Lack of elaboration in IR spectra of compounds
In IR, wavenumbers below 1500/cm are called the &quot;fingerprint&quot; region. It's very tricky to assign anything there, because the transitions don't relate to specific bonds, but come from larger structures.
If you look deeper into the literature, you will surely find an explanation for that big hump at 800something inverse centimeters.
Generally, two substances with matching fingerprint region in the IR can safely be regarded as identical, but you cannot draw conclusions about the molecular structure. Hence the name.