Theoretical calculations formation energies

The absolute values of the calculated vacancy-formation energy for fee metals nickel, copper, and palladium are in good agreement with the experimental data. The difference between predicted and measured values is less than 4%. The theoretical vacancy-formation energies for bcc metals vanadium, chromium, tantalum show larger discrepancies, up to 25-36%. For niobium, molybdenum, and tungsten the theoretical values are closer to the experimental ones. 

Theoretical calculations explain the photochemical behavior of phenylthiazoles (Fig. 14) (99MI233). The RCRE mechanism cannot be invoked because the radical intermediates have higher energies than the corresponding triplet states. Furthermore, the formation of the Dewar isomer is favored in comparison with the formation of the zwitterionic intermediate. Nevertheless, the reaction conditions used by Kojima and Maeda could allow for an endothermic reaction giving this type of intermediate. The same results were obtained using 2,5-diphenylthiazole. 

In a study of the methane complex [(diimine)Pt(CH3)(CH4)]+ (diimine = HN=C(H)-C(H)=NH), relevant to the diimine system experimentally investigated by Tilset et al. (28), theoretical calculations indicate preference for the oxidative addition pathway (30). When one water molecule was included in these calculations, the preference for oxidative addition increased due to the stabilization of Pt(IV) by coordinated water (30). The same preference for oxidative addition was previously calculated for the ethylenediamine (en) system [(en)Pt(CH3)(CH4)]+ (151). This model is relevant for the experimentally investigated tmeda system [(tmeda)Pt(CH3)(solv)]+ discussed above (Scheme 7, B) (27,152). For the bis-formate complex Pt(02CH)2, a a-bond metathesis was assumed and the energies of intermediates and transition states were calculated... 

While the neutral 1,2,3,4-oxatriazoles (1) still await synthesis, some of their properties have been predicted by theoretical calculations. AMI calculations combined with a principal component analysis loading data from other related heteroaromatics have been used to estimate geometric characteristics, aromaticity, energy of formation, and N chemical shifts <90JPR885>. The oxatriazoles (1) and (7) and the 1,2,3,5-thiatriazoles, which also have not been prepared, are calculated to be in the group with the lowest classical and magnetic aromaticity. 

Theoretical studies have been done in order to understand this behavior difference. Semiempirical calculations (AMI, MNDO) of formation energy (of the hemithio-ketal-hemiketal interconversion) have shown that hemithioketals are less stable than the corresponding hemiketal (from 10 to 15 kcal/mol). This difference can be due to steric factors, connected to the respective sizes of sulfur and oxygen. Stereoelectronic factors can also be evoked stabilization that is brought about by the anomeric effect is a priori more important for a gem-dihydroxylated compound than for the hemi-thioketal. Moreover, at the kinetic level, displacement of the water molecule of the inhibitor (under aqueous conditions, the inhibitor is hydrated) by the thiol of the enzyme is a slow and disfavored reaction. In contrast, the same reaction is favored with the hydroxyl of a serine. Experimentally, equilibrium occurs very slowly with the enzyme as well as with model molecules. ... 

Theoretical calculations support the expectation that the preferred site of initial OH attack is ortho to the methyl group (Andino et al., 1996), but addition to the other positions also occurs. If the OH-aromatic adduct, which contains 18 kcal mol-1 excess energy, is not stabilized, it decomposes back to reactants, reaction ( — 62). The existence of the adduct in the case of the OH-benzene reaction has been observed spectroscopically (Fritz et al., 1985 Knispel et al., 1990 Markert and Pagsberg, 1993 Bjergbakke et al., 1996). As expected for such a mechanism, the rate constants at temperatures below 300 K exhibit a pressure dependence at lower pressures. At higher temperatures, the rate of decomposition of the excited adduct back to reactants is higher, so the net contribution of adduct formation to the overall reaction is small compared to H-abstraction. 

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