17 votes

Why does ethylamine have two infra-red spectra? Are both correct?

When looking at IR spectra of hydrogen-bonding groups, always check how IR spectra was recorded. It appears that NIST spectra is recorded using gas phase, while the first one used liquid film. In ...
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14 votes
Accepted

Looking for a dye which emits around 680 nm

There are two dyes commonly used in biochemistry research with $\lambda_\mathrm{Ex}$ of around $\pu{650 nm}$. They are Alexa Fluor 647 from ThermoFisher and Cyanine5 (Cy5) from Lumiprobe. Two example ...
13 votes
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Why does tryptophan absorb UV light?

Any molecule can absorb UV light. What the question is probably going for is why those three molecules absorb at longer wavelengths than other amino acids. This has to do with the conjugated pi bonds ...
  • 1,606
13 votes

How does molecular structure contribute in the property of fluorescence?

Keep in mind a few things that must happen for an absorption process to result in fluorescence: (1) the initial transition is to an excited electronic state that observes certain rules regarding the ...
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10 votes
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What is the purpose of UV-VIS Spectroscopy?

The broad absorption bands of the electronic transitions observed in UV/Vis spectroscopy/spectrophotometry, as well as the myriad types of electronic transitions that might lead to absorption at a ...
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9 votes
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Why Acetone does not behave like its computational values?

It seems that the results of the calculations are more or less fine and the OP just misinterpreted the NIST data. As I said in my comment above, NIST does not claim that $\lambda_{\mathrm{max}}=276 \, ...
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8 votes

How does absorption spectroscopy work?

In an absorption experiment a collimated beam of light is passed through the sample. The intensity of this is measured before and after the sample. The absorption is measured as optical density which ...
  • 27.8k
8 votes

If Beer's Law appears to apply to a high concentration (>0.01M) of a solute, is it valid to use for concentration calculation?

Internet consensus seems to define "high concentration" for Beer's Law as >0.01M Keep in mind that Beer's law is an approximation. Look at the more advanced version. Beer's law-advanced version. ...
  • 35.1k
8 votes
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How does a spectrophotometer measure absorbance based on the intensity of detected light?

There are two possibilities: If there is only one optical pathway, you record i) the blank sample for all wavelengths of interest and store the detector's intensity information in function of $\...
  • 23.9k
7 votes
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Bohr's model of atom

Since this is pretty well covered in countless number of books and other sources I will try to just emphasise the key points. The emission spectrum of atomic hydrogen was well known at that time, the ...
  • 18.6k
7 votes

Why is absorbance the negative logarithm of transmittance?

Absorbance is useful because it is additive. That is, it's absorbance which is used in Beer's law: $$A = \epsilon \cdot c \cdot l$$ While you can certainly make a version of this law which uses ...
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6 votes
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Reference for UV/Vis Spectroscopic Data request

To know the chemical name, molecular structure, or best: the unique registry number provided by the American Chemical Society (CAS-number) is beneficial in your search as trade names may vary ...
  • 23.9k
6 votes
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How to determine the iron content in spinach with a spectrometer?

Here's how I approached the problem. Plot the Data Below is the plot I quickly hacked together, and a fit of the form $g(x) = a\cdot x$. (Note that I do want the line to go through (0,0) because the ...
  • 10.6k
6 votes

Absorption coefficient for liquid water

What the plot shows is the reciprocal path length needed for a given absorbance to apply, for example slightly less than 100 m in the visible to $10^{-8}$ m in the uv, as you point out, but this is ...
  • 27.8k
6 votes
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Why decarbonate soda for spectroscopy experiment?

The primary reason is that when soda warms up, the $\ce{CO2}$ would escape slowly as bubbles. These bubbles would significantly scatter light affecting the absorption spectra. Usually, the effect of ...
  • 1,562
6 votes
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When making a calibration curve, do we have to use the wavelength at maximum absorbance?

No it doesn’t. Measurements can in principle be made at any wavelength. If there are two colored specimens in the same solution, it is actually better to measure at the wavelength where the ...
  • 645
6 votes

Why does ethylamine have two infra-red spectra? Are both correct?

Honestly I have doubt about your textbook version of IR spectrum. However, I can suggest by the experience I have gained that the shapes of the peaks depends on the method you have used to obtain the ...
6 votes

The reason for selection of wavelengths in the spectrophotometry of Quinoline Yellow SS

The experiment in your link pg 31 is "Path Length Dependence of Absorbance Values". As you stated, quinoline absorption spectrum has a dip near 337 nm. The reason for choosing the wavelength ...
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5 votes
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Which is most reliable excitation energy: LUMO-HOMO energy or TD-DFT excitation energies?

When you can, use TDDFT, in part because of the oscillator strengths I like to point out to my group that optical excitations consist of two things consistently (e.g., in a UV/Vis spectra). There are ...
5 votes
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Absorption coefficient for liquid water

The plot is from Wikipedia, where important/helpful links to the source information can be found, and is worth a look. A similar and more detailed plot can be found on this page of Martin Chaplin's ...
  • 5,876
5 votes

Why is absorbance the negative logarithm of transmittance?

Beer's law relates transmitted intensity to concentration $[c]$ of the solution at each wavelength $\lambda$ as $$ I_{trans_\lambda}=I_0e^{-\epsilon_\lambda [c]L}$$ where L is the sample path-...
  • 27.8k
5 votes

Determining Kc for the equilibrium involving iron thiocyanate using spectrophotometric data

The problem is a bit strange since the details of the absorbance measurement aren't detailed very well. I'll assume the book defines the relationship as: $$A = \epsilon bc$$ where: A=absorbance $\...
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5 votes
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Determining Kc for the equilibrium involving iron thiocyanate using spectrophotometric data

To start with, its often helpful to make an ICE table when dealing with problems of chemical equilibrium. $$\begin{array}{cccc} \begin{array}{c|ccc} \hline & \ce{Fe^3+} & \ce{SCN-} & \ce{[...
  • 11.5k
4 votes
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Lambert Beer's law deviations

A compound S follows the Lambert-Beer law if the absorbance $E_\lambda$ at a particular wavelength $\lambda$ is proportional to the concentration $c$ of S. \[E_\lambda = \left(\frac{I_0}{I} \right) = ...
4 votes
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Molar absorption coefficient - why are the molar absorptivities defined to be the same?

An isosbestic point is found by maintaining the sum of the concentrations of the two (or more, though it's very unlikely) species the same and varying the individual contributions of each, and looking ...
4 votes

Molar absorption coefficient - why are the molar absorptivities defined to be the same?

Actually that's not the correct definition of the isosbestic point. From IUPAC goldbook: Isosbestic point - Wavelength, wavenumber or frequency at which the total absorbance of a sample does not ...
  • 14.1k
4 votes

How to calculate vibrational spectra of organic molecules?

You can build molecules and calculate vibrational frequencies with Molcalc. Molcalc does have a molecular size limit, so for bigger molecules you must download some programs. I have written about ...
  • 4,779
4 votes

How to calculate vibrational spectra of organic molecules?

The usual approach is: Find the equilibrium geometry (within the Born-Oppenheimer app.). Expand the energy in Taylor series till second order term. The first order will be 0 because of the minimum ...
  • 1,977
4 votes

How do auxochrome groups work?

Generally speaking, the typical auxochromic groups possess (at least) one pair of non-bonded $n$-electrons and -- if taken alone -- do not absorb in the UV. If attached, for example to a benzene ring,...
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