Band envelope study of vapour phase FTIR spectra of acetamide aided by DFT

The low resolution vapour phase infrared spectrum of acetamide has been recorded over the range 1800–1200 cm−1. The spectrum is complicated due to the presence of two internal rotors CH3 and NH2. In the present work, few clear band contours have been identified. The structural parameters, vibrational wavenumbers, rotational constants and the transition dipole moment vectors of the vibrational modes have been estimated from the density functional theory calculations. The PR separations have been measured and compared with the quasi-rigid model of band envelopes. The optimized molecular geometry, vibrational frequencies, rotational constants, dipole moment of acetamide have been investigated using density functional theory (DFT) at the B3LYP level with basis set 6-311++G(d,p). The results of the DFT calculations were used (i) to analyze the experimental PQR structures in the A, B, C and A/B type band envelopes for six vibrational modes in different regions, (ii) to estimate the barrier to internal rotation of internal rotors CH3 and (iii) to calculate tilt angle of the CH3 rotor’s axis. The results are in good agreement with the experimental values.