Questions tagged [ir-spectroscopy]

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

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3
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1answer
31 views

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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1answer
66 views

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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1answer
52 views

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 $cm.molecule$). ...
1
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1answer
53 views

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) ...
6
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1answer
199 views

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 ...
7
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1answer
139 views

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.). ...
2
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0answers
34 views

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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0answers
37 views

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, ...
3
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1answer
234 views

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 ...
3
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2answers
78 views

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....
3
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1answer
142 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 ...
2
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0answers
32 views

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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2answers
64 views

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 ...
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0answers
28 views

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 ...
-5
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1answer
168 views

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
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1answer
860 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
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1answer
67 views

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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0answers
18 views

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 ...
0
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1answer
36 views

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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0answers
42 views

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 $...
0
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1answer
56 views

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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0answers
26 views

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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2answers
170 views

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 ...
0
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1answer
43 views

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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25 views

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 ...
-1
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1answer
129 views

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 ...
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2answers
1k 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 $\ce{C=O}$? ...
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62 views

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) ...
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1answer
51 views

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 ...
2
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1answer
219 views

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 ...
1
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1answer
170 views

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
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1answer
389 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
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1answer
194 views

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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51 views

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
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1answer
87 views

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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0answers
57 views

$Q_3$ 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 ...
3
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1answer
564 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 ...
2
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0answers
285 views

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: ...
3
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0answers
20 views

Any large databases of IR Spectra that you are able to download many spectra in bulk [closed]

I am currently working on a machine learning project, in which I need access to many IR spectra, the more the better, in order to build a classifier that will used some unsupervised learning in order ...
1
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1answer
47 views

Raman scattering and "electron and proton center" of molecule

I am currently studying the textbook Infrared and Raman Spectroscopy, 2nd edition, by Peter Larkin. In a section entitled The Raman Scattering Process, the author says the following: Light scattering ...
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0answers
49 views

Explaining allene IR stretching frequency

So I was comparing the IR stretch frequency of ethyne, ethene and allene. Of course triple bond is stiffer than double bond so ethyne has a higher stretch frequency. The problem then is where allene ...
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0answers
2k views

The IR/ H-NMR analysis of benzophenone?

I think I have figured out the IR spectrum for benzophenone. Would it be that the sharp peak in the middle (1700 cm^-1) is for the ketone and the little peaks pass 2900 cm^-1 rang are the sp2/sp3 ...
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0answers
30 views

Which one is the correct molecule of the given properties of IR Spectroscopy?

$\ce{CH3CH=CHCO2CH3}$ $\ce{CH3COOCH=CHCH3}$ $\pu{2950 cm-1}, \ \pu{2870 cm-1}, \ \pu{1740 cm-1}$ are the observed peaks. No peaks observed $\pu{1700-1500 cm-1}$. The formula of the molecule is $\ce{...
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0answers
100 views

What are the alternatives we have to the VEDA 4 vibrational energy distribution analysis software?

Said software is very useful when you need to assign the peaks in vibrational spectra to different vibrational modes found in a hessian calculation for a given molecule, as described in Jamróz, Michał ...
2
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1answer
48 views

Do the spectra of compound materials combine linearly with respect to the ratios of the constituent compound spectra?

My question stems from reading this article on the FTIR spectrum of oil that has been mixed with various contaminants. If one has the spectrum for the original uncontaminated oil and the spectrum for ...
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0answers
419 views

Aromatic Stretches C=C in Salicylamide

I want to ask a question about the C=C streching modes of Salicylamide. I was presented with the following spectra and asked to comment on the bonds causing the peaks. I identified the molecule ...
7
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1answer
378 views

Why does the binding of DMSO to metal via the S atom (k-S) strengthen the S=O bond?

So, DMSO (dimethyl sulfoxide) can form metal complexes in both $\ce{\kappa-O}$ and $\ce{\kappa-S}$ mode i.e. binding with the oxygen or the sulfur respectively. The general explanation given is that ...
2
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1answer
68 views

Frequency region and "uncoupled oscillators" in vibrational spectroscopy

I am currently studying the textbook Infrared and Raman Spectroscopy by Larkin. In the first chapter of the textbook, the author presents the following table and accompanying explanation: The ...
0
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1answer
67 views

Near-IR spectroscopy measures the broad overtone and combination bands of some of the fundamental vibrations (only the higher frequency modes)

I am currently studying the textbook Infrared and Raman Spectroscopy by Larkin. In the first chapter of the textbook, the author opens (probably prematurely) with the following statement: Near-IR ...
3
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2answers
266 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 ...