Hydride affinities

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... 

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. 

Table S3. Solution Hydride Affinity of Some Carbocations...

The phenyl cation is an extremely unstable cation, as is reflected by the high hydride affinity shown in Table 5.2. In this case, the ring geometry opposes rehybridization so the vacant orbital retains sp character. Because the empty orbital is in the nodal plane of the ring, it receives no stabilization firom the n electrons. 

Table 8.6 shows the hydride affinities calculated for a related series of compounds at... 

Hydride Affinity (HA) The 298 K enthalpy required to remove a hydride to form the neutral molecule... 

Other Ion Affinities Binding affinities for many different types of ions to neutrals are defined analogously to hydride affinity, as the 298 K enthalpy required to dissociate the complexed species. The ion can be a cation or an anion. Conversely, ion affinities can be described in terms of the dissociation. 

Although this discussion refers to properties of neutral molecules, the substrates A and AH can in principle be ionic. Eor example, the hydride affinity can also be described for a cation... 

Positive ion Approaches The positive ion approaches commonly used to determine BDEs in saturated hydrocarbons are less amenable to the investigation of BDEs in radicals mainly because of the challenge in measuring the cation hydride affinities (HA(R )). However, photodissociation has been used in select cases. Alternatively, cation enthalpies of formation can in principle be obtained by dissociation of disubstituted precursors... 

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]. 

Thermodynamic properties related to R3SiH can also be obtained from the hydride-affinities of RsSi" ". The following thermochemical cycle (cf. Scheme... 

Ion cyclotron resonance (ICR) spectroscopy has been used to determine the reaction enthalphy (A//r) of hydride-transfer reaction of silanes with various hydrocarbons having known hydride affinities (Reaction 2.19). The hydride affinities of R3Si+, D//(X3Si+—H ) = Affbase, were obtained from Equation (2.20) and are summarized in Table 2.6 [30,31]-... 

The use of CO2 as a reagent for synthetic purposes would be highly desirable, due not only to the vast availabiUty of this gas but also its environmental concerns. The stoichiometric activation of CO2 has been achieved with the iridium-PCP complex 29 comprising an alkyl rather than an aryl skeleton (Scheme 12.12) [32]. The addition of CO2 to the dihydride complex results in C=0 insertion into the iridium-hydride bond, and affords the formate complex 30. However, this complex is not stable and disproportionates spontaneously into the virtually insoluble bicarbonate complex 31 and the carbonyl dihydride 32. Such disproportionation is suppressed when the iridium metal center is replaced by rhodium [33], which is generally assumed to have a lower hydride affinity than iridium. 

The hydride affinity scale is also applicable to aqueous solution. In analogy with Kr we can take H30+ as reference as in Equation (8). 

Perhaps less obviously, the hydrocarbon also provides a reference for the carbocation. It is worthwhile examining the implications of such a reference, by considering briefly thermodynamic measurements of carbocation stabilities in terms of heats (enthalpies) or free energies of formation. Mayr and Ofial contrast our ability to measure the relative energies of tertiary and secondary butyl cations with the significant differences in relative stabilities of secondary butyl and isopropyl cations derived from different equilibrium measurements, namely, hydride, chloride, or hydroxide ion affinities. It is convenient to focus on this example and to assess the effectiveness of hydride affinities for comparing the stabilities of these three ions. 

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.

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