Questions tagged [ir-spectroscopy]

Use this tag for questions regarding the principles and application of infrared spectroscopy.

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11
votes
2answers
62k views

Is carbon dioxide IR inactive?

Wouldn't CO2 be IR inactive because of its non-polar bonds?
2
votes
1answer
7k views

IR spectra and hydrogen bonds

Normally we see IR signals that correspond to the vibration of covalent bonds. Can a (strong) hydrogen bond (or a vibration thereof, to be precise) correspond to an IR signal as well? How about other ...
11
votes
4answers
18k views

Why are some molecules unable to absorb infrared radiation?

Methane $\ce{CH4}$ is symmetrical and non polar, and so is oxygen $\ce{O2}$. But $\ce{O2}$ doesn't absorb IR. Why is that? People say that if a molecule can change its polarity then it will absorb IR, ...
9
votes
3answers
617 views

How do I figure out how many hydrogens my compound actually has using a mass and NMR spectrum?

Question 3: It said $m/z = 122,$ and $m/z = 124$ is in a $3:1$ ratio, so I figured that meant that chlorine is present. Then I thought $m/z$ was the actual compound's molecular mass. So I used ...
5
votes
1answer
1k views

Aromatic Overtones

For IR Spectroscopy, I noticed that there are aromatic overtones or "benzene fingers" a bit below the $\mathrm{2200~cm^{-1}}$ region. Why do these occur in IR spectroscopy? And is there any way to ...
4
votes
3answers
3k views

How to identify an unknown compound with spectroscopic data?

I need some help identifying the following unknown compound. Any suggestions or hints regarding the possible compound would be highly appreciated. So far I am thinking the broad peak on the IR is an ...
2
votes
1answer
7k views

How to systematically find the reducible representation of a molecule? (eg: XeF4)

I am attempting to use the reduction formula to find the irreducible representation of $\ce{XeF4}$ to determine its IR stretching vibrations. I know the point group of $\ce{XeF4}$ is $D_{4h}$ and have ...
6
votes
2answers
289 views

If vibrational modes are quantised, why is infrared spectroscopy smooth looking?

I have heard that the vibrational energy level in a molecule is quantised. However, when looking at the infrared spectroscopy graph, the curves look highly continuous and smooth - not something I ...
5
votes
2answers
2k views

How do I find out what benzene containing compound this is with IR and NMR?

I have the following IR and NMR of an unknown substance. Can someone tell me what is a possible structure of the compound or point me in the correct direction. So far I am thinking of a benzene ring ...
3
votes
1answer
2k views

Rotational Constant for CO2 from an IR Vibrational-Rotational spectra

I have been trying to calculate the rotational constants ($B$) for $\ce{CO}$ and $\ce{CO2}$ from IR Vibrational-Rotational spectra. I know that for $\ce{CO}$ the peak spacing is approximately $2B$ (...
2
votes
1answer
272 views

Wavenumber range reversed in IR spectroscopy

I recently stumbled upon a result table from an IR spectroscopy, that is an image full of peaks. The goal was to decide which peak belongs to which group by looking up the intervals in a related table....
2
votes
1answer
77 views

Comparison of two spectra in order to find whether the irradiated sample has significantly different resulting spectra

I would like to compare two absorption spectra ( or interferograms) and conclude whether between these two there are statistically significant differences at particular wavelength intervals. At the ...
2
votes
2answers
6k views

O-H infrared band dependence of alcohol concentration in water

With regard to IR spectroscopy, how would the O-H band change as an alcohol solution was diluted with water? I am referring to any alcohol, but for the sake of example we can use ethanol. I think ...
1
vote
1answer
128 views

Methods used to compare two spectra in order to determine the effects of irradiation of sample

I would like to compare two absorption spectra (or interferograms) and conclude whether between these two there are statistically significant differences at particular wavelength intervals. At the ...