Hydride-affinity Cycles

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]. 

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... 

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

Of course, the gas-phase thermochemistry of ions is not solely restricted to the measurement of the quantities described above a wide range of other ion affinities have been measured, including methyl cation affinities, hydride affinities and halide affinities . Further, such measurements can often be related to unknown neutral thermochemistry via the appropriate thermochemistry cycle. For example, the phosphorus-carbon double bond strength (the sum of the a and n bond contributions) in HP=CH2 was recently estimated via mass spectrometric measurements to be 101 7 kcaF (ref. 43). 

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. 

The next step is the hydride transfer which occurs in the same way as before. The calculated barrier is only 3.0 kcal/mol. After the minor proton motion as described in Section III.A, there is again an electron and proton release. The calculated electron affinity is now 90.7 kcal/mol corresponding to a redox potential of —0.26 V. The proton affinity of the product is 289.9 kcal/mol. The entire catalytic cycle for the case with the protonated His77 is shown in Fig. 9. 

The reaction involves the hydride transfer from the substrate to the pyridine C-4 position of NAD(P)+. This transfer is usually stereospecific, being the oxidoreductase either anti or syn, depending on the rotation by 180° of the nicotinamide ring with respect to the ribose moiety. During the catalytic cycle, when formed, NAD(P)H dissociates and is replaced by an incoming NAD(P)+ indicating that NAD(P)H exhibits a weaker enzyme affinity. An oxidoreductase usually consists of two domains, a coenzyme-binding domain, like the Rossmann fold (8, 9), and an adjacent catalytic domain where the substrate binds (Fig. 2). 

The enthalpy of activation for the turnover-limiting hydrogen-atom transfer step depends upon the affinity of tire anthracene substrate for free radicals. The 9,10-dihydroanthracene is formed as the product because the radical is most stable in these positions, and the mixture of stereoisomers is formed because the process does not involve migratory insertion. This mechanism also explains why simple benzene derivatives are imreactive. The 1,4-hexadienyl radical lies too far uphill of benzene and the cobalt hydride to be a product from a reaction of a catalytic cycle that occurs under mild conditions. 

The bond in the H2 molecule is short and very strong, much stronger than in any halogen molecule, and the electron affinity of the hydrogen atom is much smaller than that of any halogen atom. In a Bom-Haber cycle, these two effects are the chief cause of the lower stability of hydrides with respect to their constituent elements. 

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