Theoretical studies energetics

Theoretical studies of the relative stabilities of tautomers 14a and 14b were carried out mostly at the semiempirical level. AMI and PM3 calculations [98JST(T)249] of the relative stabilities carried out for a series of 4(5)-substituted imidazoles 14 (R = H, R = H, CH3, OH, F, NO2, Ph) are mostly in accord with the conclusion based on the Charton s equation. From the comparison of the electronic spectra of 4(5)-phenylimidazole 14 (R2 = Ph, R = R3 = H) and 2,4(5)-diphenylimidazole 14 (R = R = Ph, R = H) in ethanol with those calculated by using ir-electron PPP method for each of the tautomeric forms, it follows that calculations for type 14a tautomers match the experimentally observed spectra better (86ZC378). The AMI calculations [92JCS(P1)2779] of enthalpies of formation of 4(5)-aminoimidazole 14 (R = NH2, R = R = H) and 4(5)-nitroimidazole 14 (R = NO2, R = R = H) point to tautomers 14a and 14b respectively as being energetically preferred in the gas phase. Both predictions are in disagreement with expectations based on Charton s equation and the data related to basicity measurements (Table III). These inconsistencies may be... 

In our detailed theoretical study of reaction pathways of the model A-azido-A-methoxyformamide 82b we showed that decomposition by loss of nitrogen was the energetically most favourable process with an EA of only 5.3 kcal mol-1 at B3LYP/6-31G. 36 In addition this step is exothermic by 42-44kcalmol-1. Thermal decomposition of 68b to methyl formate 67b and nitrogen has an EA of only 2.9 kcal mol-1 and is exothermic by 95 kcal mol-1. Overall, the conversion of 82b to methyl formate 67b and two molecules of nitrogen is thus predicted to be exothermic by 137-139 kcalmol-1. 

A theoretical study on the structures, magnetical properties, and energetics of phosphorus analogues of borazine, E2G3H6 191 (E = B, Al, Ga G = N, P, As), compared to those of other borazine analogues, E3J3H3 192 (J = O, S, Se) and to phosphazene (194),234 shows that results could vary with function of the reference system. Some compounds with P and As heteroatoms were found to prefer nonplanar geometries (Scheme 74). Contradictory results were... 

In view of the abundant experimental studies on the boration reactions catalyzed by transition metal complexes summarized above, theoretical studies to the detailed reaction mechanisms have also been carried out [25-28]. The main focus of these quantum chemical calculations has been to provide detailed structural and energetic information on the proposed reaction mechanisms. 

There have been a small number of theoretical studies of cation ordering in LDHs. First principles molecular dynamics calculations [43] on [Mg3Al(OH)8]Cl LDHs discussed in Sect. 3.2.6 suggested that structures with adjacent aluminum cations were energetically less favorable than one without, although the chosen arrangement for the latter lacked either hexagonal or rhombohedral supercell. 

A number of theoretical studies have been performed to improve our understanding of the adsorption and conversion of methanol on acidic zeolites 245,283-288). Applying non-local periodic density functional calculations. Gale and co-workers 284) suggested that both pathways, described in Eqs. (27a, b) and (28), are energetically reasonable routes. By contrast, Blaszkowski and van Santen 286) found... 

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