Background: Chalcones are part of a family of small phenolic compounds that are being extensively studied for presenting a diversity of molecular structures and biological activities. In this paper, two chalcones, (E)-1-(2-hydroxy-3,4,6-trimethoxyphenyl)-3-(3-nitrophenyl)prop-2-en-1-one (1), (E)-1-(2-hydroxy-3,4,6-trimethoxyphenyl)-3-(4-nitrophenyl)prop-2-en-1-one (2), were synthesized by Claisen–Schmidt condensation. Methods: The molecular structures of these chalcones were determined by Nuclear Magnetic Resonance and characterized by infrared, Raman spectroscopy, and electrochemical analysis at room temperature. Vibrational wavenumbers were predicted using Functional Density Theory (DFT) calculations, and their normal modes were analyzed in terms of potential energy distribution (PED). Besides this, DFT calculations were performed to obtain the molecular orbitals and their quantum descriptors. The UV-Vis absorption spectrum of the synthesized chalcones was measured and compared with each other. In addition, analyses of antimicrobial activity and modulation of antibiotic resistance were carried out to assess the antibacterial potential of these chalcones. Results: The vibrational spectra of polycrystalline chalcones obtained by ATR-FTIR, FT-Raman and DFT calculations allowed a complete assignment of the vibrational modes, and revealed the quantum chemical parameters. Both chalcones did not show good responses when associated with the antibiotics Ciprofloxacin and Cephalexin against S. aureus 10 and E. coli 06 strains. However, a significant potentiating of the Gentamicin activity against S. aureus 10 and E. col 06 strains was observed for chalcone 2. On the other hand, when associated with Norfloxacin, an antagonistic effect was observed. The results found for EtBr suggest that, although the tested chalcones behave as efflux pump inhibitors, probably inhibiting other efflux pumps, they were not able to inhibit NorA. Thus, these synthetic chalcones are not recommended for use in association with Norfloxacin against strains of S. aureus 1199-B that overexpress the NorA gene. Conclusions: Spectroscopic data confirmed the structure of the chalcones, and chalcone 2 showed potential as an adjuvant in antibiotic therapy.