Gas-phase hydride affinity

There is an excellent correlation between these data and the gas-phase data, in terms both of the stability order and the energy differences between carbocations. A plot of the gas-phase hydride affinity versus the ionization enthalpy gives a line of slope 1.63 with a correlation coefficient of 0.973. This result is in agreement with the expectation that the gas-phase stability would be somewhat more sensitive to structure than the solution-phase stability. The energy gap between tertiary and secondary ions is about 17kcal/mol in the gas phase and about 9.5 kcal/mole in the SO2CIF solution. 

Mutual interchanges between pKr values, gas-phase hydride affinities, and ethanolysis rate constants can, in principle, be derived from Eqs. (9-12). Because of the use of more extended data sets and of uncorrected ethanolysis rate constants, the correlations presented in Ref. 19 are slightly different. 

The relative reactivity of tertiary bridgehead systems toward solvolysis is well correlated with the increase in strain that results from conversion of the ring structure to a carbocation, as calculated by molecular mechanics. " This result implies that the increased energy associated with a nonplanar carbocation is proportional to the strain energy present in the ground state reactant. The solvolysis rates also correlate with bridgehead cation stability measured by gas phase hydride affinity andMP2/6-31 IG MO calculations. ... 

These results, which pertain to stable Jon conditions, provide a strong case that the most stable structure for the norbomyl cation is the symmetrically bridged nonclassical ion. How much stabilization does the cr bridging provide An estimate based on molecular mechanics calculations and a thermodynamic cycle suggest a stabilization of about 6 1 kcal/mol. Molecular orbital methods suggest a range of 8-15kcal/mol for the stabilization of the nonclassical structure relative to the classical secondary ion. " An experimental value based on mass spectrometric measurements is 11 kcal/mol. Gas phase hydride affinity and chloride affinity data also show the norbomyl cation to be especially stable. 

Stability of Carbenium Ions. In terms of thermodynamic stability, carbenium ions can be ranked on the basis of gas-phase hydride affinity, heats of ionization, ionization equilibrium in solution, and Sn 1-solvolysis rate (21). The stability scales obtained by these different methods correlate reasonably well... 

Figure 5.10 Absoiute (data below the structures) and relative (data near the arrows) gas phase hydride ion affinities for selected carbocations. All energies are in kcal/mol and taken from ref 21.

Figure 5.12 (a) Gas phase hydride ion affinities for carbocations with a-heteroatoms. All energies are in kcal/mol and taken from ref 26. (b) Cost of rehybridization at a heteroatom X is included (as a penalty) in stabilization provided to a carbocation by the lone piiir of X. The different trend for compounds of Group V originates from the higher cost of planarization for the heavier elements (Norbital size of X increases. ... 

Gas Phase Hydride Ion Affinities (HIA, in kcal/mol) for Selected Carbocations (in kcal/mol) ... 

Because these stability measurements pertain to the gas phase, it is important to consider the effects that solvation might have on the structure-stability relationships. Hydride affinity values based on solution measurements can be derived from thermodynamic cycles that relate hydrocarbon p T, bond dissociation energy and electrochemical potentials. The hydride affinity, AG, for the reaction... 

Thermochemical information about neutral species can also be obtained from measurements of ions. Indeed, accurate bond dissociation energies for neutral molecules have been obtained from gas-phase ion chemistry techniques. In this section, we will summarize both the negative-ion and hydride-affinity cycles involving silicon hydrides (RsSiH) which are connected to electron affinity (EA) and ionization potential (IP) of silyl radicals, respectively [22-24]. 

A further popular scale in the gas phase is hydride ion affinity (HIA)2,37 for which X =H. ... 

Fig. 2 Plot of hydride affinities (HIA) in the gas phase against values in aqueous solution at 25°C. Filled circles, alkyl cations open circles, methoxyalkyl cations triangles, vinyl cations.

Computational methods have also been used frequently to estimate the thermodynamic stabilities of superelectrophiles. These calculations have often involved the estimation of barriers to gas phase dissociation or deprotonation, and the proton affinities of conventional electrophilic intermediates. Other useful studies have calculated the heats of reactions for isodesmic processes. An interesting example of these calculations comes from a study of the protoacetyl dication (Cf COH2"1- ).42 In calculations at the 6-31G //4-31G level of theoiy, the protoacetyl dication (83) is estimated to react with methane by hydride abstraction with a very favorable... 

The influence of cyclopropyl on the gas phase stability of carbocations as measured by ion cyclotron resonance is shown in Table 14, along with data for some reference compounds. The results are given as gas phase basicities, GB, and proton affinities, PA, defined as AG° and AH°, respectively, for dissociation of the protonated molecule, as in equation 11. In addition hydride affinities D(BH H ) for some cations defined as — AH° for equation 18 are included. For the gas phase basicities and proton affinities the products B are alkenes, amines, nitriles or carbonyl compounds, and thus for these values the stability of the cation is compared to a derivative where the substituent is conjugated with a carbon-carbon or carbon-oxygen double bond, or a nitrogen lone pair, whereas for hydride affinities the products are saturated. 

TABLE 14. Gas phase basicities, proton affinities and hydride affinities for cations... 

By employing hydride ion affinity (HIA, the enthalpy change for the reaction RH -> R-hH ) [13] of a carbonyl group as a useful theimochemical correlate, the extent to which the complexation of CH3CHO to BF3 makes the carbonyl electrophilic in the gas phase was also clearly demonstrated. 

Carbocation stability in the gas phase can be measured by mass spectrometry and reported as hydride affinity, which is the enthalpy of the reaction ... 

Cubane has had an interesting place in the discussion of the correlation between C-H acidity and carbon hybridization. Its acidity was measured by the H exchange NMR technique and found to be about 6.6 x 10 as reactive as benzene. An experimental gas phase measurement of the proton affinity (PA) as 404kcal/mol is available. (See Tables 3.14 and 3.38 for comparable data on other hydrocarbons.) Both of these values indicate that cubane is somewhat more acidic than expected on the basis of the carbon hydridization. There appears to be unusual hybridization of the anion in this case. An AIM analysis suggests that the C—C bond paths in the anion are less than 90°, suggesting that the bonds bend inward toward the center of the ring. Sauers also noted an increase in s character on going from the hydrocarbon to the anion. Of the 28 deprotonations he examined, only cyclopropane and bicyclo[l.l.l]pentane also showed increased s character in the anion. 

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