Question

What type of compounds cannot be detected using infrared spectroscopy and why?

Answer 1

In general, compounds that are symmetrical AND cannot form a polar shape (namely, all homonuclear diatomics), like `"N"_2` and `"O"_2`, cannot be detected in IR spectroscopy.

Note that `"CO"_2` is symmetrical, and yet it creates peaks in an IR spectrum, since it has vibrational motions (stretches and bends) that cause its dipole moment to shift.

This change in dipole moment is detected by IR spectrometers, and so, peaks will show up (specifically near `"2360 cm"^(-1)`) for `"CO"_2` on an IR spectrum.

Note that the first vibrational motion has no dipole change, so it is IR-inactive. The remaining three are IR-active.