Interpreting the Raman, IR, and NIR Vibrational Fingerprints of Chlorothalonil Using Anharmonic DFT Analysis

Near‐infrared (NIR) spectroscopy probes overtone and combination bands, yet its application in pesticide residue analysis is often limited by chemometric models that ignore the vibrational origin of the signals. Herein, the anharmonic NIR spectrum of chlorothalonil was computed with second‐order vibrational perturbation theory using B3LYP and B2PLYP functionals and interpreted against experimental data. Two spectrally distinct overtone features were resolved: a weak envelope near 4400 cm −1 associated with the ν (C≡N) fundamental (≈2240 cm −1 ) and a sharp, intense band at 4551 cm −1 originating from an aromatic ring 3 ν (C–C) stretching mode. Pairwise Birge–Sponer analysis quantified these behaviors. For ν (C≡N), the calculated pairwise constants were small and negative ( = −2.6 ± 2.0 cm −1 ) and harmonic estimates clustered tightly ( = 2235 ± 6 cm −1 ), consistent with a near‐local mode. An external nitrile reference, CCl 3 CN, exhibited a similar pattern, supporting the use of ν (C≡N) as an internal reference. In contrast, the aromatic ring C–C stretching modes displayed large, positive pairwise constants ( = 190 ± 7 cm −1 , = 226 ± 45 cm −1 ) accompanied by widely dispersed harmonic estimates ( = 674 ± 23 cm −1 ; = 589 ± 138 cm −1 ). These features indicated substitution‐induced quartic distortions and strong vibrational coupling with torsions and out‐of‐plane motions. These anharmonically enhanced transitions were observed experimentally and served as chemically specific markers. Guided by these markers, principal component analysis (PCA) restricted to the corresponding spectral subregions successfully differentiated spiked from unspiked kale samples, whereas full‐range PCA failed. The results establish that theory‐guided analysis isolates compound‐specific NIR features, improving the robustness of spectral residue screening in complex food matrices.