DFT computation and experimental analysis of vibrational and electronic spectra of phenoxy acetic acid herbicides

An absolute vibrational analysis has been attempted on the basis of experimental FTIR and NIR-FT Raman spectra with calculated vibrational wavenumbers and intensities of phenoxy acetic acids. The equilibrium geometry, bonding features and harmonic vibrational wavenumbers have been calculated with the help of B3LYP method with Dunning correlation consistent basis set aug-cc-pVTZ. The electronic structures of molecular fragments were described in terms of natural bond orbital analysis, which shows intermolecular OH⋯O and intramolecular CH⋯O hydrogen bonds. The electronic absorption spectra with different solvents have been investigated in combination with time-dependent density functional theory calculation. The pKa values of phenoxy acetic acids were compared.

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► Spectral investigation has been carried out herbicide compound DMPAA.
► The intermolecular hydrogen bonding in DMPAA enhances the herbicidal activity.
► Hyperconjugation and induction of methyl group causes herbicidal activity.
► DFT method is used to predict the optimized structure and vibrational wavenumbers.