IR Spectroscopy

I was wondering why the functional groups of a complex compound usually determine its infrared spectroscopy. Do the other molecules just not interact with infrared?

What do you mean with "the other molecules"?

Those that are not the functional group of the compound.

So basically you're just wondering why C-H bonds don't have a degree of freedom with an infrared mode?

Sorry, I might have gotten this all wrong. So basically how I understand it is that a compound consists of many molecules. What I understood was that when we look at the interaction of a compound with infrared, we tend to rather look at molecules like H2O and how it absorbs infrared rather than the more complex ones in the compound.

I still don't quite understand what you're asking but I think we're getting closer. H2O has a well documented IR spectrum so I don't know why its spectrum would tell you much about what's happening around it.

What are you talking about? What do you mean by "complex compounds?"

Aren't these "complex compounds" just comprised of simpler compounds like alcohols, alkyl, c-o bonds, like ether or carbonyls?

You use IR to determine whether or not this complex compound is comprised of these functional groups that make up the overall molecule.

So what distinguishes a functional group from other molecules?

not helping you on your Gen. Chem II. Already got a bad grade because I forgot to turn in my reflection, now I'm angry, and I want others to do poorly

The molecule is one single thing. A compound is a molecule. A molecule can further be divided into smaller groups like COOH, ketones, OH, etc. Like the other user said, these groups each give rise to a specific peak in the IR spectra.

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.

So do inorganic compounds just not interact with infrared?

They do. Every molecule does.
The OH bond in water will interact differently than the OH bond in an organic compound because of the other things it's bonded to and how it's bonded (single or double).

Functional groups themselves are irrelevant to IR spectroscopy. The molecule's symmetry is what determines their peaks, i.e. which point group a molecule belongs to. Clearly an alcohol has different symmetry than it's homologous alkane counterpart, and that affects the spectroscopic fingerprint in IR.

IR spectroscopy is based around molecule bond vibration caused by absorbing light of certain wavelengths (4000-400 cm^-1).

The instrument can only detect bonds that form dipole moments, in other words bonds that are symmetrical (homonuclear) are the only one that won't be detected.

Each functional group will vibrate at a certain frequency which is what's detected and shown on the resulting graph.

>I am going to ignore most aspects of functional IR to sperg out on something that just confuses the conversation

Functional groups do matter. Symmetry is one aspect of how a bond is excited by light but the character of the molecules that form the bond decides a lot too.

There's a reason we can tell the difference between a nitrile and an alkyne in IR.

Adding on, the other bonds (like C-H) are detected also, it's just that they're display on an infrared spectra is so indistinct and difficult to make out that they aren't considered. Functional groups will produce distinct, clear peaks and so are much more reliable.

When you take the IR of a complex compound (with many different functional groups), you do get a messy bunch of peaks all over the place.

Your chemistry professor is just trying to make things easier to understand.

It's based on molecular vibration. If a bond is capable of changing the dipole moment of the molecule by vibrating, it will show up in the IR at the resonant frequency that causes the vibration. There are ways of predicting the IR spectrum of a molecule through inorganic chemistry that is a very long and boring process but is based off of looking for symmetry, making matrices describing all possible molecular motions, picking out which motions are vibrations and then looking at which vibrations will cause a change in charge distribution.