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.
638
questions
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Why does the absorbance of Ca decrease in the presence of certain metals?
I'm trying to find a reason for an experimental observation, I noticed that when the absorbance of $\ce{Ca}$ is measured with FAS(flame atomic spectroscopy) that it decreases when metals such as $\ce{...
1
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0answers
32 views
What is the best way to go about matching proton-NMR spectra to molecules?
I've attatched an example question. I have already solved this one, I'm wondering about what, in your experience, is the most time-efficient way of approaching a question like this one. I can solve ...
8
votes
1answer
256 views
Empirical Evidence and Assignment of Orbital Energies (Electron Filling)
I see a lot of questions on electron configurations from students at various levels here. Students mechanically remember how to fill orbitals and sorry to say many teachers do the same. Not a single ...
0
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0answers
64 views
Term symbols for excited nitrogen (2s2p^4 configuration)
Looking at the NIST levels listing for neutral nitrogen atom, one of the excited configuration is $2s2p^4$ configuration.
https://physics.nist.gov/cgi-bin/ASD/energy1.pl?encodedlist=XXT2&de=0&...
5
votes
2answers
354 views
Can all NMR spectrometers perform special techniques such as COSY, NOESY, HETCOR,..? [closed]
Do you need special spectrometers for:
probing nuclei other than hydrogen or carbon?
performing 2D spectroscopy such as COSY, NOESY, HETCOR, HSQC,..
Or are these techniques usually available on ...
2
votes
1answer
45 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 ...
0
votes
1answer
55 views
Why do certain signals in proton NMR experience extensive splitting despite only having 1 neighbour?
I came across the compound 3-methylpentan-2-ol. Looking at its proton NMR spectra, it seems there is a lot of splitting for the signal at 4 ppm.
I'm assuming that the 4 ppm represents the proton in ...
1
vote
0answers
70 views
Effect of ionization on dipole moment
Suppose we have a non-dipolar molecule such as carbon dioxide. If carbon dioxide is ionized by some means in the gas-phase, will it develop a dipole moment? I am asking with reference to microwave ...
4
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1answer
71 views
What exchange timescales are needed to cause decoupling in NMR?
I'll try to explain the source of my confusion as clear as possible:
(*) The methyl protons in toluene are decoupled because the C-C bond between the methyl group and benzene allows very fast ...
19
votes
3answers
2k 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 ...
0
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0answers
75 views
why is the H-NMR chemical shift of methane lower than that for ethane?
If we replace one hydrogen in methane by a methyl group we arrive at ethane. Now we can say:
Carbon is more electronegative, and so it withdraws electrons which leads to deshielding. This is why the ...
6
votes
1answer
108 views
Radiationless energy transfer: how to arrive at the equation that give R0 between two chromophores?
I have a homework question where I should calculate $R_0$ of an energy transfer between two chromophores. However, I don't understand how my professor arrived at the equation that we should apply (...
5
votes
2answers
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 ...
2
votes
1answer
121 views
Different force constant in ground vs excited state in spectroscopy
The force constant of iodine in the first excited state is $k=41.78$ N/m, and in the ground state it is $170$ N/m.
I know that the formula for calculating $k$ is:
$$ k= (\frac{ĪE_{vib} *2Ļ}{h})^2 Āµ$$...
0
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0answers
26 views
Why does peak decoupling occur in H-NMR spectra?
For example, when we analyze the conversion of some chemical compound, certain characteristic areas tend to decrease (ok).
However, why can a peak (i.e., one peak) generate smaller peaks (i.e., more ...
4
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3answers
3k 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 ...
3
votes
0answers
49 views
Enantiomers to diasteroisomers (rotational spectra)
Enantiomers have identical rotational spectrum because their mass distributions are identical. Recently a trick has been introduced in microwave spectroscopy in which a enatiomerically pure gas chiral ...
6
votes
1answer
123 views
Question about a line from Rosenfeld's paper on optical rotation
There are a lot of papers on optical rotation which cite Rosenfeld's (German) 1928 paper "Quantum mechanical theory of natural optical rotation..." [Quantenmechanische Theorie der naturlichen ...
11
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2answers
61k views
6
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1answer
96 views
Beer's law: UV-Vis absorbance values exceeding 2
In classical analytical chemistry, absorbance values in Beer's law can theoretically range from 0 to infinity.
As general rule of thumb neither high, nor very low absorbance are used for ...
9
votes
3answers
11k 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 ...
3
votes
0answers
73 views
Why does Larmor precession occur in NMR?
Larmor precession is the phenomenon that in NMR the spin (and by default also the magnetic dipole moment) of protons does not line up with the applied magnetic field but rather precesses around this ...
3
votes
1answer
740 views
How can every atom have unique spectral lines?
Why does every element have unique spectral lines?
I have studied that spectral lines help us to identify the properties of an element. This is because after excitation when the excited electron(s) ...
4
votes
1answer
137 views
How to find the lifetime of sodium atoms in the 3p using information on the spectral line width?
I am trying to find the lifetime of sodium atoms at the 3p state using the relationship between the spectral linewidth and the uncertainty principle:
$$ \Delta E \times \Delta t = h / 4\pi $$
The ...
5
votes
0answers
56 views
Is there an anisotropy factor (g factor) for TDDFT Abs and CD calculations?
Experimentally, anisotropy factor is calculated by dividing the CD spectra by the absorbance spectra multiplying by a factor of 32980 (in order to get a nondimensional quantity)
Theoretical ...
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3answers
847 views
Luminescent materials under IR radiation?
is there a database somewhere of materials that are luminescent when exposed to Infra Red radiation?
1
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0answers
18 views
LIF spectrum with laser-coupled levels lines as the strongest
Why, if I excited the OH molecule Q-branch transition, my LIF spectrum (recorded during and after the laser pulse) does not result in only Q line being the most prominent, but also P and R lines with $...
0
votes
1answer
143 views
Emission spectra (Be and Mg)
The line says
Alkaline metals and their salts impart characteristic colour to an oxidising flame. This is because the heat from the flame excites the outermost orbital electron to higher energy level....
2
votes
2answers
215 views
How can the formula of WD-40 remain a trade secret?
Wikipedia says
WD-40's formula is a trade secret
How can it remain a secret today? Can't mass spectrometers, NMR and other equipment be used to determine the structure of the compounds that go ...
0
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0answers
39 views
Interpretation of Carbon-13 NMR Spectra for Chemically Similar Carbon Atoms
I am analyzing the Carbon-13 NMR data from a reaction I will be modifying. There is not NMR data available for the specific compound I am attempting to synthesize; however, the compound I am ...
1
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0answers
50 views
In UV photoelectron spectroscopy why are filled orbitals seen at a higher binding energy?
I understand that the partially filled Ni intersects the Fermi level as there is a change from filled to unfilled orbitals. But what is the explanation for the lower binding energy of the filled 3d Cu ...
1
vote
0answers
24 views
Difference in transition lifetimes for near-resonant v/s non-resonant Raman effect
I have read that a virtual state is created when Raman scattering occurs. Since this state is forbidden, the molecule upon absorbing a photon (not enough to take it to the next electronically excited ...
9
votes
1answer
138 views
Hybrid orbitals, photoionization and time dependence
Wikipedia, in the article on Orbital Hybridisation, states the following:
One misconception concerning orbital hybridisation is that it incorrectly predicts the ultraviolet photoelectron spectra of ...
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votes
1answer
52 views
Difference between equations for spectroscopy wavenumbers
What is the difference between these two equations and when is appropriate to use which one?
We were told that $F(J)$ in this case is
$F(J)= E/h$ for units in frequency
And
$F(J) = BJ(J+1)$
$\...
0
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0answers
27 views
Rotational spectroscopy application of different equations [duplicate]
So we were given two problems and I don't understand why one was solved one way, and another was solved another way
Estimate the bond length of 12C16O (pure rotational spectrum) given J''=3 (15.356 ...
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vote
1answer
113 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 ...
2
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0answers
42 views
Can stimulated Raman processes be strong enough to drive out of equilibrium?
In the (spontaneous) Raman process, incident light $\hbar \omega_1$ scatters and transfers some energy $\hbar \omega$ to a vibrational excitation of molecule or solid.
Typically this is a very rare ...
3
votes
2answers
136 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 ...
0
votes
1answer
106 views
Does Mie Scattering occur in Liquid or is Rayleigh scattering?
Rayleigh scattering occurs when the dimensions of the scatter is much smaller than the wavelength of the incident electromagnetic radiation.
Mie scattering occurs when the dimensions of the ...
0
votes
1answer
82 views
Size of atomic shells
Is the distance of some $n^{th}$ shell ($n$ $\neq$ outermost shell) from the nucleus of an atom different for different elements?
If so, then how much is the difference or how could we calculate it?
2
votes
1answer
130 views
Spectroscopic Determination of an Equilibrium Constant [closed]
I got values of $\pu{5.0E-4}$, $\pu{2.5E-4}$, and $183$ for the following problems. However, these seem to be inaccurate and I'm not sure what to do.
Here is the procedure:
Obtain 6 disposable ...
4
votes
2answers
151 views
Raman Scattered (inelastic) photons rescattering outside of capture area
Imagine that you are focusing the raman laser right at the center of the vial sample with water (or any liquid) using 532nm laser.
(from "Photometric Standards for Raman Spectroscopy)
For raman 180 ...
2
votes
1answer
860 views
Right Raman Spectrum of Cyclohexane
In this site, https://www.princetoninstruments.com/userfiles/files/assetLibrary/Technical%20posters/Poster-Aberration-Free-Raman-Spectroscopy-with-the-IsoPlane.pdf the raman spectrum of cyclohexane ...
1
vote
2answers
56 views
Size of target substance in IR and Raman Spectroscopy
Usually what is the size (in nanometer) of the sample or target substance in IR and Raman Spectroscopy?
Which one has largest size of sample it can scan, how many molecules or atoms are involved? If ...
4
votes
2answers
277 views
Determine structure from NMR
So I've come across this lovely spectrum of a compund where only its chemical formula is given with C5H9NO.
I have made the pictures of the spectrum available below.
obviously there is some pretty ...
1
vote
0answers
30 views
UV-Vis absorbtion spectra of a radical cation/anion. Is it different compared to its even counterpart?
I am studying gas-phase Uv-Vis spectra of radical molecules. If one electron is removed/captured from the neutral even electron molecule, what can I expect from the absorption spectra? Will new peaks ...
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0answers
104 views
What is the effect of applied magnetic field on absorption spectrum?
Upon application of a weak magnetic field, a line in the microwave absorption spectrum of rigid rotor splits into $3$ lines. The quantum number $(J)$ of the rotational energy level from which the ...
2
votes
1answer
178 views
Spectra from Dipole Moments [closed]
If there is a change in dipole moments such as absorption spectroscopy, there are infrared absorptions (and/or transmittance) corresponding to them. Is the reason change in dipole moments are in ...
8
votes
1answer
198 views
Any way to see “cubic-active” vibrational modes in molecules?
For molecular complexes with a moderate amount of symmetry there are usually vibrational modes that are neither infrared nor Raman active. Sometimes these modes transform as some cubic polynomial in x,...
2
votes
2answers
306 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 ...
