So if you have a simple molecule, a diatomic simple molecule like,
say, H-Cl, then the only motions that the atoms can do in there
is they can move back and forth.
They can have what they call a stretching of the bond, a lengthening and
compressing.
So when you get more complex molecules, like you get in biological systems.
Then you have other types of modes.
And likely two of the molecules here on the right,
you can see that the stretching there of all the bonds.
And also, on the bottom, you can see bending of the atoms.
So, they're stretching, and there's bending going on.
And when you measure an infrared spectrum,
you're measuring the energy of these movements really.
So the vibrational motions, like these.
The energy needed to excite them, from their ground state or their excited state,
which is what all spectroscopy is about.
That energy that you need for that,
comes within the infrared region of the electromagnetic spectrum.
So, if you remember again,
support the electromagnetic spectrum from the first day.
You have the high energy regions, the gamma rays and the X rays.
Then you come along to what we talked about the last day, the UV and
then you move into the visible region.
And then lower energy from that, you have the infrared region.
So this is the region of the electromagnetic spectrum,
that you observe vibrations of molecules.
And of course in practical applications as we move to later on,
what you're trying to do, you're trying to gain from spectroscopy,
you're just trying to get some information about the structure, the geometric
structure and sometimes maybe the electronic structure of the molecule.
So that's the main points on that slide there.
So now we're gonna move into the, the theory behind it, because you need to have
an appreciation of that as well as just being able to assign assign spectra.