Losartan (LOS) is a drug mainly used to treat high blood pressure. It is also used for diabetic kidney disease, heart failure, and left ventricular enlargement. In the present investigation, the quantum chemical approach employed to determine the molecular structures was the Becke three-parameter hybrid method using the Lee-Yang-Par correlation functional. The basis sets used for calculations were 6-311++G(3df,2p) or pseudopotential for lanthanide atoms. The IR spectra were obtained using harmonic vibrational frequencies. They were determined by the analytic evaluation of the second derivative of the energy with respect to nuclear displacemen. Solvent effects (DMSO) were described by SMD model and D3 version of Grimmes’s dispersion with Becke-Johnson damping. The molecular calculations performed in this study were carried out using the Gaussian 16 program. The theoretical molecular geometries showed that the interaction of lanthanide ions (La, Ce, Pr, Nd, Sm, and Eu) with the 3 losartan ligands occurs through the tetrazole ring and the number of water molecules varies in the coordination sphere (6 or 7): [La(LOS)3(H2O)7], [Ce(LOS)3(H2O)6], [Pr(LOS)3(H2O)6], [Nd(LOS)3(H2O)6], [Sm(LOS)3(H2O)6], and Eu(LOS)3(H2O)6]. It is possible to observe a decrease in the wave number (displacement) of the bands referring to the vibrational modes of the tetrazole ring, ~1500 cm-1 (asymmetrical stretching), ~1300 cm-1 (angular deformation of the scissoring type) and ~800 cm- 1 (wagging-type angular deformation) in all molecular systems. These calculations are similar to those obtained for all synthesized metal compounds. Experimental data of FTIR of losartan compounds indicated vibrations stretching of NCN at 1492 cm-1 to [La(LOS)3(H2O)7] and 1491 cm-1 to others LOS compounds. The major variation of experimental data of LnLOS compounds, the angular deformation was observed at 1355 cm-1(LaLOS, NdLOS, SmLOS), 1354 cm-1(PrLOS, EuLOS), 1352 cm-1(CeLOS). So, our results can bring some light on the data performed by experimentalist groups.