IR spectroscopy

[Maxxxx]

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So in IR spec, you shoot infrared at a molecule, and certain wavelengths are absorbed. So what exactly absorbs those wavelengths? Bond strength? Bond polarity? Resonance?

Specifically, I have no idea how to answer this question from TPR.

The carbonyl group is one of the most easily identified peaks in the infrared spectrum. The strength of the carbonyl band is most likely due to:

A.
stabilization of the molecule.

B.
the carbon-hydrogen stretching modes.

C.
steric hindrance imposed by neighboring alkyl groups.

D.
large changes in dipole moment as the carbon-oxygen bond contracts and stretches.

Correct Answer
Explanation:
D. Choice A is incorrect because IR stretching frequencies never arise from stabilization of the molecule. Choice B is wrong because the carbonyl band has no C—H bonds, and choice C is wrong because IR spectroscopy does not detect steric hindrance. The answer is D.

 

[phantomcello]

The molecule absorbs at a particular frequency, because that is the frequency that the molecule itself resonates at. When you have IR absorption the bonds do all sorts of fun things like stretching, moving in and out, vibration, etc. The carbonyl bond itself contract when there is double bond character between the C and the O, and stretches when the electrons in the double bond move to the oxygen - creating a large change in the dipole moment.

 

[Maxxxx]

Would it be correct to say the more a bond stretches, the stronger the absorption signal? And do you know what makes a signal appear more broad or more distinct?

 

[phantomcello]

Would it be correct to say the more a bond stretches, the stronger the absorption signal? And do you know what makes a signal appear more broad or more distinct?

Well you can't generalize to that extent, because there are many different types of bonds. But if you take a look at C=O molecules (carbonyls, aldehydes, ketones) you should be able to notice a trend.

Sure. So let's look at broadness. One of the best known examples is the -OH functional group that has a distinct broad signal in the 3100+ range or so. When you're taking an IR you have many molecules in the sample. Since you have so many molecules in the sample, they exhibit a whole range of different characteristics. Some might be more strongly H-bonded than others, resulting in different IR frequencies for the same bond. When you have the possibility of producing such a range you get a broad signal, which is basically an average of the frequencies.

 

[rlior84]

is there a good website to practice
IR Spectroscopy?