| Popis: |
We have characterized tautomers and conformers (T/Cs) of 4-methyl, 4-chloro, 4-nitro, N α methyl histamines (4(5)-2′-aminoethylimidazole), amthamine (2-amino-5-(2′-aminoethyl)-4-methylthiazol) and amthamine derivatives (2-amino-5-(2′-aminoethyl)-thiazole, and 4-methyl-5-(2′-aminoethyl)-thiazole) neutral and as monocations by means of the RHF wavefunctions at the 6-31G ** //6-31G ** level, using analytical gradients. All these compounds are agonists of the H 2 receptors of histamine. Most stable T/Cs of histamine derivatives are gauche with an internal hydrogen bond (HyB). These HyBs have been characterized with the help of the topological analysis of the charge density. Monocations are also gauche with stronger HyBs, in both N (1,3) –H and N (3) –H tautomers. Amthamine and amthamine derivatives show trans conformations as the most stable structures. N (3) –H gauche monocations are the most stable structures. gauche conformers present electrostatic internal interactions characterized by means of topological analysis of the charge density, giving bond critical points. We have calculated proton affinity (PA) differences relative to ammonia with the help of isodesmic reactions. All histamine derivatives are strong bases in the gas phase, especially methyl derivatives. Amthamine yields a conformer structure and PA very close to that of the histamine. Two other amthamine derivatives yield lower PAs than that of amthamine. Reactivity of histamine as well as substituent effects are discussed. Possible interactions with the H 2 receptors of histamine are also discussed. |
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