MBN is a disubstituted naphthalene: a bromine atom occupies the 6-position and a methyl carboxylate group occupies the 2-position. Before this work, the authors reported no published crystallographic structure and no experimental or computational study of MBN’s structural, vibrational, optical and electronic properties. The resulting findings are a molecular characterization of a synthesis intermediate, not evidence of clinical performance or a demonstration of a working optical device.

At the B3LYP-D3/6-311++G(d,p) level of theory, a relaxed potential-energy scan generated 1,369 optimized molecular geometries by varying the C3C2C11O14 and C11O14C12H22 dihedral angles in 10-degree increments from 0 to 360 degrees. Three equivalent global minima were found, each with an energy of −3187.4993 Hartree, corresponding to the same preferred molecular geometry because of the threefold symmetry of the methyl group. No imaginary frequencies were obtained from the subsequent harmonic analysis, confirming that the optimized structure was a minimum on the calculated potential-energy surface.

The calculations treated an isolated molecule, however, so they did not include intermolecular interactions or crystal-packing effects. That limitation matters because no X-ray structure was available for direct comparison. The researchers instead compared calculated bond lengths and angles with experimental values reported for the related compound methyl 1-bromo-2-naphthoate.

The authors described general agreement but did not present the comparison as a direct structural validation of MBN. For the aromatic carbon–hydrogen bonds, isolated-molecule calculations overestimated distances relative to the comparison data, with a root-mean-square error of about 0.154 angstroms. MBN’s carbon–hydrogen stretching frequencies were nevertheless higher than those of naphthalene, and potential-energy-distribution analysis identified all six as distinct modes.

The strongest changes occurred near the bromine and ester substituents, which removed the local equivalence among naphthalene’s carbon–hydrogen sites. The vibrational comparison drew on an FT-IR spectrum recorded from a potassium bromide pellet at room temperature over 400–4,000 inverse centimeters, and a Raman spectrum measured with 785-nanometer excitation over 250–3,200 inverse centimeters. The principal aromatic carbon–hydrogen stretches were calculated near 3,047–3,079 inverse centimeters, with observed bands at 3,063, 3,040 and 3,058 inverse centimeters.

The strong carbonyl feature appeared at 1,705 inverse centimeters in FT-IR and 1,716 inverse centimeters in Raman and was assigned 87% to the C=O stretching contribution. A band at 1,096 inverse centimeters in FT-IR, with a Raman counterpart at 1,098 inverse centimeters, was characterized as a mixed carbon–oxygen stretching and in-plane carbon–hydrogen bending mode. The researchers applied harmonic calculations to the 400–1,770 and 2,880–3,100 inverse-centimeter regions, where fundamental bands dominate, and used anharmonic calculations for the 1,770–2,880 and 3,100–4,000 inverse-centimeter regions.

The latter treatment separated overtones from combination bands and allowed weak experimental features to be assigned. A relatively resolved FT-IR band at 3,410 inverse centimeters was assigned to the overtone of the carbonyl vibration, with a calculated position of 3,443 inverse centimeters. An observed band at 2,847 inverse centimeters was matched to a calculated overtone at 2,837 inverse centimeters.

Below 2,200 inverse centimeters, overlapping overtone and combination bands were assigned individually at experimental positions of 2,292, 2,183, 1,944, 1,913, 1,829 and 1,775 inverse centimeters. The study indicates that anharmonic calculations can recover information missed when analysis is restricted to conventional fundamental-frequency windows, but it does not establish that every weak band has a single unique physical origin. Ultraviolet–visible measurements were performed on six MBN solutions in chloroform spanning reported concentrations from 10⁻² to 10⁻⁷ M.

Spectra at the highest concentrations were broad and noisy, while a sample of 2.305 × 10⁻⁵ M produced the clearest structured absorption profile, which was used for comparison with calculations. The experimental spectrum showed bands at 291, 279, 246 and 239 nanometers. Time-dependent density functional calculations at the same B3LYP-D3/6-311++G(d,p) level predicted solution-phase transitions at 316.48, 298.02, 254.21 and 228.04 nanometers.

Among three tested approaches, B3LYP-D3 gave the smallest relative error in the experimental maximum absorption wavelength, 3.34%, compared with 5.60% for CAM-B3LYP-D3 and 6.36% for wB97XD. Frontier-orbital calculations assigned the predicted 316.48-nanometer shoulder mainly to a HOMO-to-LUMO transition with an 84% contribution, while the strongest calculated transition at 254.21 nanometers, with an oscillator strength of 1.1840, combined HOMO−1-to-LUMO and HOMO-to-LUMO+1 contributions. The calculated HOMO–LUMO energy gap was 4.405 eV in solution and 4.426 eV in the gas phase.

The study also estimated a total dipole moment of 2.0676 debye, a mean polarizability of 26.8085 × 10⁻²⁴ electrostatic units and a first hyperpolarizability of 11,902.1478 × 10⁻³³ electrostatic units. These calculated values were respectively about five and 31 times the corresponding reference values for urea, and the hyperpolarizability was nonzero unlike that of centrosymmetric naphthalene. These are theoretical molecular properties; the research did not fabricate a nonlinear-optical device or measure device-level performance.

Calculated reactivity descriptors gave a vertical ionization potential of 8.070 eV, vertical electron affinity of 0.815 eV, electronegativity of 4.442 eV and absolute hardness of 3.628 eV. Relative to values cited for naphthalene, the results characterize substituted MBN as less basic, more electron-accepting and less resistant to electron-density perturbation. Overall, the experiments and calculations provide a detailed baseline for MBN’s geometry, spectra and electronic structure, including assignments that would be inaccessible from a standard harmonic analysis alone.

Their potential relevance to drug, dye and optical-material synthesis lies in improving molecular understanding; the study did not test MBN in a finished pharmaceutical, establish biological efficacy, or demonstrate an application beyond the laboratory characterization.

Subject of Research: Experimental and computational characterization of methyl 6-bromo-2-naphthoate using spectroscopy and density functional theory.

Article Title: Conformational, harmonic/anharmonic vibrational frequencies, and molecular electronic investigations of methyl 6-bromo-2-naphthoate: when experiments (FT-IR, Raman, UV–Vis) meet DFT calculations

Article References: Benalia, A., Amrani, R., Boukaoud, A., Chiba, Y., Sebbar, D., Krid, A., Lekoui, F., & Abdelouahed, L. (2026). Conformational, harmonic/anharmonic vibrational frequencies, and molecular electronic investigations of methyl 6-bromo-2-naphthoate: when experiments (FT-IR, Raman, UV–Vis) meet DFT calculations. Results in Chemistry, 31, Article 103973. https://doi.org/10.1016/j.rechem.2026.103973

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DOI: 10.1016/j.rechem.2026.103973

Keywords: methyl 6-bromo-2-naphthoate, density functional theory, FT-IR spectroscopy, Raman spectroscopy, UV–Vis spectroscopy, anharmonic vibrations, molecular geometry, electronic structure, nonlinear optics, naphthalene derivatives, Conformational, harmonic