The syntheses, antioxidant activities, acidity properties, experimental and theoretical investigations of vibrational spectra (FT-IR and micro-Raman), C-13 and H-1 NMR chemical shifts and electronic properties of 3-alkyl-4-[3-methoxy-4-(4-methylbenzoxy)benzylidenamino]-4,5-dihydro-1H-1,2,4-triazol-5-one (Me, Et and n-Pr) molecules have been presented for the first time. The new compounds were analyzed for their potential antioxidant activities in three different methods. The calculations of molecular structures, vibrational frequencies, C-13 and H-1 NMR chemical shifts and electronic absorption wavelengths of the title molecules were computed by using the DFT/B3LYP method with 6- 31G(d) basis set which was used to have the structural and spectroscopic data about the mentioned molecules in the ground state and the results calculated were compared with experimental values. Furthermore, gauge invariant atomic orbital (GIAO) H-1 and C-13 NMR chemical shifts in different solvents (gas phase, DMSO and cholorofom), UV-vis. TD- DFT calculations in ethanol solvent, the highest occupied molecular orbital (HOMO-1, HOMO), lowest unoccupied molecular orbital (LUMO, LUMO+1), molecular electrostatic potential map (MEP), atomic charges and thermodynamic properties of the title compounds have theoretically verified and simulated at the mentioned level. In addition, the calculated infrared intensities and Raman activities of the compounds under study have also been reported.