Questions tagged [ir-spectroscopy]

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

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Help with chemical bond associated with 1500cm-1 and 1600cm-1 [closed]

I have IR spectra of polystyrene with characteristic peaks at 1450cm-1. 1500cm-1 and 1600cm-1. What chemical bond is associated with 1600cm-1 and 1500cm-1?
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IR / Raman factor group analysis (site and factor group determination)

I'm currently trying to interpret some IR and Raman spectra of solid state compounds. They show a simple molecular unit of which the expected bands are split due to the coordination in the crystal ...
4 votes
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How exactly thermal infrared (e.g. FLIR) cameras image methane leaks? Are they measuring a temperature difference or an emissivity difference or...?

The story Infrared camera shows the impact of vehicle emissions says: In an effort to visually demonstrate the potentially fatal impacts of air pollution, FLIR Systems (Wilsonville, OR, USA; www.flir....
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What Is This Small Peak That Always Occurs Within Ketone Infra-Red Analysis?

After looking at a bunch of IR spectrums for ketones, I have noticed this tiny peak that I circled on the diagram below. Does anyone know what it actually is and if I can always use it to distinguish ...
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Idenfitying material composition of a polymer membrane (burn test?)

I have a polymer composite membrane. It has a porous layer and a dense layer. I know the dense layer's composition. However, I don't know the composition of the porous layer. I want to figure out if ...
3 votes
1 answer
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Why my Infrared data acquired in transmission mode looks like it was acquired in absorbance mode?

I'm analyzing some carbon nanotubes with DRIFTS. I mix the CNT with KBr powder and proceed to measure the spectrum of the powder. After acquiring the KBr background (in %T, peaks facing down) I ...
1 vote
1 answer
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Effect of electron-donating and electron-withdrawing groups on the C=O bond

My notes states this: An electronegative element tends to draw in the electrons between the carbon and oxygen atoms through its electron-withdrawing effect. This strengthens the C=O bond. An electron-...
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Why does the ν1+ν4 combination band for methane occur?

I am looking at the normal modes for methane and have attached a figure of them below: The only IR active modes are $\nu_3$ and $\nu_4$ which I understand because the other two modes do not change ...
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2 votes
1 answer
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Paracetamol infrared spectrum issues

I was studying Infrared Spectroscopy and i analyze different polymers. When i prepared paracetamol making a pill mixing 2mg of it with 200mg of $\ce{KBr}$ on a vacuum-connected press when i put it in ...
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A Multistep Reaction Provided with IR Spectra (Guessing Started with Esterification)

Below, IR spectra for this sequence are provided: For compound E, I suggest the formation of a conjugated ester with the elimination of $\ce{-OH}$ group. The IR peak at $\pu{1732 cm-1}$ is a ...
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Why is this graph of vibration spectroscopy like this?

This graph shows the absorbance of different wavenumbers of infrared radiation causing different forms of vibrations of the water molecule. I would like to know why the graph turns out the way it is. ...
9 votes
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Why are there two infrared vibrational frequencies for the terminal CO ligands of Fe2(CO)9?

I am struggling to rationalise why there are two infrared vibrational frequencies ($\pu{2082 cm^-1}$ and $\pu{2019 cm^-1}$) for the stretching of the terminal $\ce{CO}$ ligands of $\ce{Fe2(CO)9}$. ...
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Error in calculating G-matrix

I have a question regarding the calculation of the $G$-matrix in the classic "Molecular vibrations" textbook by Wilson (1955, McGraw Hill). On page 63 it gives an example for the calculation ...
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1 answer
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Why the count of the IR band of H2O and CO2 is different? [closed]

Why $\ce{H2O}$ shows three and $\ce{CO2}$ shows two fundamental bands in IR while they both are triatomic molecules?
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1 answer
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Conversion from Spectral Line Intensity to Absorbance (HITRAN)

The HITRAN spectral database gives access to the spectral absorption lines of various gaseous species. These lines are given as $S$ values in units of $cm^{-1}/(molecule·cm^{-2})$ (or $cm.molecule$). ...
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Vibrations of metal complexes (polyatomic molecules)

I am reading about vibrational spectroscopy of polyatomic molecules and specifically about metal complexes. What I don't understand is what happens on the vibrational modes of the ligand (e.g. water) ...
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1 answer
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How is the vibration of multiple atoms treated in vibrational spectroscopy?

Most of the advanced undergraduate textbooks on physical chemistry/ instrumental analysis show the case of a diatomic harmonic oscillator and derive formulae for predicting the frequencies. The ...
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7 votes
1 answer
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How to interpret the relation between CO stretch frequencies and backbonding strength for polycarbonyl compounds?

I am working on a series of octahedral complexes of the type $\ce{[M(CO)2(CH3)X3]-}$ where $\ce{M} = \ce{Co}, \ce{Rh}, \ce{Ir},$ and $\ce{X}$ is a range of ligands (halide, cyanide, nitrosyl, etc.). ...
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2 votes
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Why is the intensity of the IR spectra of IBr higher than that of ICl?

Why is the intensity of the IR spectra of $\ce{IBr}$ higher than that of $\ce{ICl}$? I know that the IR spectra intensities are proportional to the derivative of the dipole moment with internuclear ...
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Should Birge–Sponer plots always have a negative gradient?

I have a set of data I would like to use the Birge–Sponer extrapolation with, but the data is the wavenumbers of the bands in the vibrational-electronic spectrum of a diatomic molecule for $v' = 0, 1, ...
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3 votes
1 answer
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Comparing the zero point vibrational energies of DCl and HCl

(a) Compare the zero point energy (i.e. $E_0)$ in joules for $\ce{H^{35}Cl}$ and $\ce{D^{35}Cl}$ (assuming both are anharmonic oscillators). (b) Explain the effect this will this have on their ...
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3 votes
2 answers
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How does the force constant change when a heavier atom is used in vibrational spectroscopy?

(iii) The fundamental band in the IR spectrum of $\ce{^1H^{127}I}$ is found at $\pu{2308 cm^-1}.$ Calculate the force constant for the bond. Assume the molecule can be treated as a harmonic oscillator....
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3 votes
1 answer
410 views

Why is the ester carbonyl peak in benzocaine IR spectrum so low?

Having looked at multiple IR spectra for benzocaine, I am struggling to understand why the carbonyl C=O peak is so low in the spectra compared to where an ester carbonyl group usually is on other IR ...
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2 votes
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Why are Methyl vibr. modes called r modes and methylene vibr. modes called d modes

I am coming from a chemical background and currently work in the field of vibrational spectroscopy. When I talk to fellow physicists or chemists I always encounter the same question. "Why are the ...
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Seeking safe, water-soluble chemicals for an IR spectroscopy experiment

I am working with a custom-built device which is intended to perform some mid-infrared chemical measurements. It consists of four thermopile sensors with optical windows. In front of each window is ...
1 vote
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Comparing bond strength by IR frequency: symmetrical modes

In literature the stretching mode of $\ce{CO}$ is often said to be the diagnostic criteria for strength of back-donation by virtue of weakening the $\ce{CO}$ bond. However, when speaking about ...
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-5 votes
1 answer
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Why does the infrared spectrum of HCl only have one peak? [closed]

Suggest one reason why HCl has only one peak? Does this have something to do with fact that peaks (in IR spectroscopy) represent areas of the spectrum where specific bond vibrations occur and ...
3 votes
1 answer
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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 ...
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3 votes
1 answer
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Lack of elaboration in IR spectra of compounds

I've recently started learning about IR spectroscopy, and a common trend I'm seeing in my textbook and other sources is that not all the features in the IR spectrum are explained in detail. The focus ...
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pH Dependency of IR Stretching Frequency in CO-binded Heme Protein

If one consumes CO, then CO binds with Heme Protein forming a Fe-CO bond. My question is if there will be any pH dependency on IR Stretching Frequency, i.e. ν(CO) and ν(Fe-CO). And if there is, then ...
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1 answer
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What functional groups can 900-2500nm IR spectrometers detect?

3700 cm-1 corresponds to about 2,700 nm 650 cm-1 corresponds to about 15,000 nm. Many cheap IR spectrometers ($7000 or so like the ones at Stellarnet) has range of 900nm to 2400nm (or 11,111 cm-1 to ...
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Finding wavelength of the centre of Stokes Ro-vibrational spectrum

I have the following rovibrational Raman spectrum for $N_2$: and I am trying to calculate the "wavelength where the centre of the Stokes ro-vibrational spectrum would be observed" if the $...
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1 answer
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IR absorption chart

I am taking a course in structural determination of organic compounds. Right now, we are giving the IR spectra. I am looking for a table, as detailed as possible, of the absorptions. I hope you can ...
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1 vote
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How can I cluster infrared spectra?

This is sorta a cross-post from my post in StackOverflow. Although, rather than seeking help with the code I'm here asking for help with a chemical/statistical problem associated with the code. I'm ...
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-1 votes
2 answers
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How to determine the unknown molecule using the infrared spectroscopy?

An unknown sample was analysed using infrared spectroscopy. What is the compound? So I am having difficulty in using the infrared data to determine the unknown compound. I keep on misinterpreting the ...
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1 answer
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Does increasing the number of interferometers increase signal in FT-IR?

I'm trying to understand why most modern FT-IRs have three interferometers. Does this improve the signal in someway or does it have another effect?
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Using the method of differential absorption to determine the ethanol content of an unknown sample [closed]

I have completed a whole set of FITR-ATR spectra analysing 6 solutions of ethanol-water mixtures, all at different concentrations of each. I have now been instructed to use the method of differential ...
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1 answer
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Is acetylene IR active? [closed]

I've a very straight forward question, I'm confused because I've been told that a atom which has a oscillating transient/permanent dipole is IR active. But there are few asymmetric stretching/bending ...
7 votes
2 answers
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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 $\ce{C=O}$? ...
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Estimate the relative intensities of absorption given the split in energy levels

Estimate the relative intensities at $\pu{25 °C}$ of absorption originated in the ground state and the first excited state when the energy levels involved are separated by: (a) $\pu{10000 cm^-1};$ (b) ...
-2 votes
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Detecting carbon dioxide concentration using mosquito neurons

At the Mauna Loa Observatory the concentration of atmospheric carbon dioxide is measured using the absorption of infra-red light. This is a statistical technique that requires constant careful ...
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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 ...
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1 vote
1 answer
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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 ...
2 votes
1 answer
766 views

XPS/EDS vs FTIR/Raman [closed]

So I have been studying the four methods the past couple of weeks. To my understanding, XPS is a very surface-sensitive technique. It can give information about the composition of the surface but also ...
1 vote
1 answer
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Can you measure an IR spectrum of a sample inside a glass vial?

If I have a sample of some compounds dissolved in water inside a glass vial (let's say quartz). Would it be possible to get a usable measurement through this glass vial without too much absorption by ...
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Raman or FTIR spectroscopy to identify metals?

Metals are opaque, so doesn't most of the light reflect? And does the IR source even cause vibrations in the metal? Same goes for Raman spectroscopy, is it useful for use of metal identification? Is ...
3 votes
1 answer
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PE expression for vibrational spectra of molecules using Newton's second law

I am currently studying the textbook Infrared and Raman Spectroscopy: Principles and Spectral Interpretation, second edition, by Peter J. Larkin. Section 10. Calculating the Vibrational Spectra of ...
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1 vote
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$Q_3$ is symmetric with respect to the $\sigma_v$ operation?

I am currently studying the textbook Infrared and Raman Spectroscopy: Principles and Spectral Interpretation, second edition, by Peter J. Larkin. Section 8. Symmetry: Infrared and Raman Active ...
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3 votes
1 answer
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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 ...
2 votes
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What is the precise meaning of "in-phase" (and "out-of-phase") in this context?

I am currently studying the textbook Infrared and Raman Spectroscopy, 2nd edition, by Peter Larkin. In a section entitled Symmetry: Infrared and Raman Active Vibrations, the author says the following: ...
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