FT-Raman and FTIR spectra, normal coordinate analysis and ab initio computations of (2-methylphenoxy)acetic acid dimer

The Fourier Transform Raman and infrared spectra of the crystallized herbicide (2-methylphenoxy)acetic acid (MPA) have been recorded in the region 4000–400 cm⁻¹. The geometry, intermolecular hydrogen bond, and harmonic vibrational frequencies of MPA have been investigated with the help of B3LYP density functional theory (DFT) methods. The calculated molecular geometry has been compared with the experimental data obtained from XRD data. The assignments of the vibrational spectra have been carried out with the aid of normal coordinate analysis (NCA) following the scaled quantum mechanical force field methodology (SQMFF). The strong doubly hydrogen bonded interface of the dimerized system is well demonstrated by the red shift in OH stretching frequency concomitant with the elongation of bond length. The most stable structure of the dimer possesses center of symmetry and interaction energy of −83.642 kJ mol⁻¹ after the basis set superposition error (BSSE) correction by the counterpoise (CP) method. The natural bond orbital analysis (NBO) ascertains that the delocalization of unpaired electron of oxygen atom onto the CO bond causes double bond character.