Stable ion structures

A stable ion structure is one which corresponds to a local minimum on a potential energy surface of an ion. It is implied that a stable ion structure in its ground vibronic state is stable with respect to unimol-ecular reaction. A great deal is known of the possible stable ion structures of particular formulae [54, 401, 517, 595]. The relevance of stable ion structures to reaction kinetics is, of course, that it is their vibrationally excited states which are assumed to be the reactants (and products) in the unimolecular decompositions and isomerisations. Identifying stable ion structures is an essential prerequisite to determining energies (heats) 

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Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

Electron delocalization can be important in ions as well as in neutral molecules Using curved arrows show how an equally stable resonance structure can be generated for each of the following anions... 

Three equally stable Lewis structures are possible for nitrate ion The negative charge in nitrate is shared equally by all three oxygens... 

The most stable Lewis structure for cyanate ion is F because the negative charge is on its oxygen... 

Write the most stable resonance structure for the cyclohexa dienyl anion formed by reaction of methoxide ion with o fluoronitrobenzene J... 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

The first step in the addition of an electrophile such as HBr to an alkyne involves protonation and subsequent formation of an intermediate vinyl cation. Where does propyne protonate Compare energies of 1-methylvinyl and 2-methylvinyl cations. Which is more stable Why Measure CC bond distance in the more stable cation. Does the cation incorporate a full triple bond (as in propyne) or a double bond (as in propene). Examine atomic charges and electrostatic potential maps to locate the positive charge in the two cations. Is the more stable ion the one in which the charge is better delocalized Use the charges together with information about the ions geometry to draw Lewis structures (or a series of Lewis structures) for 1-methylvinyl and 2-methylvinyl cations. 

The structures shown for ions, such as in Equations 7 and 8, are entirely speculative since the mass spectrum gives only relative abundance of the various m/e-groups formed. Chemical reasoning, meta-stable-ion peaks, peak shifts in the deuterated analogs, and published mass spectra of similar compounds are the bases for the proposed modes of fragmentation and for the postulated structures.)... 

Chelants, or chelating agents, typically are organic chemicals (although inorganic chelants exist) that react with polyvalent metal ions to form stable ring structures that incorporate the metal ion within the molecule. Chelants tie up metals and deactivate them. 

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