Questions tagged [ir-spectroscopy]

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

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12 views

Dynamic dipole moment [closed]

Dynamic/transition dipole moment is defined as the electric dipole moment associated with the transition between the two states. It has to be not equal to 0 for a molecule to have a vibrational ...
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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 ...
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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 ...
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1answer
270 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 (...
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2answers
149 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 ...
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78 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 ...
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18 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 ...
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35 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 ...
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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 ...
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24 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.
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1answer
135 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 ...
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1answer
45 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}...
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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.
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1answer
105 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 ...
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2answers
144 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 ...
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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 ...
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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 ...
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1answer
179 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 ...
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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 ...
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2answers
85 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 ...
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1answer
112 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 ...
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158 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 ...
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5answers
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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 ...
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1answer
176 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 ...
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1answer
181 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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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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217 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. ...
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1answer
122 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
201 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
186 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
127 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
46 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
269 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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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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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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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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398 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
175 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
93 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
59 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
131 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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111 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
3k 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
152 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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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-...
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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 ...
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1answer
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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
352 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
447 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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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,...