TBR Spectroscopy

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In table of IR absorbances there are only 5 bond types that exhibit bending. They are:

O-H (H-bonding)
C-H aromatic
C-H alkenes
C-H alkanes
O-H acids (H-bonding)

What is it about these bond types that allows them to exhibit stretching and bending while the other listed bond types only have stretching? Another question that I had is why C-H aldehyde has two bands and none of the other bond types do.

 

[t5Nitro]

Somebody more experienced with IR spectroscopy may be able to answer your question more fully, but it is far too in-depth for MCAT purposes.

There is the near IR and far IR spectrum. Near IR ranges between about 0.75 micrometers to 20 micrometers. Far IR ranges between 20 micrometers and 1000 micrometers. Near IR (closer to the visible spectrum) has molecular transitions of rotations and vibrations. The far IR is usually restricted to rotations. Vibrations are your bends and stretches. Each frequency of IR EMR has a unique energy associated with it, and it can cause these molecular transitions within the molecules. Imagine a single atom, it won't bend. Molecules will be able to bend because of the electron domain between two atoms. These electrons can be excited into higher energy orbitals, causing these transitions. I believe bond strength can definitely play a role in stretching, but you may need to look this up.

The numbers you identify those key peaks are measured by wavenumber, the inverse of wavelength (you'll see it as cm^-1), and a specific energy corresponds to each.

Overall, maybe this helps clarify some things, but I am no expert in IR spectroscopy, and you definitely wouldn't have to be for the MCAT.