Vibrational, NMR spectrum and orbital analysis of 3,3′,5,5′-tetrabromobisphenol A: A combined experimental and computational study

In the present work, the experimental and theoretical studies on the structure, vibrations, NMR and HOMO–LUMO analysis of 3,3′,5,5′-tetrabromobisphenol A (TBBPA) are presented. The FT-IR (400–4000 cm−1) and FT-Raman (100–4000 cm−1) spectra of TBBPA were recorded. The molecular geometry, vibrational frequencies were calculated by using density functional theory (DFT) method with the 6-31G(d) basis set. The optimized geometric properties, scaled vibrational wavenumbers, IR intensities, Raman activities show good agreement with the experimental data. The assigned vibrational modes of the IR and Raman spectra were compared with the corresponding properties of the polybrominated diphenyl ethers (PBDEs). Comparative analysis indicated that the red shift of C–Br vibration could probably be ascribed to the further electronic density equalization due to the p–π conjugation between O atom and the benzene. The natural bonding orbital (NBO) analysis demonstrated that the intermolecular hyperconjugative interactions are mainly formed by the orbital overlap between σ (O–H), σ* (C–C), π (C–C), π* (C–C) bond orbitals. Compared to the higher E(2) value (33.65–34.82 kcal/mol) originated from LP(2)O to π* (C–C), the one (E(2): 8.23–9.73 kcal/mol) from LP(3)Br and π* (C–C) contributes to the preferential tendency of C–Br breakage to the C–O breakage in the transformation. The calculated NMR results obtained on the 6-31G(d) level proves good agreement with the experimental data (r2 = 0.999). Analysis of isosurface of the related orbital shows that all the main excitation exhibit π–π* character localized on the benzene rings.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The calculated structure and vibration correspond well with the experimental data. ► The calculated NMR showed good agreement with the experimental data (r2 = 0.999). ► The p–π conjugation between O and benzene leads to the red shift of C–Br vibration. ► The lower E(2) (LP(3)Br → π* (C–C)) leads to the preferential tendency of C–Br breakage. ► The main excitation by TDDFT exhibit π–π* character localized on the benzene rings.