Hyperconjugation, definition

Another set of terms considered by Eyring, et al. consisted of structures having a double bond between carbons. These should be expected to be involved to some small extent—small because they have one less real bond. This is essentially invoking hyperconjugation. These authors did not reach a definite conclusion regarding the importance of these structures and decided that they probably would stabilize the eclipsed configuration, which was not known at that time to be the incorrect one. It is still not simple to make a reliable estimate of the importance of this contribution. 

Brown (1959) has presented a charge transfer model of the transition state for electrophilic reactions which differs appreciably from that proposed by Fukui and his collaborators and leads to the definition of a new reactivity index termed the Z value . The model is based on a more conventional formulation of the charge transfer mechanism, which avoids the complete transfer of electrons associated with v = 0,1,2 in Fukui s model. There is no dependence on the formation of a pseudo tt orbital in the transition state, nor is hyperconjugation invoked. A wave function for a charge transfer complex is written as a linear combination of a wave function < o describing the unperturbed ground state of the molecule under attack, and a function which differs from (Pq in the replacement... 

Mulliken s model is entirely compatible with the descriptions given previously in this Section, since charge shifts that result from polarization are already taken into account in forming the cr-complex by subsequent changes in hybridization. Fukui s model, upon which the definition of the superdelocalizability is based, resembles Mulliken s only in the use of a pseudo-TT orbital , and the formulation of the hyperconjugation problem is quite different, since )3 is taken by Fukui to be small, so that 8f can be defined by perturbation formulae. In particular, the bonding of the pseudo-TT orbital in Fukui s model primarily involves the least bound, or frontier orbitals, whereas in Mulliken s model the most bound MO is involved. 

Since the Es value is determined by the relative activation free energy from the unsaturated initial state to the saturated tetrahedral intermediate state of the ester hydrolysis, Hancock and his coworkers considered that a hyperconjugation effect of a-hydrogen may contribute to the estimate of Es values 19). To separate the hyperconjugation effect from the true steric effect , they defined the parameter E° (corrected steric) as Eq. 20, assuming that the hyperconjugation effect is proportional to the number of a-hydrogen atoms, nH. By definition, E (Me) = 0. 

We have also proposed a general definition of the concept of stabilization energy applying to any compound, as shown by the numerous applications described in this survey article. Using this concept, we have been able to carry out a critical analysis and to offer a unified definition of various former notions such as hyperconjugation, resonance, ring strain, and steric repulsion energies. 

Several objections have been raised for this interpretation17 Unlike in the 260 m/x band, no definite trend has been found in the position of the 200 m/x bands in alkyl benzenes or p-alkyl substituted benzenes. In many cases, the observed wavelengths are actually found to be in the reverse order (i.e., in the inductive order) to that predicted by C-—H hyperconjugation. At this point it may be stated that the evidence for hyperconjugation from the absorption spectra of alkyl substituted benzene derivatives is not definitive. 

Orbital interactions are complex and depend on the combination of stabilizing delocalizations, sterics, and electrostatics. Again, in the spirit of the definition of stereoelectronic effects in the introduction, we will concentrate on stabilizing effects, such as hyperconjugation and conjugation. 

We have suggested " that a Class 3 force field be defined as one which contains chemical effects, in addition to the physical effects so far discussed. Chemical effects would include such things as hyperconjugation, the electronegativity effect, the anomeric and Bohlmann effects, and so on. These effects depend upon exactly which atom occupies a position, that is, they depend not only on ordinary mechanical quantities, but also on specific properties of oxygen, for example, compared with nitrogen, or with carbon. These effects can, of course, be properly represented by suitable cross terms in the force constant matrix. The origins of these terms, however, have a definite chemical basis. 

Delocalization of a electrons is the most basic definition of hyperconjugation. In organic molecules one typically thinks of the electrons in a cr-type chemical bond as being highly localized (as compared to, say, the electrons in conjugated 7T system,s). However, when sufficient energetic driving force... 

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