Questions tagged [ir-spectroscopy]
Use this tag for questions regarding the principles and application of infrared spectroscopy.
112
questions
2
votes
0answers
32 views
Right Raman Spectrum of Cyclohexane
In this site, https://www.princetoninstruments.com/userfiles/files/assetLibrary/Technical%20posters/Poster-Aberration-Free-Raman-Spectroscopy-with-the-IsoPlane.pdf the raman spectrum of cyclohexane ...
6
votes
1answer
167 views
Neither Raman nor IR Active vibrational modes
Is there any molecular vibrational mode (or activity) that is neither Raman nor IR active?
In other words. Is there any activity in the molecules that can't be captured by either Raman spectrometer (...
0
votes
0answers
23 views
I need to match a compound from this scheme to each of the 2 infrared spectra. Which compound goes with which spectrum and why? [closed]
I need to decide which compound in the mechanism matches spectrum A and which matches spectrum B and explain why. I am struggling to understand infrared analysis.
2
votes
2answers
92 views
Why does the dipole moment of molecules go from positive to negative? [duplicate]
In physics the dipole moment is said to go from the negative to the positive pole, check for instance https://en.wikipedia.org/wiki/Electric_dipole_moment (Elementary definition).
This makes sense ...
0
votes
2answers
74 views
Could dimerization account for weak IR signals?
How would you interpret the IR spectrum of your product if it's peaks match the literature spectrum, but the signals are weak? Could the argument that the sample was very dilute and lots of water was ...
0
votes
0answers
15 views
How can I determine the type of IR active mode on a complex molecule from molecular geometry arguments?
I am trying to synthesize 2,6,9,10-tetracyanoanthracene and I need to discriminate it from other regioisomers. I am hoping to take advantage of the C2h symmetry under an IR spectrometer.
I like to ...
1
vote
0answers
33 views
Increase of peak intensity in case of hydrogen bonding
Typical motif in case of hydrogen bonds is charge transfer from electron rich acceptor Y to (usually) anti-bonding X-H σ* orbital. If so, the X-H bonding distance increases, bond weakens, stretching ...
20
votes
2answers
2k views
Why does the weaker C–H bond have a higher wavenumber than the C=O bond?
My understanding is that a stronger bond has a higher wavenumber in IR spectrum. But why does the C–H vibration have a higher wavenumber than the C=O vibration? The latter is a double bond, so I think ...
0
votes
0answers
23 views
FTIR Band of C-C
I am looking at FTIR tables, and no one list aliphatic C-C bonds. Why?
If e.g. I have a spectrum of a short alkane, won't I see a vibration mode of the C-C bonds??
Thanks.
1
vote
1answer
63 views
Why do Ketones Have Lower Wavenumbers than Esters?
The average wavenumber for a ketone is about $\pu{1720 cm-1}$ and the average wavenumber for an ester is about $\pu{1740 cm-1}$. This, however, does not make sense, as the carbonyl group of an ester ...
1
vote
1answer
33 views
CO-vibration in metal carbonyl
I'm struggling to understand the trend of $\ce{CO}$-vibration in isoelectronic metal carbonyls.
$$
\begin{array}{lr|lr}
\hline
\text{Compound} & ν/\pu{cm-1} & \text{Compound} & ν/\pu{cm-1}...
1
vote
1answer
28 views
Where can I find the positions of the infrared band for phosphorus compounds in general? Specifically sodium hexametaphosphate [closed]
Specifically I am looking for the IR of the sodium hexametaphosphate compound.
2
votes
1answer
79 views
Why is UV-Vis Spectroscopy not like IR Spectroscopy
UV/Vis Spectroscopy is where electromagnetic radiation in the UV to Visible range is shone at a sample to be analysed. The absorbance of each wavelength is recorded and then plotted (very similar to ...
5
votes
2answers
120 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 ...
3
votes
0answers
31 views
Ozone cross-section in the near-infrared
I am searching for ozone absorption cross-section in the wavelengths range 1.1 μm to 10 μm. So far I looked in the following databases:
HITRAN
Max-Planck Institute for Chemistry Mainz
and of ...
1
vote
0answers
24 views
Alkyl insertion with cobalt and CO
Reaction of $\ce{(Cp)Co(Me)2(CO)}$ in the presence of excess $\ce{CO}$ yields two different organic products, $\ce{A}$ and $\ce{B}.$ The only metal-containing product is $\ce{(Cp)Co(CO)2}.$ The IR ...
3
votes
1answer
175 views
Can anyone help me with this question on vibrational modes of free base porphyrin (C20H14N4)?
4 a. The structure of free-base porphyrin $(\ce{H2P}$, chemical formula: $\ce{C20H14N4})$ is shown below. Given that this molecule belongs to the $D_\mathrm{2h}$ point group (character table given ...
1
vote
0answers
21 views
Is there a simpler way to detect alcohol in a blood sample using IR spectroscopy?
I have seen all sorts of ways to detect alcohol using NIRS (Near InfraRed Spectrscopy) or FTIR, but they are all quite complicated in that they are using and analyzing a very wide spectrum. In ...
1
vote
2answers
77 views
What am I doing incorrectly with this IR/ Mass spec data? [closed]
Hello, I am trying to determine the chemical formula for the following compound. From the IR spec, I see there is a carbonyl present. Moving on to the mass spec, I see an M+ peak of 142, with what ...
1
vote
1answer
71 views
How can one distinguish an IR spectroscopy alcohol band on an organic molecule versus the alcohol bands from water in the sample?
I'm reading this IR spectra and trying to determine if the large broad band at 3388 is alcohol from an intermediate or from water in the sample. The details of the reaction, conditions, and mechanism ...
0
votes
0answers
101 views
Distinguish 1-bromobutane and 2-bromobutane with IR
How can I distinguish 1-bromobutane and 2-bromobutane by their IR spectrums?
In the 1-bromobutane spectrum I think the $\ce{-CH3}$ peak is shorter, but I don't know if that is actually reasonable or ...
17
votes
5answers
4k views
Is it “common practice in Fourier transform spectroscopy to multiply the measured interferogram by an apodizing function”? If so, why?
The recent paper in Nature Independent confirmation of a methane spike on Mars and a source region east of Gale Crater is interesting and the basis is a reanalysis of 2013 data taken by a X ...
3
votes
1answer
72 views
Why are N-H stretching vibrations often sharp and not broad?
It is known that hydrogen bonding results in the broadening of peaks in the infrared spectrum of a molecule. Hydrogen bonding results in a spectrum of different bond lengths with different stretching ...
2
votes
1answer
108 views
electron donation affecting carbonyl bond strength
I have been told that electron donation decreases carbonyl bond strength and thus decreases the bond stretch frequency, but electron withdrawal increases it.
This does not seem to make any sense as ...
2
votes
0answers
48 views
CO2 IR extinction coefficients near the 15 micrometer band
I have searched multiple times for a spectrum or a table of the gaseous carbon dioxide Lambert-Beer extinction coefficients in the IR Earth thermal region.
All I found was absorption spectra.
Is ...
0
votes
0answers
109 views
Predict the aldol condenation product given IR and H-NMR spectra
An aldol condensation product would have a ketone and at least one alkene in the form of R1-C(=O)-C(R2)=C(R3)-R4
H-NMR: 7.09ppm 1H doublet, 7.42ppm 3H triplet, 7.64ppm 2H quartet, 7.75ppm 1H doublet. ...
5
votes
1answer
115 views
HCl rota-vibrational lab: ab initio simulation of rotation constants
I'm trying to set up the simulation portion of the classic undergraduate lab studying the rotavibrational spectrum of HCl and DCl. In terms of simulation (using Gaussian 16), the equilibrium ...
3
votes
1answer
185 views
Raman transition and IR transition
In a character table one can find the reducible representation of vibration of a molecule. In such a table we also see things like $xy,z,x...$. If this corresponds with an irrep we say that IR ...
9
votes
1answer
173 views
Dominant contribution of v = 0 → v = 1 transition in vibrational spectroscopy
Recently I had studied vibrational spectroscopy using the quantum harmonic oscillator model.
After stating the gross selection rule [that vibrational oscillation demands a change in dipole moment for ...
1
vote
1answer
120 views
Which organic compound's NMR and IR are these? Can't find out. (Data available: Melting Point ~ 116 °C)
Which organic compound's NMR and IR are these? I have figured out that the compound is a 1,4 substituted benzene. And the substituents are probably not -COOH and -OH from the spectroscopy. Kindly help ...
1
vote
1answer
42 views
How to distinguish two molecules with same formula but different geometry with vibrational spectroscopy
There is a sample that we know is all made up by only one of these two molecule:
how you can see they have the same formula but two different geometries. The question is if it is possible to know by ...
0
votes
2answers
186 views
Infrared Spectroscopy of N2O [closed]
Would you be able to identify the stretch and bend in the spectrum? I can not find much information about the $\ce{N2O}$.It's solved, does anyone know the symmetry of these vibrations?
2
votes
0answers
658 views
What is the peak at 3200 cm⁻¹ in the IR spectrum of Benzocaine?
Benzocaine has the following IR spectrum:
and here is it's structure:
From what I understand:
The two leftmost peaks are the antisymmetrical and symmetrical stretchs of the amine group.
Just before ...
2
votes
0answers
52 views
Why is the Infrared spectrum usually drawn using transmittance rather than absorbance?
Infrared radiation can be used to identify compounds but I noticed that the spectra are usually drawn using percentage transmittance rather than percentage absorbance . Why is that ?
1
vote
0answers
43 views
structure information form IR and Raman spectra? [closed]
Is it possible to gain the idea of full molecular structure from IR or Raman spectroscopy?
1
vote
0answers
315 views
Calculation of average bond length from force constant and IR absorption?
I need some help beginning a problem set problem (just want hints, I'd like to figure it out myself).
The problem gives me the IR absorption for a diatomic, as well as the force constant for the ...
-2
votes
1answer
97 views
Is there a detailed explanation as to why polarity of a molecule affects the absorption intensity of IR radiation? [closed]
Polar molecules have an electric field due to there being a net dipole moment. The electric field will be stronger with greater polarity. The stronger electric field in the polar molecule will ...
1
vote
1answer
82 views
Determination of Urea and Hydrogen Peroxide in Cow Milk by ATR-FTIR(Attenuated total reflection-Fourier Transform Infrared Spectroscopy) [closed]
I am doing a research on cow milk adulteration. I am checking Urea and Hydrogen Peroxide content in milk by ATR-FTIR(Attenuated total reflection-Fourier Transform Infrared Spectroscopy). I am using $\...
1
vote
1answer
57 views
How do I get Mass spectrometry graph, history, and Infra-red spectroscopy graph for Secnidazole?
My chemistry teacher wants me to create an academic poster for a recently-discovered biologically important organic molecule with chirality that is relatively small and is also recent.
Secnidazole (...
2
votes
0answers
121 views
Spectral database in the UV-Vis/NIR and IR regime
I'm searching for a bunch of spectra (some tens), preferably with intense bands in the Vis spectrum, to play around with (calculating chromaticity, etc.).
They should range from
250 nm to 2500 nm (UV-...
2
votes
0answers
96 views
Creating an IR Spectrum using Gaussian ADMP Calculation
I have a Gaussian 16 trajectory for a small cluster of water molecules carried out with ADMP technique. I was wondering if anyone knows how to compute the IR spectrum from the information in the ...
2
votes
1answer
2k 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 ...
3
votes
1answer
111 views
Where can I find example MS, IR, NMR spectra for teaching molecular spectroscopy?
I am teaching a course on analytical chemistry, and I would like to give my students MS, IR and NMR spectra to practice with.
It would be ideal if they could analyze spectra in a more "realistic" way ...
1
vote
0answers
80 views
Absence of anti-Stokes line in Raman spectrum
If we have a molecule 1H-25X which shows a band at $3000~\mathrm{cm}^{-1}$ in the IR spectrum and you want to record a Stokes and anti-Stokes Raman spectrum with a laser of 1000 nm, why is the anti-...
2
votes
1answer
2k views
Differences between Liquid and Solid Infrared Spectrometry [closed]
In general, when should I use liquid or solid IR spectrometry to analyse my compound? What should I expect to see? Would I see higher or lower frequency, broader or narrower transitions? My specific ...
2
votes
1answer
89 views
Origin of broad CO stretch in infrared spectrum of tricobalt cluster
I'm quite confident that the peaks around $\pu{2000 cm^{-1}}$ are $\ce{CO}$ stretches. But I cannot find any reference on that massive broad stretch following the $\ce{CO}$ stretches. Does it have ...
0
votes
1answer
289 views
Why do the unsaturated ketones have C-O bond weaker than saturated ketones? How it is related to resonance structures?
I am studying Infrared Spectroscopy and the book says that unsaturated ketones have a carbonyl group which has weaker bond because it's more single-bond character than single bond because it has these ...
0
votes
1answer
392 views
Stretching frequency of nitro and nitrito
I got the values of the stretching frequency of nitro and nitrito ligands from this webpage.
I understand why stretching frequencies are close in nitro and well separated in nitrito. How to explain ...
1
vote
0answers
41 views
Need help with an IR spectrum (polymer identification)
I have been given an unknow polymer and I have to identify it.
DSC measurements showed no glass transition (which means it's below room temperature) and a melting temperature at around 100-120°C. So,...
1
vote
0answers
125 views
Which bands are considered significant in the fingerprint region of IR spectra?
I'm currently writing my characterisations for the experimental section of my dissertation and can't decide which peaks from the fingerprint region of my IR are worth reporting. Is there a threshold ...
