Carbocation electronic structure

Vinylic carbocation, electronic structure of, 263 electrostatic potential map of. 

It should be kept in mind that quantum chemical calculations of structures and magnetic properties generally are done for the isolated carbocation without taking into account its environment and media effects such as solvent, site-specific solvation or counterion effects. This is a critical question since NMR spectra of carbocations with a few exceptions are studied in superacid solutions and properties calculated for the gas-phase species are of little relevance if the electronic structure of carbocations is strongly perturbed by solvent effects. Provided that appropriate methods are used,... 

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

Before we move on from the hybrid orbitals of carbon, we should take a look at the electronic structure of important reactive species that will figure prominently in our consideration of chemical reactions. First, let us consider carbanions and carbocations. We shall consider the simplest examples, the methyl anion CHs and the methyl cation CH3+, though these are not going to be typical of the carbanions and carbocations we shall be meeting, in that they lack features to enhance their stability and utility. 

As described in Chapter I in this volume, electronic structure theory calculations are also very useful for computing C chemical shifts in carbocations. The predicted C NMR spectra and the experimental spectra, obtained in superacids under conditions where the carbocations are stable, can be compared. These comparisons are particularly helpful in differentiating between classical and nonclassi-cal structures for these electron-deficient species. 

Hypervalent carbocations have received some attention this year.14 The concept of three-centre, two-electron bonding in these entities is supported by a topological bifurcation analysis of the electronic structure of CH5 15 this and the related species CHg+ and CH3+ are also the subject of a review.16 The CHj + H2 reaction has been studied theoretically.17... 

K. Raghavachari, R. A. Whiteside, J. A. Pople, and P. v. R. Schleyer, ]. Am. Chem. Soc., 103, 5649 (1981). Molecular Orbital Theory of the Electronic Structure of Organic Molecules. 40. Structures and Energies of C1-C3 Carbocations, Including Effects of Electron Correlation. 

This carbocation has to have a monocyclic structure and is the simplest potentially degenerate (CH) carbocation. It can either be a degenerate cyclopropenyl cation [339] rearranging as in (221) or a static aromatic 2 t-electron structure [340], i.e. the simplest aromatic system (222). The ion... 

X, Y, and Z may all be carbon atoms, as in the case of an allylic carbocation (resonance structures A and B), or they may be heteroatoms, as in the case of the acetate anion (resonance structures C and D). The atom Z bonded to the multiple bond can be charged (a net positive or negative charge) or neutral (having zero, one, or two nonbonded electrons). The two resonance structures differ in the location of the double bond, and either the charge, the radical, or the lone pair, generalized by [ ]. 

The electronic structure of a carbocation. The trivalent carbon is sp -hybridized and has a vacant p orbital extending perpendicular to the plane of the carbon and three attached groups. 

In terms of the discussed approach to the quantitative description of carbocation rearrangements it proved possible to cover by general correlations the MT of different migrants and its dependence on the electronic structure of the MCR of the rearranging carbocation The basis of these correlations is a simple model and the corresponding terms Ej which describes the loosening of the O —R bond, and E2 which describes the affinity of the migrant to the carbocation centre. 

C, indicate that the mesomeric effect e.g. MejC- -F Me2C=F ) of the halogens involved decreases in the order F > Cl > Br. All this and related work by Olah s group on fluorinated carbocations has been reviewed, and the results of MO studies of fluorinated methyl, allyl, or trans-2 vinylcyclopropylmethyP cations and radicals have been presented in detail. The electronic structure of difluoromethane and the partial double-bond character of C— F bonds in CHjF, CHjFj, and CHF, have received attention at the hands of MO pundits. 

The isotopic perturbation method is very useful for studying fractional bonding and hypercoordination in coordinatively unsaturated and electron deficient compounds such as transition metal complexes or carbocations. The 2-norbornyl cation and the bicyclobutonium cation are the most prominent examples of carbocations whose structures have led to much controversial discussion (the so-called classical-nonclassical ion controversy). The application of the isotopic perturbation methed is likely to be the most decisive piece of nmr evidence for the hypercoordinated structure of these two cations in solution. 

From the standpoint of molecular and electronic structure, carbocations present a much greater variety and more significant conceptual challenges than carbanions, radicals, or carbenes. Many aspects of carbocation chemistry have been at times quite controversial. Here, we describe the bonding models for basic carbocation structures. First, however, we must explain some nomenclature. 

Further research into the reaction mechanism revealed that the reaction rate was correlated with the electron structure of the sulfoxide the more electropositive sulfoxides were the better oxygen donors. Excellent correlation of the reaction rates with the heterolytic benzylic carbon-hydrogen bond dissociation energies indicated a hydride abstraction mechanism in the rate-determining step to yield a carbocation intermediate. The formation of 9-phenylfluorene as by-product in the oxidation of triphenylmethane supports this suggestion. Further kinetic experiments and NMR showed the formation of a polyoxometalate-sulfoxide complex before the oxidation reaction, this complex being the active oxidant in these systems. Subsequently, in a similar reaction system, sulfoxides were used to facilitate the aerobic oxidation of alcohols [29]. In this manner, benzylic, allyUc, and aliphatic alcohols were all oxidized to aldehydes and ketones in a reaction catalyzed by Ke jn-type... 

A theoretical smdy of the structures and NMR properties of a number of /r-hydrido-bridged carbocations and carbodications has been carried out. The C-H —H bond is reported to have a non-linear, three-centre two-electron structure. Results fairly consistent with the experimental data were obtained.The carbocations derived from... 

Consider the electronic structure of the carbocation that would result from the loss of a hydride ion from 1,3-cyclopentadiene that is, cyclopentadienyl cation. How many electrons are in the 7t system Are they all paired Is this ion aromatic What are the relative energy levels of this ion ... 

---