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Vibrational analysis of the tautomers of the -adrenergic agonist clonidine agent and their protonated species

Elida Romano, Silvia A.Brand�?�?�?�?�?�?�?¡n

In the presentework, a theoretical study on the structural and vibrational properties of three tautomers of the-adrenergic agonist clonidine agent and their protonated species was reported by using the hybrid B3LYPmethod together with the 6-31G* and 6-311++G** basis sets. The inter-conversion between the three tautomers of clonidine and the reaction paths connecting these three structures with the corresponding transition states were studied by using the Synchronous Transit-Guided Quasi-Newton (STQN) method and those two levels of theory. TheNaturalBondOrbital (NBO) andAtoms inMolecules theory (AIM) calculations were employed in order to evaluate their atomic charges, bond orders, stabilization energies and topological properties. The analysis of the frontier orbitals suggest that the pres-ence of two Cl atoms in the phenyl rings is essential for increase the reactivity of a hydrochloride species. Moreover, theN-Hgroup between the two phenyl and imidazoline rings increase the reactivity of clonidine hydrochloride, as compared with tolazoline hydrochloride. The study of the frontier orbitals predicted that the protonated form of clonidine is the less reactive in the gas phase. The tentative vibrational assignments for the four studied species were reported together with the force constants whichwere comparedwith those obtained for similar antihypertensive agents.