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CH [Formula: see text] X (X = N, O, or Cl) hydrogen bonds formed intramolecularly in 2-methyl-4-(2-chloro-4,5-dimethoxyphenyl)thiazole (Ia), 2-amino-4-(2-chloro-4,5-dimethoxy phenyl)thiazole (Ib), 2-amino-4-(2,4,5-trimethoxyphenyl)thiazole (Ic), and 2-methyl-4-(2,4,5-trimethoxyphenyl)thiazole (Id) were studied by means of all-electron calculations performed with the B3LYP/6-311++G(d,p) method. Computed ground states, in the gas phase, show the presence of a single H-bond, CH [Formula: see text] Cl, in each Ia and Ib moiety, and two H-bonds, CH [Formula: see text] N and CH [Formula: see text] O, for each Ic and Id molecule. H [Formula: see text] Cl, H [Formula: see text] N, and H [Formula: see text] O distances are shorter than the sum of the X and H van der Waals radii. H-bond energies of ≅2.0 kcal/mol were estimated for Ia and Ib and ≅4.0 kcal/mol for Ic and Id. These results agree with those of the theory of atoms in molecules, since bond critical points were found for these H [Formula: see text] X bonds. Finally, the chemical shifts in the (1)H NMR were calculated by the GIAO method; in Ia and Ib they are merely due to the different topological positions of the H atoms. But in Ic and Id the shifts of H [Formula: see text] N and H [Formula: see text] O have signatures of H-bond formations.

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There are still gaps in the theory of supposedly well-known chemical reactions. For example, there is no explanation why there is a notorious preponderance of one of the expected isomers in some electrophilic aromatic substitutions. The preferred ortho orientation of acetyl nitrate has been used widely to obtain ortho nitro compounds; however, there is not causal explanation of this phenomenon. In this communication, an explanatory discussion is given to provide a complete theory on the regiochemistry of some of the mentioned reactions. To illustrate our theory, acetanilide, toluene, and anisole have been chosen as substrates in nitrations performed with nitric/sulfuric acid mixture and with acetyl nitrate.

This Review is a brief account of our theoretical contributions in seven research communications in the field of reaction mechanisms. Some mechanisms were corrected as in the case of the Baeyer-Drewsen indigo synthesis. When two very different reaction mechanisms had been proposed, as in the Clemmensen Reduction, a unified theory was provided. In other cases there were no reaction mechanisms at all, as in the Baeyer-Emmerling synthesis of indigo and in the Froehde Reaction for opioids. This deficit has been solved. The reaction that controls fructosazone regiochemistry has been described, and an internal process in a mixed osazone formation has been explained. All the proposals are based on well known reactivities and we provide complete and coherent reaction series with commented steps.