Questions tagged [ir-spectroscopy]

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

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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 ...
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1answer
829 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 ...
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0answers
72 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 ...
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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 ...
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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 ...
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1answer
235 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 ...
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1answer
161 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 ...
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1answer
58 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 ...
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2answers
556 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?
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2k 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 ...
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93 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 ?
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50 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?
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772 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 ...
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1answer
648 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 ...
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1answer
135 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 $\...
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1answer
68 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 (...
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0answers
156 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-...
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143 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 ...
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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 ...
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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 ...
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93 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
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1answer
3k 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 ...
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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 ...
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1answer
800 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 ...
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1answer
580 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 ...
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64 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,...
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137 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 ...
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1answer
2k views

Distinguishing propan-2-ol from ethanol using IR spectroscopy

A question requires me to distinguish between propan-2-ol and ethanol from IR spectra. To me, they both look nearly identical, even though I know both of them contain the same functional group and ...
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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 ...
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1answer
717 views

In vibrational spectroscopy, what affect do overtones have on the spectra? [closed]

I've been in a lecture about vibrational overtones in anharmonic oscillators. How would these affect an IR spectrum?
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100 views

Theoretical limits of (NIR) spectroscopy for quantitative analysis

Edit (months later): In hindsight, I was asking about the theoretical limits of multivariate curve resolution (see Lawton & Sylvestre: Self-modeling curve resolution) in chemometric applications. ...
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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 ...
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0answers
31 views

Trend in the infrared stretching of molecular hydrogen to group 8 transitions metals?

While going through old notes, in relation to the Dewar-Chatt-Duncanson model, it states that the stretching frequency for $\ce{N2}$ to the group 8 transition metals ($\nu(\ce{NN})$) proceeds as: \...
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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 ...
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36 views

rovibrational spectroscopy: why do intensities of rotational lines increase with J? [closed]

how and why do the intensity of rotational lines change with J value? in the simplest terms possible please
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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 ...
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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 ...
3
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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 ...
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0answers
54 views

How to effectively induce the bending motion of carbon dioxide?

I would like to know how to effectively induce the bending motion of $\ce{CO_2}$ with femtosecond IR lasers. In other words, how to effectively populate bending vibrational states in $\ce{CO_2}$ ...
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0answers
981 views

IR spectroscopy in CCl4 Solution v/s Liquid film [closed]

Why there are differences in these spectra ? Which one is more useful and accurate ? What is the significance of using these two different methods ?
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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 ...
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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 ...
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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: ...
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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. ...
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1answer
5k views

What do s, m, w, m-w, and w-s mean in the context of infrared spectroscopy?

I am referring to Donald Pavia's Introduction to Spectroscopy, and I found this table that tells about the intensities of the peaks of vibrations of certain bonds. I can't understand what ...
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1answer
421 views

Where can I find (citable) FTIR spectra of CuCl and CuCl2?

I can't seem to find any published/citable infrared (FTIR) spectra in the 4000-400 cm-1 range of CuCl and CuCl2. Does anyone know of a source available, either free or paid?
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1answer
471 views

Expected accuracy of experimental IR spectra compared to literature reference

I have a table from a journal which indicated signals of an alpha chitin that I used as theoretical values. I did my own analysis to identify a chitin sample and it had signals that slightly varied ...
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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, ...
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1answer
228 views

Slight FTIR band intensity changes across several different copper salt spectra: relevant for characterization?

I have several spectra of different samples which are probably all copper (neutral and/or basic) acetate(s) and which are very similar. I present them overlayed and (some of them) stacked: My ...
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1answer
808 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 ...