Lack of elaboration in IR spectra of compounds

I've recently started learning about IR spectroscopy, and a common trend I'm seeing in my textbook and other sources is that not all the features in the IR spectrum are explained in detail. The focus is primarily on features whose results are known, and there is quite a bit of backward reasoning in deriving the structures of compounds from their IR spectra.

Consider the IR spectrum for Decane:

Here, what are the features causing absorption in the $$\pu{720 cm^{-1}}$$ range? Morrison Boyd states that C-C stretching causes absorption in the $$\pu{1500-1600 cm^{-1}}$$ range, and the absorption in the $$\pu{3000 cm^{-1}}$$ is explained in the figure, but I am unable to explain the other absorption.

Another example is the IR spectrum for Butanamine:

Here, what is causing the large absorption in the sub-$$\pu{1000 cm^{-1}}$$ region? Wouldn't such a large absorption at this frequency indicate unsaturation? How are we deducing then that it is an alkane?

The lack of explanation is a common feature for a lot of IR spectra presented in the text. Is this normal for someone learning IR spectra at this level? Also, any suggestions for other references for learning about IR/NMR spectra in more detail are appreciated.

• Hesse, Meier, Zeeh is the standard OC spectroscopy textbook. – Karl Jan 7 at 9:32
• chemistry.stackexchange.com/questions/60019/… – Karl Jan 7 at 9:33
• Spectroscopic Methods in Organic Chemistry by Williams & Fleming is a good textbook for this springer.com/gp/book/9783030182519 – Waylander Jan 7 at 12:17
• Please remember that IR spectroscopy is just a tool. It is mostly used for identifying important functional groups. Hence, instead of analysing the fingerprint region, which is tricky, we could simply use other methods, such as HNMR (which could be explained in depth). – cngzz1 Jan 8 at 0:18