Hyperconjugation effects, bond

MM2 was, according the web site of the authors, released as MM2 87). The various MM2 flavors are superseded by MM3, with significant improvements in the functional form [10]. It was also extended to handle amides, polypeptides, and proteins [11]. The last release of this series was MM3(%). Further improvements followed by starting the MM4 series, which focuses on hydrocarbons [12], on the description of hyperconjugative effects on carbon-carbon bond lengths [13], and on conjugated hydrocarbons [14] with special emphasis on vibrational frequencies [15]. For applications of MM2 and MM3 in inorganic systems, readers are referred to the literature [16-19]. 

Valence bond representation of the hyperconjugation effect which leads to a lengthening of the C—H bond icetaldeyde. 

The interaction of the lone-pair electrons on an amine nitrogen with adjacent C—H bonds is another example of a hyperconjugative effect that can be described in MO language. The lone-pair electrons, when properly aligned with the C—H bond, lead to a... 

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

Hyperconjugative effects on carbon-carbon bond lengths in molecular mechanics (MM4), J. Comp. Chem. 17 747 (1996). 

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

Group IV unsaturated molecules, indicates the significance of tt-bonding effects. However, no differentiation of the orbitals involved in the bonding and, thus, no differentiation between d-orbital effects and hyperconjugative effects is possible. Recently, several groups have performed semiempirica SCF-MO calculations which, to some extent, overcome this difficulty (29, 85, 112). 

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