A series of progression bands observed in the infrared spectra of nylon-m/n and their model compounds have been interpreted in a new manner on the basis of simply coupled oscillator models of zigzag alkyl chains. Nylon-m/n possesses the methylene sequences of (CH2)(m) and (CH2)(n-2), and so the effective models of m and n-2 coupled oscillators, respectively, had previously been assumed for the methylene rocking-twisting mode, for example. However, the spectral patterns of progression bands predicted by this previously proposed. model have been found to be inconsistent with those observed for many kinds of nylon samples with various m and n values. It is rather reasonable to assume that the effective numbers of oscillators should be m-2 and n-4 for the methylene rocking, twisting, and wagging modes of the (CH2)(m) and (CH2)(n-2) sequences, respectively. In other words, the infrared progression bands observed for methylene local modes of nylon-m/n may be interpreted reasonably with the data of n-alkane molecules with the chemical formulae CH3(CH2)(m-2)CH3 and CH3(CH2)(n-4)CH3. For the C-C stretching modes, the equivalent n-alkanes are CH3(CH2)(m-1)CH3 and CH3(CH2)(n-3)CH3, respectively. In the simply coupled oscillator model, the vibrational mode of one methylene group is represented by an oscillator, for example. Our new concept is to isolate the terminal oscillator adjacent to the amide group from the other oscillators in the inner parts of the methylene zigzag sequence. This corresponds to a physical situation in which the methylene group adjacent to the amide group shows a different vibrational behavior of larger amplitude than those of the inner methylene sequence, as supported by broad-line NMR data and molecular dynamics calculations reported in the literature. Another possibility is a difference in the electron structure of the methylene unit adjacent to the amide group from that of the inner methylene sequence, resulting in a difference in the force constant and giving a vibrational decoupling between these two types of methylene units. (C) 2003 Wiley Periodicals, Inc.