We have synthesized 4-(1-trifluoromethylheptyloxycarbonyl)phenyl-4’-nonylcarbonyloxybiphenyl-4-caboxylate with deuterated achiral chain (TFMHPNCBC-d(19)), which enantiotropically exhibits the antiferroelectric hexatic SIA* phase, and studied the orientational structure of the alkyl chains by polarized IR spectroscopy. The extremely bent chiral alkyl chain structure that was already observed in SC* and SA is confirmed even in STA*. Hence, contrary to the previous conclusion by Neundorf et al. [J. Mater. Chem., 4, 997 (1995)], the antiferroelectricity both in SCA* and SIA* results from the same type of intermolecular interactions among dipole moments near the smectic layer boundaries.