An experimental and theoretical study on the hydration in aqueous medium of the antihypertensive agent tolazoline hydrochloride

Tolazoline hydrochloride was characterized by infrared and Raman spectroscopies in aqueous solution phases. Optimized geometries and relative stabilities for the monocation (protonated), chloride (deprotonated) and dimeric forms of the compound in aqueous solution have been calculated by means of the Density Functional Theory (DFT) method using the hybrid functional B3LYP together with the 6-31G* basis set. The solvent effects were investigated in terms of the self-consistent reaction field (SCRF) by using the Onsager and polarized continuum (PCM) models. For a complete assignment of the IR and Raman spectra in aqueous solution phases, the calculations were combined with Pulay’s Scaled Quantum Mechanics Force Field (SQMFF) methodology in order to fit the theoretical frequency values to the experimental ones. An agreement between theoretical and available experimental results was found. The presence of tolazoline monocation, chloride and dimeric forms in aqueous solution was detected in the infrared spectrum by means of the characteristic bands at 2988, 2705, 1610, 1590, 1293 and 1046 cm−1. Also, the possible charge-transfer and the topological properties for tolazoline hydrochloride were studied by means of Natural Bond Orbital (NBO) and Atoms in Molecules theory (AIM) investigation.