Benzimidazole nucleus is present in many biologically active compounds. The synthesis and characterization of new benzimidazole derivatives is driven by the search for enhanced physiological activity. In this contribution, three 1-propenyl isomeric derivatives of 2-methylbenzimidazole (2-MBZM) prepared from the reaction of 2-MBZM with allyl chloride, using a strong base as a catalyst. Gas chromatography-mass spectrometry (GC-MS) was selected as the method of chemical analysis to identify and quantify the products of the reaction which allowed the characterization of two diastereomers: E-1-(1-propenyl)-2methylbenzimidazole and Z-1-(1-propenyl)-2-methylbenzimidazole and one constitutional isomer: 1-(2-propenyl)-2-methylbenzimidazole. The structures of the compounds were confirmed by 1 H and 13 C Nuclear Magnetic Resonance. Also FT-Raman, FT-IR and UV-VIS spectral data in a condensed phase have been included. For the interpretation of the experimental results, the structural and spectral characteristics of 1-propenyl-2-methylbenzimidazole have been studied by performing DFT calculations for energies, geometries, vibrational frequencies and shieldings constants (GIAO method). Using the basis set 6-311+G ** and the hybrid density functional B3LYP, the experimental and calculated results were consistent with each other.