How could IR spectroscopy be used to distinguish between the following pair of compounds?

Question

CH2=CHCH2(CH3)2 and CH3CH2CH2CH(CH3)2

Answer 1

The first one will give you a C=C double bond stretch and the second one won't. It's been a while, but if I recall my Instrumental Analysis class correctly, the double bond is stronger and shorter, resulting in more energy and a higher frequency needed to stretch it.

Answer 2

A double bond is actually lower in energy than the single bond. (It is true however that a double bond is stronger, and shorter, than a single bond).The double bonded carbons are lower in energy because they are sp2 hybridized and the single bonded carbons are sp3 hybridized (and an s orbital is lower in energy than a p orbital, and since in sp2, s orbitals account for 1/3 of the orbitals versus 1/4 in the sp3 hybridized carbons). Therefore it takes less energy, and a higher wavelength to bend the sp2 orbitals than sp3 orbitals (E=hc/wavelength, therefore wavelength is inversely proportional to energy).
Therefore: you will see a peak for the C-H bond further to the left, at a higher wavelength, for the molecule with double bonded carbons in it, using IR spectroscopy.
Hope this helps :)