By deuterizing the chiral and achiral alkyl chains separately, we have observed the ir absorbance vs polarizer rotation angle for the C[Formula Presented] and phenyl ring stretching peaks in a prototype antiferroelectric liquid crystal, 4-(1-methylheptyloxycarbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC). The angle between the chiral chain and core axes is more than 54.7° (the magic angle) even in the smectic-[Formula Presented] (Sm-[Formula Presented]) phase and the precession appears to be biased or hindered toward the tilt-plane normal in the chiral smectic-[Formula Presented] (Sm-[Formula Presented]) phase. © 1996 The American Physical Society.