Questions tagged [ir-spectroscopy]

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

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19
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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 ...
18
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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 ...
15
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1answer
5k views

In rovibrational spectra, when exactly is the Q branch forbidden?

I'm aware that the Q branch ($\Delta j = 0$) is usually not observed in rotational-vibrational spectra of diatomic molecules in the gas state, however, I've heard different things about when exactly ...
14
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2answers
8k views

Why does the carbonyl group in an acid anhydride have two stretching frequencies?

The carbonyl stretching frequencies for an acid anhydride are approximately $1820~\mathrm{cm^{-1}}$ and $1760~\mathrm{cm^{-1}}$. These are both higher frequencies than a simple ketone owing to the ...
12
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2answers
16k views

Why do molecules have to have a change in dipole moment in order for them to be IR active?

My chemistry textbook keeps saying that in order for a molecule to be able to absorb infrared radiation, it has to have a change in dipole moment when the bond vibrates. I don't understand why that ...
11
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3answers
2k views

Why does the C–C bond have extremely weak absorptions?

My book, Pavia's Introduction to Spectroscopy, tells me this about the infrared spectrum of alkanes: $\ce{C-C}$ : Stretch not interpretatively useful; many weak peaks. This is very interesting. ...
11
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2answers
62k views

Is carbon dioxide IR inactive?

Wouldn't CO2 be IR inactive because of its non-polar bonds?
11
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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, ...
11
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1answer
546 views

How to determine conformation using IR

In my advisor's Ph.D. research, she used IR spectroscopy to (help) determine the conformation of a compound of interest (its structure is known.) In organic chemistry, I learned about using IR as a ...
11
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1answer
128 views

What equipment is used for two-dimensional IR spectroscopy and how can the molecular structure of a compound be resolved?

On the Wikipedia page about two-dimensional IR spectroscopy it is said that 2D IR spectroscopy can be used to study the molecular dynamics with femtosecond time resolution. According to Wikipedia ...
9
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3answers
615 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 ...
9
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1answer
7k views

In infrared spectroscopy, why is there no C=C reading for E alkenes?

For the (E)-2-pentene, there was no C=C reading, about 1600. For the (Z)-2-pentene, there is a C=C. Why is there a reading on the Z but not on the E?
9
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1answer
233 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 ...
9
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0answers
3k views

Why do asymmetric stretches occur at higher wavenumbers than symmetric stretches? [duplicate]

I learned that the $\ce{N-H}$ bonds in a primary amine lead to two signals the infrared spectra at $\sim \pu{3400 cm^-1}$, one for symmetrical stretching and another for asymmetrical stretching. For ...
8
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2answers
214 views

Harmonic frequencies (IR) and basis set size

Usually vibrations (harmonic), derived from harmonic approximations are bigger than for experimental values. The question is, how does this depend on the size of the basis set. Many papers give that ...
7
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2answers
804 views

Why does ethylamine have two infra-red spectra? Are both correct?

The one I have in my textbook is this: Shouldn't the first trough at $3500$ to around $3000$ mean that it's an $\ce{O—H}$ group with a $\ce{C—H}$? And then the other trough at $1700$ be a $\ce{C=O}$? ...
7
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1answer
3k views

What does the peak separation of carbon dioxide IR rotational-vibrational spectrum corresponds to and why?

Spectrum of $\ce{CO2}$ at $2349\space \mathrm{cm^{-1}}$ region For some other molecules (for example $\ce{CO}$) the peak separation corresponds to $2B$ (where $B$ is the rotational constant ...
7
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1answer
807 views

Why does nitrous oxide have 300 times the global warming potential of CO2?

Both nitrous oxide and carbon dioxide have roughly the same atmospheric lifetime (nitrous oxide slightly longer), so I thought that it would have to do with the infrared absorbance associated with ...
6
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2answers
288 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 ...
6
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1answer
599 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 (...
6
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1answer
203 views

How to interpret intervals on a FT-IR spectrogram of soil samples?

I am studying to analyze the IR spectra of soil samples. And I'm interested in the question - how to interpret the intervals in (1750..1820) and (3020 ... 2820) of the two samples on the graph? The ...
5
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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 ...
5
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2answers
2k views

How is the position of a peak on an IR spectrum determined?

In an IR spectrum, all sources describe the position of a peak as a range. The ranges seem to be defined arbitrarily. Take, for example, the IR spectrum of ethanol (which I can't upload for some ...
5
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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 ...
5
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2answers
10k views

Use of water in IR spectroscopy

I am familiar with the process of IR spectroscopy and the preparation of IR samples. However, I am curious as to why you cannot use hygroscopic or protic solvents for IR spectroscopy. Is it just ...
5
votes
1answer
207 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 ...
5
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1answer
4k views

Assignment of the IR spectrum of 2,4-pentanedione (acetylacetone)

The IR spectrum is shown here: My assignments so far: The 1728 and 1708 stretches correspond to $\ce{C=O}$ stretches in the keto form - the symmetric/asymmetric stretches respectively. The 3003 ...
5
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0answers
171 views

FT-IR background lines

I started doing IR spectroscopy a few months ago and have run into the following problem: Here's the IR background that everyone knows and loves: CO2 peaks in the middle, water lines on either side. ...
5
votes
1answer
623 views

What is the signal at around 2090 wavenumbers in the nitroguanidine FTIR spectrum?

There is a signal at around 2090 apparent in the FTIR spectrum of nitroguanidine from Sigma-Aldrich (first image). It's also apparent in FTIR spectra we've taken in the lab (second image), but when ...
4
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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 ...
4
votes
2answers
26k views

Why is the carbonyl IR frequency for a carboxylic acid lower than that of a ketone whilst an ester is higher than a ketone

The approximate stretching frequencies are as follows: Acid: $1710$, Ketone: $1715$, Ester: $1730$ (all $\mathrm{cm^{-1}}$) This would seem to suggest that the acid decreases the carbonyl bond ...
4
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1answer
964 views

Explanation for increasing wave number trends for hydrocarbons

At my university, we had an organic chemistry exam question that stated,"The trend in the values of the ir absorbtions for the c-h bonds of methane ($\ce{CH4}$: $2995\,\mathrm{cm}^{-1}$), (ethylene: $...
3
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1answer
1k views

What exactly is the long chain band?

I have progressed quite enough in infrared spectroscopy, and now I'm looking at the spectrum of alkanes in detail. I found out that straight chain alkanes have this long chain band in the spectrum: ...
3
votes
1answer
242 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 ...
3
votes
1answer
285 views

Identify the structure of compound with C₉H₁₀ formula using IR and NMR data [closed]

Use IR and NMR data to define the structure of the molecule, the formula of the given structure $\ce{C9H10}.$ On what basis do we conclude that its structure is that (the answer) only? On what points ...
3
votes
1answer
213 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 ...
3
votes
2answers
5k views

Why does an OH bond absorb a higher frequency than a CH bond?

I'm wondering why a bond between an O and an H is going to absorb a higher frequency than a bond between a C and an H (any hybridization). O is heavier than C in terms of atomic masses, and I'm ...
3
votes
1answer
328 views

Reasonably complete infrared spectral database with data in CSV form?

I'm currently developing a neural network that analyses IR spectroscopy signatures. I've found a very complete data set at The Spectral Database of Organic Compounds, but data is only available as ...
3
votes
1answer
23k views

How to determine force constant of C-C bond in ethanol?

How would one determine the theoretical force constant of the C-C bond in ethanol? We are asked to calculate the frequency of the C-C bond (theoretical), but to do this we need a force constant, which ...
3
votes
1answer
2k views

Difference between torsion, out of plane, coplanar and perpendicular bends?

In Infrared spectroscopy, stretches are easily understandable. But, how do I visualize (or conceptually understand) the difference between dihedral (torsions), out of plane bends, coplanar bends, and ...
3
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1answer
68 views

Atmopsheric Gas IR-Spectra

I am looking for normal resolution IR-spectra of: $\ce{O2}$ and $\ce{N2}$ at high concentrations and; common minor gas in atmosphere: $\ce{H2O}$, $\ce{CO}$, and $\ce{CO2}$ at environmental ...
3
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1answer
1k 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 ...
3
votes
1answer
201 views

Wavelength-dependence of refractive index in internal reflection spectroscopy?

In many literature sources (web example), only a single value for the refractive index is assumed for the infrared element, and another is typically assumed for the sample substance. However, should ...
3
votes
2answers
68 views

How do IR spectrometers produce light spanning the entire infrared spectrum?

An IR spectrum covers thousands of wavenumbers, typically $\sim 4000$ to $\pu{400 cm-1}$. How are the spectrometers able to generate different wavelengths of light covering this entire range of ...
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 $2B$ (...
3
votes
1answer
94 views

Identifying the molecular structure of compound with sum formula C8H14O3 formula using NMR and IR spectrometry data

I am looking to identify the molecular structure of a compound with the sum formula $\ce{C8H14O3}$ using the NMR and IR spectrometry data below. I was able to figure out the molecule as My friend ...
3
votes
2answers
165 views

H-NMR with 10H integration?

I have the following two spectra (IR and 1H-NMR) see below. The chemical formula is: C16H16O2 Thus DBE = 9 My approach: From the IR, I can see that there's no broad peak around 2500-3300, thus I ...
3
votes
1answer
349 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 ...
3
votes
1answer
96 views

NO2 gas was detected in FTIR

We have a laser chamber full of $\ce{F2}$ gas (no air should be in the chamber) and several parts including steel, wires, ceramics, zinc.... This is for laser generation so the temperature is pretty ...
3
votes
1answer
148 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 ...