Enthalpy, 1,161 group additivity

The chemistry of carbenes in solution hits been extensively studied over the past few decades.1-5 Although our understanding of their chemistry is often derived from product analyses, mechanistic details are often dependent on thermodynamic and kinetic data. Kinetic data can often be obtained either directly or indirectly from time-resolved spectroscopic methods however, thermochemical data is much less readily obtained. Reaction enthalpies are most commonly estimated from calculations, Benson group additivities,6 or other indirect methods. 

The group additivity values were taken from Revised Group Additivity Values of Enthalpies of Formation (at 298 K) of Carbon-Hydrogen Compounds , by N. Cohen, prepared for National Institute of Standards and Technology, 1994 as Aerospace Report No. ATR-94(7263)-l. The... 

Very different enthalpies of addition are found for the acylnitrone PhC(0)CHN(0)Ph. Instead of the above exothermicity of ca 82 kJmoH for the addition of PhCHN(0)Ph and its substituted derivatives to a maleimide, the acylated nitrone adds to a variety of norbomenes with an average exothermicity of ca 120 kJmoL, some 38 kJmoH more favorable. This suggests about 40 kJmol destabilization arises from the presence of the adjacent acyl and nitrone groups. No enthalpy of formation data are available for any acylnitrone. To simulate the mutual destabilizing effect of the acyl and nitrone... 

In the method of group additivity, groups of atoms within the molecule are assigned enthalpy values. By doing this, much of the problem of the effect of interactive neighboring atoms is minimized. Only the interactive effects the groups have upon each other are left unaccounted for. These effects are taken into account somewhat by the use of structural family corrections, but in general they are relatively small. 

Equilibrium constants are close to unity, as one might expect on the basis of group additivities and from the very similar structures of the reactant and product allyl esters. However, the large reaction enthalpies and entropies suggested by the kinetic data are not reasonable, even when the rather large error limits indicated are considered. We, therefore, prefer the calculated parameters. These should be accurate to about 1.5 kcal.mole" for E . 

Kinetic results are given in Table 9. The toluene carrier technique often yields low values of the activation energies and 4-factors. Results for the benzyl esters, however, appear to be reasonably good. Note that the estimated reaction enthalpies compare favorably with the observed activation energies and that -factors are reasonable (AS 5 cal.deg mole ). Since only absolute decomposition rate coefficients were reported for the allyl esters, we have estimated the Arrhenius parameters on the assumption that the -factors were all 10 sec The activation energies so obtained are reasonable, since they compare quite favorably to those estimated by group additivities and the accepted heats of formation of the product radicals (Column 3, Table 9). 

The reaction enthalpies at 298 °K are those calculated from the corrected activation energies. Reaction entropies at mean reaction temperatures and reaction heat capacities (not shown) were estimated from group additivities and should be... 

In this section we shall briefly review the experimental data on the equilibrium (47, -47) and then move on to discuss the strong correlation between the rate coefficients for reaction (47) and the ionization potential of the alkyl radical. The direct determination of the enthalpies for reactions (47, -47) during the mid and late 1980 s produced values significantly larger than those estimated by group additivity methods [95]. The reasons for these discrepancies are discussed in the final part of this section. 

When cooperativity is ignored, contributions of AG valnes to the total free energies of binding may be added together. Approaches based on functional group additives equation (21.7) or the additivity of free enthalpy components equation (21.8) have freqnently been applied to understand and predict protein-ligand interactions. Pioneering studies in this held were performed by Andrews et al. and Lau and Pettitt. ... 

In the meantime, Benson had developed an additive approach to the thermochemistry of molecules, based on the idea that thermodynamic properties like A H29i can, at least to a certain extent, be regarded as the sum of A fH29i values ascribed to constituent parts of the molecule, such as the C-C bond or the -CH2 - group. These constituent values he called bond additivity values or group additivity values. We shall see the distinction below. Although the objective of these calculations is the standard state enthalpy of formation, superscript ° will not be used in the notation because calculated AfH29S values are approximate by definition. 

Sabbe, M. K. Saeys, M. Reyniers, M-F. Marin, G. B. van Speybroeck V. Waroquier, M. (2005). Group additive values for the gas phase standard enthalpy of formation of hydrocarbons and hydrocarbon radicals, J. Phys. Chem. A, 109, pp. 7466-7480. 

The molybdenum-alkyl mean bond dissociation enthalpies are in the expected order and in good agreement e.g. with the trend reported for Th(Cp ) R complexes (Cp = rj -C Me ) (9a). On the other hand, it is interesting to note that the group additivity rule seems to apply to the Mo-alkyl family, as shown by the excellent linear relationships 7 and 8, where N is the number of carbon atoms in the alkyl chain. 

The decomposition reactions result in a number of unsaturated oxy-hydrocarbon intermediates and radical products, for which thermochemistry is not available. The Group Additivity method (GA) [2, 20] is a fast and reliable method to estimate or check the thermochemistry of unknown or large molecules. In this work we also develop a series of new groups (for use in group additivity (GA)) to aid our evaluation of enthalpies of formation for our system. 

This study reports bond energies, enthalpy, entropy, heat capacity, internal rotation potential, and structure data for a series of unsaturated peroxides. Thermochemical property groups are developed as well for future use in group additivity estimation methods. 

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