Localized anions

Anionic SN2 Reactions Involving Charge-localized Anions... 

The reactive species under these conditions consist of tight ion pairs involving the alkoxide anion from the carbohydrate (charge localized anion). The less reactive long chain methyl laurate leads to a later TS along the reaction coordinates and the magnitude of the microwave effect is therefore increased. 

Wiberg split the stabilization of the energy barrier into two parts (a) electrostatic energy in the planar form and (b) delocalization. Electrostatic stabilization lowers the energy of the planar form because the charge is spread over three atoms rather than being localized on one carbon in the rotated form. An estimation of the electrostatic stabilization was made by calculating a model, methane, for the localized anion and yielded a 23 kealmol-1... 

Fig. 5. Reaction scheme proposed for intramolecular exchange process in [V3Oi0]5. Local anionic charges are ignored. Subscripts t and c refer to terminal and central vanadium atoms. (Reprinted with permission from Andersson, I., Petterson, L., Hastings, J. J., Howarth, O. W., J. Chem. Soc. Dalton Trans. 1996, 3357.)...

Figure 4 reveals a similar pattern for acidities in DMSO solvent versus the gas phase. Once again, the small, localized anions result in solution acidities that are relatively stronger than the gas phase counterparts. The acids with large... 

The structure of the Ion pair complex Is also sensitive to the anion, especially the extent of charge delocalization and the presence of substituents close to the anionic site. Loose Ion pair formation requires a considerable increase In the inter-ionic Ion pair distance, and a charge localized anion, therefore. 

Removal of a proton from Lj gives an anion whose lone pair is orthogonal to the it bond of the carbonyl group by virtue of the rigid geometry of the bicyclic system. Consequently the lone pair cannot overlap with the carbonyl n bond and delocalization via resonance is not possible—it is effectively a localized anion. Removal of a proton from L2 gives rise to a lone pair in a p orbital which can overlap with the carbonyl it bond and thus resonance delocalization is possible. Thus the anion from L2 is resonance stabilized and is thus formed more easily. 

Now we assume that only electric work has to be done. We neglect for instance that the ion must displace other molecules. In addition, we assume that only a 1 1 salt is dissolved in the liquid. The electric work required to bring a charged cation to a place with potential -ip is W+ = etf). For an anion it is W = -ertp. The local anion and cation concentrations c and c+ are related with the local potential ij) by the Boltzmann factor c = c0 ee /fesT and c+ = co e e /kBT. Here, cq is the bulk concentration of the salt. The local charge density is... 

It is less obvious why a less polar solvent is better for the Sjsj2 reaction. The most common Sn2 reactions use an anion as the nucleophile and the transition state is less polar than the localized anion as the charge is spread between two atoms. 

It was proposed that the tethers of the pendant ligands were too short to place lithium on the axis normal to the center of the allyl plane but instead put coordinated lithium off the axis normal to the allyl plane at Ci49d. As a result what would ordinarily be a delocalized allyl anion becomes a polarized and partially localized anion, building up negative charge at Ci due to the proximity of coordinated Li+49d. The possible contribution of a small degree of covalence to the C—Li bond could not be evaluated. 

The ion pair status is not predictable only on the basis of the cation used. For highly delocalizable carbanions like 9-fluorenyl in THF, the CIP fraction was found to increase rapidly in the order Li < Na < K < Cs [22] and the caesium salt was found to be the most bathochromic in the series. The degree of association of carbanion ion pairs in non polar solvents is highly dependent upon the extent of charge delocalization [23]. With highly delocalizable anions SSIP will be formed with Li cation. A completely different sequence of ion pairs was shown in the case of naphtholate anions the CIP fraction increases in the order K < Na < Li [24], These anions are thus to be considered as more localized anions in order to agree with the preceding observations. 

The formation of the polymeric carbanions 81 of the fluorenyl-type is successful starting from the poly(para-phenylene) ladder polymer 71 with butyl-lithium as metallating agent. The degree of lithiation lies in the range of 90-95% (NMR). The UV/VIS absorption spectrum of these polymeric anions (81) is comparable with that of the 9-phenylfluorenyl anion and indicates the presence of mostly localized (anionic) sub-structures [101]. 

In electron bound complexes of C60 with aromatic hydrocarbons, the intensity of the aromatic hydrocarbon anion from collisional ionization is larger than that for the aromatic hydrocarbon. However, it is known that the adiabatic electron affinities of the aromatic hydrocarbons are significantly lower than that of C60. In these cases the relative intensities were attributed to a localized anion. When the benzanthracene and phenanthrene electron bound dimer is dissociated, the intensity of the phe-nanthrene anion is only about one-half that of the benzanthracene anion, while the electron affinity of benzanthracene is 0.72 eV and that for phenanthrene is 0.30 eV. The experimental data cannot be disputed. The ion ratio gives a AEa of 0.08 eV or the Ea of benzanthracene = 0.30 + 0.08 = 0.38(10) eV. Therefore, some other explanation must be presented. In the case of C60 excited anion states are observed and three degenerate LUMO + 1 are calculated with positive electron affinities. Benzanthracene has an excited-state MO at 0.38 eV that gives an Ea of 0.38 eV. Just as C60 has multiple LUMOs, so also does benzanthracene. Because the collisional ionization values are the most recent, they are returned by a search of the NIST tables for CH molecules. These values are fisted in Chapter 10 [61-63]. 

V is the ion volume). A larger contraction in perpendicular direction to the surface of the cation distances than that of the anions creates a highly asymmetric local anion environment, enhancing the local electrostatic field which the anion experiences. This decreases the surface energy because of the term Eq.(4.4). Puckering of the surface gives rise to surface dipole layer. 

Polar mechanisms via polar transition states. If the polarity of a system is enhanced from the ground state to the transition state, acceleration can result from an increase in material-wave interactions during the course of the reaction. The most frequently encountered examples are unimolecular or bimolecu-lar reactions between neutral molecules (because dipoles are developed in the TS) and anionic reactions of tight ion-pairs, i.e. involving charge-localized anions (leading to ionic dissociation in the TS). 

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