Theoretical study of molecular structure, tautomerism, and geometrical isomerism of moxonidine: Two-layered ONIOM calculations

The geometries of various tautomers and rotamers of moxonidine in both anio nic and protonated forms were optimized using the two-layered ONIOM(B3LYP 6 -311+G(d,p): AM1) method. The calculations showed that, in agreement with e xperiments, moxonidine exists in a more stable imino tautomer. The tautomer containing,the amino group is less stable by about 19 kJ/mol. The computed stable conformation for the moxonidine species is characterized by the pyr imidine and imidazolidine rings being in the mutual gauche conformation to one another. In contrast to the parent neutral molecule of moxonidine, ioni zation caused considerable geometric changes in the anions compared to the neutral species. In the neutral form and anion of the parent drug, an intra molecular hydrogen bond stabilizes the structure and makes the most stable conformations more planar. The primary protonation site is the imidazolidin e part of drug. The proton affinity of moxonidine was computed to be 1004 k J/mol. The moxonidine base was found to be less lipophilic than the base of parent clonidine.