A comparison of infrared spectra obtained experimentally and reproduced using the density functional theory (DFT) calculations for an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl) phenyl 4'-octylcarbonyloxybiphenyl-4-carboxylate (MHPOCBC) is being presented. The material has been prepared for investigations using infrared spectroscopy by exchanging a single carbon atom with the C-13 isotope in order to examine the carbonyl vibrational modes separately. Polarised infrared studies of the homogenously aligned cell show that the peak assigned to the stretching vibration of the carbonyl group in the non-chiral terminal chain exhibits high dichroic ratio. This effect may be explained if the transition dipole moment of this carbonyl band is considered to be exactly perpendicular to the long molecular axis. This has been obtained from the simulated molecular geometry. Such a geometry may play an important role in stabilising the anticlinic order.