Activation entropy theoretical calculations

Hcuts et a .,64 while not disputing that penultimate units might influence the activation energies, proposed on the basis of theoretical calculations that penultimate unit effects of the magnitude seen in Ihe S-AN and other systems (i.e. 2-5 fold) can also be explained by variations in the entropy of activation for the process. They also proposed that this effect would mainly influence rate rather than specificity. 

The theoretical calculations described have recently been supported by an extraordinary kinetic analysis conducted by Vanrysellberghe and Froment of the HDS of dibenzothiophene (104). That work provides the enthalpies and entropies of adsorption and the equilibrium adsorption constants of H2, H2S, dibenzothiophene, biphenyl, and cyclohexylbenzene under typical HDS conditions for CoMo/A1203 catalysts. This work supports the assumption that there are two different types of catalytic sites, one for direct desulfurization (termed a ) and one for hydrogenation (termed t). Table XIV summarizes the values obtained experimentally for adsorption constants of the various reactants and products, using the Langmuir-Hinshelwood approach. As described in more detail in Section VI, this kinetic model assumes that the reactants compete for adsorption on the active site. This competitive adsorption influences the overall reaction rate in a negative way (inhibition). 

Theoretical calculations by Yamabe <2001PCA7281> supported Olsen s conclusion of an unsymmetrical yet concerted biradical as transition state. Theoretical calculations for activation and entropy change for tetramethyl diazetines 66b are in good agreement with the experimentally observed value (AH cxi = 35.3 kcal mol-1, AH exp = 31.7 kcal mol-1 = 0.94 cal mol-1 AY xp = 0.3 0.8calmol-1). Tetramethyl diazetine 66b was... 

K. N. Houk, N. G. Rondan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivities, Tetrahedron 1985, 41, 1555. Calculations on carbene addition reactions led to a general explanation of why it is possible for very exothermic, bimolecular reactions to have negative activation enthalpies. 

Adapted from Cortright et al. (55). Difference between theoretical A G° (see Table V) and the fitted A G° of this table. Fitted parameter is the rato of surface entropy to the DFT-predicted entropy for the adsorbed species. The value of this parameter was found to be 1.4 0.15. Parameters maintained at values predicted by DFT calculations (see Table V). Value constrained such that adsorbed hydrogen does not exceed two degrees of translational mobility. - Fitted parameter is the ratio of surface entropy of the activated complex to the DFT-predicted entropy of that activated complex. The value of this parameter was found to be 0.86 0.13. Value constrained to be within 50 kJ/mol of the theoretical value. 

Theoretical estimates of the height of the barrier to inversion have been made by Shoppee (1946), 9-10 kcal mole-1, and Beckett et al. (1947), 14 kcal mole-1. A more recent theoretical estimate is that by Hendrickson (1961), 12-7 kcal mole-1. The symmetry of the intermediate boat form assumed in these calculations suggests an entropy of activation of 4-9 e.u. The diagram of Jensen et al. (1962), given as Fig. 21, is undoubtedly quite... 

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