IR Spectroscopy

This user again, it's getting late and I'm gonna go, but I'll just throw out my understanding of IR spec before I go to bed. ( has it right)

When you shine IR light (or any light for that matter) on a molecule, some of it will be absorbed. What parts of it? Just the parts that match certain vibrational (or twisting/rocking) modes of each bond. There are a shit-ton of degrees of freedom for complex molecules, but the takeaway is that for each of these degrees of freedom (and the elements connected to the place where this motion is happening), there is a very specific wavelength of light that gets absorbed.

Side note: this actually happens in stringed instruments, look up sympathetic vibrato. Basically things that are vibrating tend to transfer energy to things that have the same harmonics, so if you hit a tuning fork, other tuning forks of the same note or octaves of it will vibrate. This is the basic idea. The bonds (and any kind of rocking/twisting structure) have modes (harmonics) that are determined by their structure and what they connect. There's a deeper quantum reason but I don't have time for that.
When light that has the same wavelength as these modes hits a mode structure, it gets absorbed.

EVERY molecule has an IR signature. These are just the combination of all the modes that get absorbed. Astronomers use the same basic principles to determine the chemical composition of the atmospheres of distant planets (that's more about individual atomic spectra, but they do it for gas molecules too but I'm getting sidetracked).

Basically, H2O will absorb some IR light, but in different places than a functional group. In fact, a functional group's signature can never be the same as H2O because it has different modes. In fact, all R-group signatures are different for that reason.

Does that help?

You are not making sense. What you call a compund (water, taking your example) is made of molecules (H20 molecules in the particular case of water). A molecule (of any compund) has functional groups, which determine whether the compound you are studying is capable of having any significant interaction with IR energy, and therefore suitable for IR spectroscopy.

Those functional groups determine the IR of the molecule because this specific technique allows you to study the change in rotational/vibrational energy of a chemical bond when infrared energy is applied to a sample of the compound. Because every functional group is different, such is the specific type of bonding between the atoms: these little details should be visible in IR spectroscopy

So the functional group of H2O would be the OH bond, specifically its vibration frequency?

Obviously, this is an oversimplification.

So specific bonds of a molecule determine its interaction with infrared? Example: the C=O bond in CO2.

The OH bond is not a functional group in the molecule of water, because water is not an organic compound.

This is correct.

Thank you, this clarifies it a lot. But why can inorganic compounds not have functional groups?

Exactly. Also the arrangement of the bond. Each degree of freedom adds to the spectrum, so if it can twist at a certain frequency, that frequency gets absorbed. If the bond can stretch, that vibrational frequency gets absorbed. The Wikipedia page has some good animations to get a picture of what this looks like.

H2O is not an organic compound (). It doesn't have "functional groups" by definition, but you will still see its footprint show up in IR. Usually it's ignored because it's assumed to be there. H2O as a molecule actually has at least a couple more degrees of freedom (one is like you are pinching the two hydrogens together), so the footprint is more complex than just the vibration frequency.

This is just by the definition of an organic molecule. Inorganic compounds can have the same structures, but they aren't called functional because they literally don't function in the same way.