanism. The sign of constant B is determined by the sign of the spin density on orbitals C2 and C 2. There are grounds to believe that this density is small and negative. It may therefore be expected that B < 0 and that the contribution made by the second term of equation (2) is smaller than that made by the first term. 

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