Questions tagged [uv-vis-spectroscopy]
For questions that are related to the ultraviolet and visible spectrum of compounds, mostly due to electronic transitions between orbitals.
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Why is absorption spectrum of beta-carotene different from Zeaxanthin?
Here are the molecular structures of beta-carotene and zeaxanthin
These molecules only differ in the OH groups and the conjugated backbone length is exactly the same. I read that the absorption ...
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30 views
How do elemental halogens get their colour?
I learnt that in halogens the colour is due to transfer of electrons from HOMO to LUMO during excitation but I have trouble understanding what that means. Also I don't understand how iodine is ...
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Why is the iron inter-system crossing non-negligible at high temperatures?
Inter-system crossing (ISC) is a radiationless process involving forbidden transitions between singlet and triplet state. These are more common with heavy atom molecules, so I understand that ISC is ...
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Are there any forbidden collisional electronic transitions?
There are spectroscopy selection rules marking some radiative electronic transitions as forbidden. For example, it is stated that single electron of H atom can not move from S2 to S1, or from P2 to S2 ...
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28 views
Why do carbonyl groups lower antibonding orbital energies of conjugated systems?
In the chapter on conjugation in Organic Chemistry by Clayden, Greeves, Warren and Wothers, there's a brief discussion of pigments with conjugated systems that includes the following paragraph (first ...
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1answer
78 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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1answer
211 views
Beer's law with a polychromatic light source
As is well known, Beer’s law (aka the Beer-Lambert law, among others) is predicated on a number of conditions, one of which is that the light source be monochromatic or, as a practical approximation, ...
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3answers
276 views
Why not use all the colours in colorimetry?
Colorimetry (and it's cousin visible-UV spectroscopy) is a form of chemical analysis that works by sending visible light of one wavelength through the sample. It then analyses how much of the light ...
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1answer
64 views
Molar absorption coefficient for potassium hydrogen phthalate
I would like to draw a comparison of the sensitivity between several spectrometers. Therefor I measure the absorption of a serial dilution of potassium hydrogen phthalate (KHP). The idea is to compare ...
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Changing of UV-Vis peak absorption after heating Ni(en)2(NO2)2
(c) The trans-isomer of $\ce{Ni(en)2(NO2)2}$ D is red $(λ_\mathrm{max} = \pu{500 nm}).$ It can be heated to give a new trans-isomer E, which is violet $(λ_\mathrm{max} = \pu{560 nm}).$ Assume that, ...
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1answer
37 views
What interesting things can be seen in spectra between 400-700nm?
The visible spectrum (400-700nm) tends to give a very blurry spectral picture, as it is not a wide band relative to the typical peak width of features you can find there. Further, the vibrational ...
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1answer
57 views
Why is the graph of fluorescence vs. wavelength continuous?
If the emission of a photon from an excited state to the ground state occurs with a precise energy, then why is it that wavelengths that differ from lambda max have values on a fluorescence vs. ...
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Determination of pKa and molar absorptivity coefficient using spectrophotometry [closed]
An indicator BPB (Brophenol Blue) is used and dissolved in solutions of various known $\mathrm{pH}$. Then, the absorbance of each solution at 590 nm was measured and the question asks for molar ...
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28 views
Standard Solutions for UV-Spectrophotometry
Can one use a mixture of compounds when making a standard solution for UV-Spectrophotometry. For instance, one is attempting to find the casein concentration of milk using UV-spec and they have milk ...
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1answer
83 views
Spin-forbidden transition iof bromine
In one of the lectures of my spectroscopy course, the professor claimed that there is a strong absorption band in the UV/Vis spectrum of bromine, associated with a singlet-state to triplet-state ...
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Nanoparticle Alloys (Gold-Silver) not forming
I have synthesised what I thought were gold-silver alloys at various different compositions.
The UV/vis data showed that nanoparticles alloys with a majority of the composition made from gold had one ...
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1answer
45 views
Characterisation of gold-silver alloy nanoparticle
I have various compositions of gold-silver alloy nanoparticles.
I would like to characterize my nanoparticles and confirm that they are alloys.
Aside from the qualitative analysis of the colour and ...
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1answer
100 views
Why we should concern with A∞?
To determine the order of one reactant at pseudo concentration of another reactant the plot of $\ln(A_t - A_∞)$ versus $t$ is used.
$A_t$ is absorbance of the intermediate at time $t$.
$A_∞$ is ...
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1answer
51 views
What is the molar extinction coefficient of azorubine (carmoisine, E 122)
I'd be grateful for the molar extinction coefficient ε of azorubine at the absorption maximum (about 520 nm), with an authoritative reference.
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1answer
486 views
what the solution of this problem, negative absorbance values in UV-vis spectrum
In my project, I take the UV-visible spectrum of reaction between pyridinium chlorochromate and N-acetylcysteine. In some wavelengths, the absorbance value is negative for example it is (-0.006, -0....
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1answer
35 views
How to measure UV index? [closed]
I am a high school student so please apologise if I say something very dumb or irrelevant. I am conducting an experiment in which I want to find UV index at different points. Such as under shade, ...
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0answers
31 views
tolerable error, CASSCF simulation of UV/Vis spectrum
Please forgive my ignorance of experimental spectroscopy. I am a computational chemist, not an experimental one. My question is: what would be considered a tolerable or reasonable difference between ...
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1answer
179 views
Slope of Graph of Wave Number vs inverse square of the principal quantum number(1/n^2) in Hydrogen Emission spectrum
What is the final expression we get for wave number in a hydrogen emission spectrum? Let us say for example from 6 to an orbit $n$. Considering that it is an emission spectrum, I think it would be ...
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1answer
59 views
Evidence for a chemical reaction
I know that the emission of light is evidence for a chemical reaction. However, are there any instances where emission of light can also be considered a physical reaction?
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1answer
468 views
confusion about exocyclic double bond [duplicate]
I believe that there are only two exocyclic bonds in this structure. But in some text its mentioned as three.
can someone clarify.
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1answer
226 views
Why does benzene have 3 π–π* transitions in the UV range, but ethene, butadiene, and hexatriene each have only 1?
Are the π and π* orbitals of benzene much closer together in energy which allows for multiple transitions? Why is this not the case for hexatriene if they both contain the same number of pi bonds?
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Equation for the equilibrium constant for the hydration of carbonyls using UV spectroscopy
I was looking through the literature when I found this source:
https://pubs.rsc.org/En/content/articlepdf/1967/tf/tf9676302131
Which states that the equilibrium constant Khyd is:
Khyd = $\frac{[...
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1answer
47 views
UV Absorption of iodine with some solvent
$I_2$ in solution is typically absorbing around $525 \; nm$ this is why it appear to be magenta, I recently learn that with some solvent :
Benzene
Toluene
$o$-Xylene
$p$-Xylene
The coordination ...
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0answers
25 views
Why absorption changes in schiff-base formation? Coenzyme PLP
Pyrodoxal-5-phosphate (PLP) in enzymes form schiff-base to lysine residue. This can be detected by the change in the absorption from 390 nm to 420 nm (free PLP to stiff-base bound PLP).
I remember ...
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1answer
35 views
Angle of Light Source for Absorbance Spectroscopy
I apologize if this question is stupid, but have been puzzled by it for a few days. Say I have a spectrometer measuring in the visible light range and we're looking to measure the absorbance on a ...
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3answers
126 views
Is a hue indistinguishable from its equivalent frequency of light?
A hue is the color characteristic of a pigment, a particular chemical molecule, when illuminated by sunlight.
Of course such a molecule will not emit a single frequency of light, but will emit a ...
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1answer
81 views
Determination of Urea and Hydrogen Peroxide in Cow Milk by ATR-FTIR(Attenuated total reflection-Fourier Transform Infrared Spectroscopy) [closed]
I am doing a research on cow milk adulteration. I am checking Urea and Hydrogen Peroxide content in milk by ATR-FTIR(Attenuated total reflection-Fourier Transform Infrared Spectroscopy). I am using $\...
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Spectral database in the UV-Vis/NIR and IR regime
I'm searching for a bunch of spectra (some tens), preferably with intense bands in the Vis spectrum, to play around with (calculating chromaticity, etc.).
They should range from
250 nm to 2500 nm (UV-...
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3answers
177 views
De-excitation of a molecule
In our chemistry lecture today on UV/vis-spectroscopy, we discussed the origin of colour in conjugated molecules due to electronic transitions from lower-energy molecular orbitals to higher-energy ...
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119 views
Evaluating Franck-Condon factors
Franck-Condon overlap integrals are given by $\langle\psi^e_{v_e}|\psi^g_{v_g}\rangle$, where $\psi^e_{v_e}$ and $\psi^g_{v_g}$ are the vibrational functions for the $v^{th}$ vibrational states of the ...
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1answer
130 views
Notation for excited states
I am reading some papers about quantum chemistry and in one I found a notation for excited states of a molecule that I don't understand at all.
In the picture below you can see the potential curves ...
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1answer
41 views
How to meausre ion interferences in solution?
Is there a way to quantitatively measure the interference of different metal ions in solution? I plan to measure concentration of Pb(II), Fe(II), and Cu(II) in solution together using a ...
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2answers
3k 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 ($\pu{253 nm}$...
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2answers
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Woodward Fieser rules for dienes - non conjugated double bond as exocyclic?
When looking for exocyclic double bonds in a diene, should a double bond not connected to the conjugated system be considered?
For example, in this compound, should the marked double bond considered ...
