From the comparison of the infrared spectra of a large number of branched normal and alkanes

the authors found that the 1157 cm-1 band is a characteristic absorption for the structure while the 973 cm-1 band is characteristic for thestructure (or H). Consequently

it appears questionable toconsider these two absorption bands as the spiral conformation of isotactic polypropylene chain. Further studies are needed to clarify this point.