Gas-phase enthalpy of binding

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

The binding energy (-AE, ) of this structure is calculated to be 28.8 kcal/mol at the SCF level, increasing to 33.0 when MP2 correlation is added. These results are smaller than the gas-phase measurement of AH° of 36.8 kcal/mol The disagreement is amplified by the fact that BSSE and ZPE corrections have not been added to obtain a theoretical enthalpy. Like several of the systems mentioned above, the noncentrosymmetric H-bond illustrated in Fig. 6.10a, changes to a more symmetric structure, and the proton transfer potential converts from double to single-well character when correlation effects are accounted for. At the... 

Examples of thermochemical considerations of cupric enolates include the study of the binding of Cu + with kojic acid (16), a cyclic a-ketoenol. Comparison was made between the divalent cations of U02 +, Cu +, Zn +, Ni +, Co +, Cd +, Ca + where these metals are listed in decreasing order of binding constants over 6 powers of 10. In this case carbon-bonded metal seems most unreasonable because it would ruin the chelation as well as any aromaticity in the pyrone ring. It is admittedly an assumption that pyrones are aromatic. There are no one-ring pyrones for which there are enthalpy of formation data for gas phase species, as opposed to the benzoannelated compounds coumarin (I7)i07a, I07b chromone (is) " " "and xanthone (19) . Plausible, but unstable, Cu(II) enolates eliminate copper and form the 1,4-dicarbonyl compounds as shown in equation 8. 

DFT calculations also provide the total energies of the structures and, combined with calculations of gas phase ammonia, it is possible to compare stabilities and reaction enthalpies of the different steps of the decomposition. This clearly shows that the binding energy per ammonia molecule is lowered as more ammonia is absorbed in the crystal. 

Organic thermochemistry usually deals with molecules in the gaseous state in order to study their intrinsic stabilities in the absence of a crystal lattice, intermolecular bindings in the liquid state, or solvation forces. Therefore the determination of the enthalpy of vaporization or the enfrialpy of sublimation is an essential step in obtaining the enthalpy of formation in die gas phase. The enthalpy of sublimation can be obtained by combination of the enthalpy of vaporization and the enthalpy of fusion (equation (12)). The entibialpy of sion is easily and reliably obtained by DSC. 

The computed standard state enthalpies of these surface reactions are 0.47 eV and -0.09 eV. If we compare these surface reactions to the gas-phase species, we can see the effect of the catalyst on the energetics. The reactions are less favorable on the surface than they are in the gas phase. This means that the surface binds the reactants more strongly than it does the products. We also observe that the surface reaction that produces nitrogen is more favorable than the one producing ammonia, contrary to the gas-phase trend. This illustrates the ability of a catalyst to influence selectivity. 

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