Orthogonal FTIR, Raman and UV-vis spectral fingerprinting with DFT-assisted discrimination of broflanilide phototransformation products.

Broflanilide (BFA), a meta-diamide insecticide, forms structurally related phototransformation products that can be difficult to discriminate by a single analytical technique. Here, six major photoproducts (PP1-PP6; isolated purities 95.3-97.4%) were constrained by high-resolution MS/MS and 1D/2D NMR and differentiated using FTIR, Raman and UV-vis fingerprints supported by DFT/TD-DFT calculations. B3LYP-D3(BJ)/6-311 + G(d,p)/SMD(acetonitrile) calculations reproduced the ordering of diagnostic vibrational features; for the characteristic peak pairs used for assignment, the mean absolute error (MAE)/root-mean-square error (RMSE) was 25.7/35.3 cm-1 for FTIR and 28.3/37.6 cm-1 for Raman. TD-DFT captured the direction of chromophore changes but showed non-negligible absolute wavelength offsets: across nine matched UV-vis bands, the MAE was 17.7 nm and the maximum absolute deviation was 31 nm. PP1 provided the strongest spectral-electronic discriminator, with an experimental long-wavelength band at 292 nm (calculated 320 nm) and the smallest HOMO-LUMO gap of 3.14 eV. MEP extrema ranged from Vmax = 38.84-80.61 kcal mol-1 and Vmin = -54.81 to -48.38 kcal mol-1; comparison with condensed Fukui responses localized candidate reactive regions at carbonyl/amide centers, halogen-bearing aromatic positions and newly introduced heteroatom sites. The orthogonal evidence supports a branched BFA phototransformation network and provides a quantitatively error-bounded strategy for differentiating closely related photoproducts when complete reference-standard libraries are unavailable.