Questions tagged [ir-spectroscopy]

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

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IR active and inactive C-H stretching

I have two molecules, ethane (CH3-CH3) and 1,1,1-Trichloroethane (CH3-CCl3) and I need to determine if the C-H stretching vibrations for both molecules lead to a net change in dipole moment, and would ...
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29 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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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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40 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 ...
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1answer
39 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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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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1answer
41 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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757 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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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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49 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 ...
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1answer
68 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 ...
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1answer
127 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 ...
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1answer
102 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 ...
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1answer
107 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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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 ...
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1answer
210 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 ...
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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 ...
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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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153 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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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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1answer
44 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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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 ...
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Why does the binding of DMSO to metal via the S atom (k-S) strengthens the S=O bond?

So, DMSO (dimethyl sulfoxide) can form metal complexes in both $\mathrm{\kappa O}$ and $\mathrm{\kappa S}$ mode i.e. binding with the oxygen or the sulfur respectively. The general explanation given ...
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1answer
32 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 ...
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2answers
123 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 ...
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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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536 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
287 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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91 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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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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436 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
73 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
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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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233 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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240 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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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
106 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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193 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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318 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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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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817 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
625 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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1answer
186 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
229 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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218 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 ...