# Tag Info

## Hot answers tagged spectroscopy

82 votes
Accepted

### Is it true that heavy water is not blue?

Based on your description, I may have found the article you originally saw, or at least one very similar. Researchers from Dartmouth College published a paper$\mathrm{^1}$ in which they report, among ...
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69 votes

### Is it true that heavy water is not blue?

This does seem to be the case. I don't have images of the different types of water, but I did find this overlaid IR-visible spectrum of water and heavy water: As you stated, the presence of deuterium ...
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52 votes

### A compound that absorbs all visible light

Substances which absorb almost all the light falling on them appear black. Therefore you are looking for the blackest known compound. The record is currently held by Vantablack[1], a substance ...
• 14.9k
52 votes

### Can hot food ever emit x-rays or gamma rays?

In theory, yes, you can heat objects to a high enough temperature to emit x-rays or gamma rays. You cannot do this to food, and you certainly cannot do this in your kitchen (or probably any kitchen). ...
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31 votes

### Can hot food ever emit x-rays or gamma rays?

It has nothing to do with what you were going for, but there is a small, but non-trivial amount of x- and gamma-ray output for most food and so the answer is trivially "yes". In particular any food ...
27 votes

### Why does hydrogen burn with a pale blue flame while its emission spectral lines are red in colour?

It is a very interesting question, but comparing a combustion spectrum with an atomic emission one is like comparing apples and oranges. A flame is a luminous gas phase chemical reaction where the ...
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22 votes

### A compound that absorbs all visible light

All metals are capable of absorbing photons of any wavelength below hard ultraviolet, as ideally there are allowed electronic transitions of arbitrarily small energy between states in the unfilled ...
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18 votes
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### Why does the C–C bond have extremely weak absorptions?

For vibrational spectra the primary transition under investigation is the $v = 1 \leftarrow 0$ excitation (because $\hbar\omega >> k_\mathrm{B}T$, so excited states have negligible thermal ...
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17 votes
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### Why is a singlet state called singlet and a triplet state called triplet?

The terms arose back in the early days of quantum physics when spectral lines that were expected to be singlets were actually observed to be more complex (doublets, triplets, etc.). An electron can ...
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17 votes
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### Is carbon dioxide IR inactive?

I am told that carbon dioxide is IR inactive. You're right, that's not true. Since carbon dioxide is linear it has $3n-5 = 4$ vibrations and they are pictured below. The symmetric stretch does not ...
• 81.9k
16 votes

### How do I determine the molecular vibrations of linear molecules?

As Tyberius noted, the projection formula does not work for infinite order groups (this is because the Hermitian form on characters is defined to be G-invariant by averaging over all elements in a ...
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16 votes
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### Relationship between the symmetry number of a molecule as used in rotational spectroscopy and point group

This is not in general true Consider molecules a point group not containing inversion symmetry, e.g. $C_2$ hydrogen peroxide The $C_2$ group has only two elements, $E$ and $C_2$, and the $C_2$ ...
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15 votes

### Why is tetramethylsilane (TMS) used as an internal standard in NMR spectroscopy?

TMS was first proposed as a reliable internal chemical shift reference in 1958 by Tiers. Back in them good ol' days, 1H NMR was called proton nuclear spin resonance, or nsr, and the tau scale was ...
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15 votes
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### Comparing the experimental and calculated UV/vis spectra for ethene

In short, there are two obvious problems with the setup OP uses for TD-DFT calculations: B3LYP functional is not a good choice for TD-DFT. 6-31G(d) basis is usually too small. At M06-2X/Def2-TZVP ...
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14 votes
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### Why is tetramethylsilane (TMS) used as an internal standard in NMR spectroscopy?

TMS has 12 protons which are all equivalent and four carbons, which are also all equivalent. This means that it gives a single, strong signal in the spectrum, which turns out to be outside the range ...
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14 votes
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### Why don't equivalent hydrogens cause splitting in NMR?

I will provide a full quantum mechanical explanation here.[1] Warning: rather MathJax heavy. Hopefully, this lends some insight into how the diagrams that long and porphyrin posted come about. ...
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14 votes
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### Uv vis and fluorescence spectroscopy: sensitivity

Fluorescence is a 'zero background' or absolute type of measurement meaning that single photons can be measured against a 'dark' background so the sensitivity is huge, and limited by the fraction of ...
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14 votes
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### 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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### Are the bonding orbitals in methane equivalent - photoelectron spectrum

This question goes along the line of what does it mean when it is said that an sp3 orbital has 25% s-character. It also intrigued me so I have tried to find answer, which would not break my hybridised ...
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13 votes

### A compound that absorbs all visible light

Any black compound absorbs photons in all the visible spectrum; that is why essentially it appears black to our eyes. So, for instance, iron(II,III) oxide, $\ce{Fe3O4}$ will do so; it is even used as ...
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13 votes
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### 1H (proton) NMR spectra for alkanes

Unfortunately, although the answer given by bon provides a very simplistic answer to a fairly common NMR-101 problem, it is not quite correct. It is fine for the propane case, but falls short for ...
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13 votes
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### What is the correct way to verify a structure's geometry, for example for benzene?

A deviation within the low picometres is nothing to worry about, there are many reasons for this. Primary literature, like peer-reviewed journals, will always publish an analysis of the obtained ...
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13 votes
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### Derivation of the Orgel diagram for octahedral d2 complexes

1. Weak-field and strong-field limits I will adopt the description used in Figgis and Hitchman's Ligand Field Theory and Its Applications (p 5), because I cannot really phrase it better: It is ...
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13 votes
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### What is the difference between quantities reported as optical rotation and circular birefringence?

It gets very complicated, there are many papers and books on the topic and this little table, from Jensen et al. 1, gives a glimpse of the anisotropic effects: An extremely short answer is this: when ...
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12 votes
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### Why are d-d electronic transitions forbidden and weakly absorbing? Why do they occur at all?

In order for an electronic transition to be allowed (occur with strong intensity), certain "selection rules" must be obeyed. You may already be familiar with the rule that the electron spin quantum ...
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12 votes
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### Interpretation of TD-DFT results

Time-dependent DFT can be used to predict excitation energies through a linear-response formulation. In this Gaussian result, beyond the first line, you are looking at the largest coefficients in the ...
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### In infrared spectroscopy, why is there no C=C reading for E alkenes?

The $\ce{C=C}$ stretch is responsible for this ir peak. For an ir absorbtion to occur, the absorption must result in a change in dipole moment. If we examine the $\ce{C=C}$ stretch in cis- and trans-...
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12 votes

### Are the bonding orbitals in methane equivalent - photoelectron spectrum

The short answer is that's not how photoelectron spectroscopy works. It's a one-photon spectroscopy. You have a sample of $\ce{CH4}$ in this case, and you shoot different energy photons at the sample....
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12 votes
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### In spectroscopy, is it possible for the sample to be excited multiple times?

Very technically? Yes. Realistically? The probability is small enough that even if it does happen, the peaks for the multiple transitions are going to be small enough that we cannot really observe ...

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