Vibrational spectra, structure, theoretical calculations of 3-Fluoro-4-Hydroxybenzaldehyde: With evidence of hydrogen bonding

Abstract

Fourier-Transform Laser Raman (3500-100 cm−1) and FT-IR (4000-400 cm−1) spectral measurements have been made for the solid 3-Fluoro-4-Hydroxybenzaldehyde. Geometry optimization and frequency calculations performed at two levels namely, RHF/6-31G* and B3LYP/6-31G* have yielded Cs symmetry with energies of −517.1388109 hartrees and −520.0240034 hartrees. A detailed interpretation of infrared and Raman spectra aided by the PEDs (potential energy distributions) for the computed frequencies has been reported. The PES scan yielded four distinct conformers out of which the most stable conformer is that with the orientation of CO of aldehyde group with respect to the hydroxyl group and fluorine in the cis form. The Fluorine substitution is seen to affect its structure and charge distributions in its vicinity as is seen by the NBO analysis and mulliken charges distribution.A broad IR band near 3130 cm−1, due to OH stretch shows evidence of hydrogen bonding. The aldehydic CH vibration predicted at 2801 cm−1 is observed at 2750 cm−1. Downshifting of the aldehydic CO stretch vibration predicted at 1725 cm−1 to an intense IR band at 1670 cm−1; increase in the frequency of in-plane bending vibration of OH as a shoulder absorption band at 1290 cm−1 from its predicted mode 1243 cm−1 are all further indications of OH⋯O bonding. Dimerization for the most stable conformer has been evaluated by B3LYP/6-31G* level. The dimer spectrum is seen to agree fairly well with the observed spectra. CF stretching vibration predicted at 1235 cm−1 is seen to couple with ring bending vibrations and is assigned to a strong Raman band at 1259 cm−1. The optimized structure and harmonic frequencies computed at B3LYP/6-31G* level are in good agreement with the experimental values.