Experimental FTIR and theoretical investigation of the molecular structure and vibrational spectra of acetanilide using DFT and dispersion correction to DFT

The FTIR spectrum of acetanilide (ACN) is recorded and analyzed. The optimized molecular structures, harmonic vibrational wavenumbers and corresponding vibrational assignments of the ACN are computationally examined by using the B3LYP density functional theory method together with the standard 6-311++G(d,p) basis set. From the calculations, the ACN is predicted to exist predominantly in trans configuration with the relative energy, rotational barrier, and population of 2.8 kcal/mol, 14.8 kcal/mol, and 99.5%, respectively. The optimized structure shows that the amide group (CO-NH) of trans-ACN adopts a planar peptide-like conformation. The effect of the incorporation of dispersion correction to the B3LYP on the calculated equilibrium structure and vibrational spectra of ACN is investigated. The highest occupied and the lowest unoccupied molecular orbitals, IR intensities and molecular electrostatic potential results are reported. In addition, reliable vibrational assignments have been made on the basis of Potential Energy Distribution (PED) using VEDA4 program. Simulated IR spectrum are compared with the experimental FTIR and FT-Raman spectra. Energy decomposition analysis (EDA) revealed that Pauli repulsion is responsible for the increased stability of the trans over the cis isomer.