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Questions tagged [spectroscopy]

Use this tag for questions about spectroscopic methods in practice and in theory, especially where more specific tags for the individual method (such as [nmr-spectroscopy] or [ir-spectroscopy]) are not available.

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Why don't equivalent hydrogens cause splitting in NMR?

When doing NMR spectroscopy, it is an observed fact that equivalent hydrogens do not split one another. Why don't equivalent hydrogens split each other's signals? For example, why is the NMR spectrum ...
Niels Kornerup's user avatar
11 votes
2 answers
80k views

Is carbon dioxide IR inactive?

Wouldn't CO2 be IR inactive because of its non-polar bonds?
Dissenter's user avatar
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36 votes
3 answers
7k views

A compound that absorbs all visible light

Is there a compound that absorbs all visible light?
oushida 's user avatar
30 votes
3 answers
6k views

Are the bonding orbitals in methane equivalent - photoelectron spectrum

The low energy portion (the part dealing with the $\ce{2s}$ and $\ce{2p}$ electrons) of the photoelectron (PE) spectrum of methane is reproduced below. (image source) The reaction being examined is ...
ron's user avatar
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13 votes
1 answer
1k views

Why is it possible to image LUMO if these orbitals are, by definition, unoccupied?

LUMO stands for lowest unoccupied molecular orbital. If it's unoccupied, how is it possible to image it? See for example in this paper ("Pentacene imaged with STM and NC-AFM").
Sparkler's user avatar
  • 4,275
13 votes
2 answers
14k views

Rotational degrees of freedom (3N-5 and 3N-6)

In spectroscopy we described the electric energy with the approximative separability of internal motions as: \begin{equation} E=E_e+E_v+E_r+E_{ns} \end{equation} (energies: electronic, vibratory, ...
laminin's user avatar
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8 votes
3 answers
7k views

How to explain the discrepancy between the calculated absorption wavelength within the particle in the box theory and the observed wavelength?

The experimental observed wavelength for this molecule is 425 nm. We are supposed to estimate this wavelength theoretically using this equation (energy change in a 1D box), where $N$ is the number of ...
Plex ASM's user avatar
  • 441
2 votes
1 answer
8k views

IR spectra and hydrogen bonds

Normally we see IR signals that correspond to the vibration of covalent bonds. Can a (strong) hydrogen bond (or a vibration thereof, to be precise) correspond to an IR signal as well? How about other ...
logical x 2's user avatar
  • 2,794
2 votes
2 answers
20k views

How does one find the ground-state term symbol for a configuration that is exactly half-filled?

For instance, the ground-state configuration of N atom is a $p^3$ configuration of all parallel spins and one electron in each $2p$ orbital, which has: Total spin angular momentum, $S = 3 * \frac{1}{...
timaeus222's user avatar
20 votes
1 answer
935 views

When simulating spectral line broadening, which convolution is preferred?

Many computational chemistry packages permit the calculation of vibrational and electronic spectra. These spectra are obtained as a set of discrete eigenvalues however they are often convolved with ...
Richard Terrett's user avatar
18 votes
3 answers
5k views

Modern open-source tools for simulation of NMR spectra

I am interested in predicting the NMR spectrum of small organic compounds. It doesn't matter to me if the prediction is very accurate. I'll eventually be comparing the prediction to experimental ...
Curt F.'s user avatar
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16 votes
1 answer
3k views

Explaining the Relative Energies of Various Vibrational Modes

Specifically, I'm wondering why it is that the asymmetric stretch in most (although there are exceptions to this I believe) molecules is higher in energy than the symmetric stretch. Similarly, I ...
jheindel's user avatar
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13 votes
2 answers
13k views

What is an exocyclic double bond?

I have been tasked to find the $\lambda_{\text{max}}$ of 3,4-dimethyl-1,2,3,4,4a,5-hexahydronaphthalene. This was supposed to be fairly easy. I used the base value for homoannular diene ($\pu{253 nm}$...
Pritt says Reinstate Monica's user avatar
13 votes
4 answers
31k 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, ...
user41793's user avatar
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7 votes
1 answer
25k views

Ring Strain and C=O Stretching Frequency

Why does $\ce{C=O}$ stretching frequency increase with ring strain? Why does conjugation decrease $\ce{C=O}$ ring strain? I am told that cyclopropanone exhibits a higher $\ce{C=O}$ stretching ...
Dissenter's user avatar
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6 votes
2 answers
4k views

How can I deduce the linearity of XeF2 from the IR spectrum?

The vibrational modes of $\ce{XeF2}$ are at $555$, $515$, and $213~\mathrm{cm^{-1}}$ but only two are IR active. Explain why this is consistent with $\ce{XeF2}$ having a linear structure. From what ...
MikhaelM's user avatar
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5 votes
2 answers
755 views

What is the correct way to verify a structure's geometry, for example for benzene?

My goal is to compare a calculated bond length with experimental data. Benzene is obviously common enough to expect sufficient experimental data to be available for it on the internet. I have found ...
Blaise's user avatar
  • 1,648
5 votes
1 answer
3k views

What is the proton NMR spectrum of p-methoxyphenol?

I think there will be two pairs of doublets, since the hydrogens adjacent to the $\ce{OCH3}$ and those adjacent to the $\ce{OH}$ group will be in different environments. The answer, however, is that ...
John's user avatar
  • 485
3 votes
1 answer
1k views

several coefficient differences in UV-vis Spectroscopy

Does anyone know what is scattering coefficient, absorption coefficient and extinction coefficient, and how to separate them experimentally?
Tianyu Zheng's user avatar
3 votes
1 answer
996 views

Spin spin coupling in a proton NMR of an ester?

I am learning about proton NMR and spin-spin coupling, and am confused about whether splitting occurs over an ester bond. Specifically, in the case of ethyl methanoate, HCOOCH2CH3, if I were to number ...
Meep's user avatar
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3 votes
1 answer
466 views

Difference between relaxation and resonance leading to an absorption spectral feature?

I need help understanding the physics behind this insightful comment below the question Does water really have strong EM absorption at 3 kHz in solid and 2 GHz in liquid? Why the huge shift?: ...
uhoh's user avatar
  • 5,888
1 vote
1 answer
2k views

Determine bond length via rotational spectroscopy

Estimate the bond length of 12C16O (pure rotational spectrum) given J''=3 (15.356 cm-1) and J'=9 (38.356) I understand that the first step is the calculate the rotational constant. So to do this I ...
Harley McFarlen's user avatar
0 votes
1 answer
107 views

laser diode problem [closed]

I'm a student studying chemistry, i'm working with diode laser to look at emission spectra for certain complexes. i'm using integrating sphere as my setup, when i use the 405nm diode laser output of 4....
chem's user avatar
  • 11
11 votes
1 answer
3k views

Difference between a "cartoon" flame test and a "real" flame test? How do chemists do flame tests correctly?

This well-received answer to Why does the same electron transition release photons of different frequencies for some elements? begins with the following intriguing passage: I am glad that you updated ...
uhoh's user avatar
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88 votes
5 answers
20k views

Is it true that heavy water is not blue?

I believe I saw this claim somewhere on the internet a long time ago. Specifically, it was claimed that the difference could be observed by filling one long, straight tube with light water and one ...
Brian's user avatar
  • 1,201
22 votes
3 answers
8k views

What is a virtual state?

In talking about Raman spectroscopy, one finds the Stokes line is simply the difference between the energy of an incoming photon and an emitted photon. This energy corresponds to a vibrational ...
jheindel's user avatar
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19 votes
4 answers
149k views

Why are there multiple lines in the hydrogen line spectrum?

Hydrogen has only one electron, yet it exhibits multiple lines in a spectral series, why is this?
Gerard's user avatar
  • 3,459
18 votes
1 answer
7k views

Interpretation of TD-DFT results

For the first time I'm doing TD-DFT calculations (wB97XD functional) in Gaussian 09 for an open-shell system and the results look like the hell of a mess for me. The molecule is rather big, so I ...
Wildcat's user avatar
  • 19.1k
14 votes
4 answers
2k views

How do I determine the molecular vibrations of linear molecules?

I know how to do it for just about every other point group, but the $D_{\infty \mathrm h}$ and $C_{\infty \mathrm v}$ character tables aren't as straightforward. In particular, I'm interested in the ...
SBoZon's user avatar
  • 261
13 votes
1 answer
34k views

Why are d-d electronic transitions forbidden and weakly absorbing? Why do they occur at all?

I am unable to understand why d-d electronic transitions are forbidden? Why are they weakly absorbing and apart from that, why do they occur at all?
Saroj's user avatar
  • 562
12 votes
1 answer
16k views

Derivation of the Orgel diagram for octahedral d2 complexes

The Orgel diagram for a typical octahedral $\mathrm{d^2}$ complex is shown below: (source: Kettle, Physical Inorganic Chemistry, p 146) I understand that the right-hand half of the graph shows how ...
orthocresol's user avatar
  • 71.5k
12 votes
3 answers
709 views

How does absorption spectroscopy work?

So a beam of white-light photons passes through a gas and various photons, having exactly the same energy as the gas's molecular energy transition state deltas, get absorbed. We then get a ...
adlibber's user avatar
  • 251
10 votes
2 answers
1k views

Terminology of atomic spectroscopy: Difference Among Term, States and Level

In A Primer on Quantum Numbers and Spectroscopic Notation Contents, the concept of a term and level is described as a) The level is the set of 2J+1 states with specific values of L, S, and J. ...
ACR's user avatar
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10 votes
3 answers
25k views

1H (proton) NMR spectra for alkanes

I do not fully understand the idea of coupling and why chemically equivalent hydrogens do not couple. I wanted to work with the alkanes, so let's take propane first: There would be a triplet signal ...
user258521's user avatar
10 votes
2 answers
3k views

How to interpret orbital transition in TDDFT?

Consider the following TDDFT run with GAMESS: ...
Aug's user avatar
  • 2,141
9 votes
3 answers
14k views

How to convert SPC files to ASCII without original spectroscopy software?

IR and Raman spectroscopy are typically saved in *.SPC files (chemical/x-galactic-spc), which are binary files. Collected data is save to SPC files by default, but ...
Sparkler's user avatar
  • 4,275
9 votes
1 answer
395 views

Why do we say "approximation" in the dipole approximation in spectroscopy?

In the dipole approximation, the following relation holds: $$ \hat{V} = -\hat{\mu} \cdot \vec{E}. $$ When we say "approximation", I guess we want to point out that we are dealing with linear ...
laminin's user avatar
  • 3,405
9 votes
2 answers
394 views

Software to draw one-dimensional PES including vibrational energy levels

I am trying to represent the difference between the an anharmonic and harmonic treatment of vibrations. The easiest way to do this is to present an example of a harmonic and anharmonic potential ...
isolated matrix's user avatar
8 votes
2 answers
649 views

FTIR Spectra: Did I synthesize ethyl gallate from gallic acid?

I successfully synthesized ethyl gallate from gallic acid and ethanol (of course there are other reagents too, like DCC and Pyridine). Before conducting FTIR analysis, I performed column ...
Jonathan's user avatar
  • 155
8 votes
3 answers
2k views

The concept of normal coordinates of a molecule

Sometime ago I had posted the What is the origin of "normal" in normal coordinates and normal modes? in math & sci history problems. Nobody was sure for the reason for using the word normal there -...
ACR's user avatar
  • 41.5k
7 votes
1 answer
418 views

Determining overtones bands of degenerate modes

I have a question of a more mathematical nature on the mathSE (Symmetric Direct Product Distributive?) that received a good answer, but I think an answer more oriented to chemists would be a useful ...
Tyberius's user avatar
  • 11.8k
7 votes
1 answer
917 views

UV/Vis spectroscopy transition reference

There are mainly fundamental transitions happening during excitation and UV/Vis spectroscopy (as n- > σ* ,π->π* ,n- > π ,..). I am looking for a good reference that shows these transitions for ...
Aug's user avatar
  • 2,141
7 votes
2 answers
942 views

Open resource for spectroscopic raw data

Is anyone aware of reliable resources (ideally public) which allow the user to download the raw spectroscopic data such as infrared spectrum of small organic molecules, or rotational spectrum of HCl/...
ACR's user avatar
  • 41.5k
7 votes
1 answer
3k views

How to determine the structure of organic molecules without spectroscopy [duplicate]

I wonder how chemists worked out the structures of organic molecules before spectroscopy (especially NMR) was invented? Surely, we could do functional group tests, but the correct connection of atoms ...
Nissa's user avatar
  • 419
6 votes
1 answer
2k views

Do acyl fluorides exhibit resonance donation or inductive withdrawal?

I know that in acyl chlorides, there is partial triple bond character in the $\ce{C=O}$ bond not because the chlorine is electronically donating (due to size mismatch with carbon) but because the ...
Dissenter's user avatar
  • 19k
6 votes
1 answer
28k views

What is the principle of diffuse reflectance spectroscopy behind band gap measurement?

As the title suggest can you explain how DRS can be used for finding band gap of a material. Below is an DRS output result: From the figure we can see that at about 465 nm the reflectance drops ...
Eka's user avatar
  • 2,768
6 votes
2 answers
411 views

Tabulated absorption spectra of greenhouse gases?

Are there any free sources of tabulated absorption spectra of greenhouse gases from UV/Vis to far infrared (say 70 micrometers)? This link shows the net absorption over the whole atmosphere (at least ...
oliver's user avatar
  • 301
5 votes
3 answers
4k views

How to identify an unknown compound with spectroscopic data?

I need some help identifying the following unknown compound. Any suggestions or hints regarding the possible compound would be highly appreciated. So far I am thinking the broad peak on the IR is an ...
Pepria's user avatar
  • 567
5 votes
2 answers
2k views

How do I find out what benzene containing compound this is with IR and NMR?

I have the following IR and NMR of an unknown substance. Can someone tell me what is a possible structure of the compound or point me in the correct direction. So far I am thinking of a benzene ring ...
oneCoderToRuleThemAll's user avatar
5 votes
1 answer
6k views

Why is the para carbon in Benzaldehyde NMR more deshielded than the ortho and meta carbons?

Assuming this image is correct, why is the Carbon-3 more deshielded than Carbon-2 and Carbon-1? From what I understand, deshielding is the greatest when the Carbon atom is closer to electron ...
oneCoderToRuleThemAll's user avatar