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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11
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1answer
27k 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?
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136 views

Effects on Stretching Frequencies in Organometallic Complexes

If you added a negative charge to $\ce{Mo(CO)_6}$ how would this effect the $\ce{CO}$ stretching frequency in an IR spectrum? My thoughts are that it would lower the $\ce{CO}$ stretching frequency ...
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2answers
588 views

Relationship between the symmetry number of a molecule as used in rotational spectroscopy and point group

While doing a problem set I noticed that the symmetry number of a molecule turns out (usually) to be half the number of symmetry elements that the point group the molecule belongs to. When I say ...
4
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1answer
176 views

Proton NMR spectrum of aminocaproic acid

I have completely unpacked the NMR spectrum for the drug aminocaproic acid (structure below) and have found a peak that I myself can not explain. The signals for the drug are simple to understand what ...
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0answers
62 views

How to recreate bioluminescence in fireflies in a lab?

I have several mg/mL of D-luciferin, luciferase (firefly), ATP solution (25 mmol) and MgSO4 (for the mg2+ ions). The experiment has to be done in cuvettes of 2,5 mL (if I get any results, it will be ...
7
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1answer
199 views

What could cause a peak to split into 2 resonances as temperature increases in NMR spectroscopy

I understand that some peaks may coalesce upon an increase in temperature due to an increase in free rotation, this does occur with some peaks in my data. Why might you see a peak split into two other ...
7
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1answer
67 views

How to determine the number of water ligands attached to Pd in palladium chloride aquacomplex PdClₓ · yH₂O?

Normally this can be achieved using UV-vis: there is a strong absorbance around wavelength 220 nm which varies based on the number of ligands. If the palladium concentration is too high, the solution ...
9
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2answers
5k views

Which are the differences between resonance Raman and fluorescence?

If the fluorescence is the re-emitting of a photon with a larger wavelenght due to the transition from a higher energy state to a lower energy state the case of resonance Raman (where there aren't any ...
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1answer
16k views

What is the maximum number of emission lines when the excited electron of a H atom in n = 6 drops to ground state?

When an electron in a higher energy state jumps to a lower energy state in Bohr’s model of hydrogen atom, it emits a photon which has energy equal to the difference in the energy between the final and ...
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1answer
159 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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0answers
84 views

Internal standard with low T1 for NMR quantification of ammonia

I am trying to quantify low levels of ammonia with NMR. The good news is that ammonia has a very low T1 (<0.5) which shortens d1 a lot. However, I cannot find any internal standard with a T1 that ...
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0answers
27 views

why do cone cells have broad responsivity spectra peaks?

My understanding of vision is that, although the cone cells in our eyes can detect three colors, there is enough overlap in the responsivity spectra (as shown here: https://en.wikipedia.org/wiki/...
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1answer
34 views

Absorbance maxima of polyenes [closed]

Can anyone tell me how to approach this question from USNCO 2013?
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2answers
155 views

Flame Test Spectrograms Not Lining Up With Reality

In my AS Level chemistry class we had tried some DIY spectroscopy of flame tests with the aim of identifying anions in certain salts. We got as far as building the spectrometer inspired by Paper Craft ...
2
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0answers
34 views

How do we determine the proportions of hydrogen to helium (to other elements) that make up the Sun given the absorption spectra?

I am reading about the composition of the Sun and how it is made up of mostly hydrogen (~90%) and helium (>10%) and other elements with much smaller quantities. The explains that different elements ...
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0answers
2k views

How to tell if a vibrational mode in benzene is IR and/or Raman active?

Consider the vibrational mode that corresponds to the boat-like bending of a benzene ring. Is it (i) Infrared active? (ii) Raman active? This question was asked in an assignment in my college. ...
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1answer
1k views

Why the chemical shift value of a proton be negative in proton NMR spectroscopy where TMS is used as standard reference?

If there's any possibility of chemical shift value of a proton be negative.And if yes...why..when I am using TMS as a standard reference.
3
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2answers
147 views

Why doesn't the current stop in neon lamps when all the atoms have been ionized? [closed]

My personal experience (which could be wrong) tells me neon lamps can theoretically run on forever or at least a very long time. If current is the flow of charge why doesn't it stop once all the atoms ...
5
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1answer
649 views

Proton NMR Spectrum for Nicotinic Acid

I have been able to slightly unpack the NMR spectrum for the nicotinic acid to understand the order of signals and what ones correspond to what protons but I am having a problem with why some of the ...
5
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3answers
295 views

Heisenberg uncertainty principle and lifetime of the excited state

A key relation in optical spectroscopy also used to derive natural bandwidth is: $\Delta E \Delta t=ℏ$ $\Delta E$ is the energy uncertainty of a state, $\Delta t$ should be the uncertainty of the ...
10
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2answers
2k views

HNMR - Peak Splitting

If you have $\ce{CH3-\color{\red}{CH2}-CH2Br}$ Then the $\ce{CH2}$ (in red) would have a peak splitting of $12$ because $(n+1)(m+1)$ where $n$ for the $\ce{CH3}$ is $(3+1=4)$ and m for $\ce{CH2}$ is $...
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1answer
40 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, ...
3
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1answer
228 views

Raman transition and IR transition

In a character table one can find the reducible representation of vibration of a molecule. In such a table we also see things like $xy,z,x...$. If this corresponds with an irrep we say that IR ...
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1answer
2k views

How to make approximation of rotational partition function of diatomic linear molecules?

Using the rigid rotor approximation to the level energies, and such other appropriate assumptions, we can approximate rotational partition function, $Q_{\mathrm{rot}}$, of linear molecules as follows: ...
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3answers
837 views

What other nuclei besides hydrogen and carbon 13 could be detected using NMR and is H1-NMR synonymus to NMR?

From my understanding NMR spectroscopy works by analyzing the resonance frequency of each structure around a hydrogen or carbon. Since every structure is unique and has unique resonance frequency it ...
0
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1answer
33 views

Potential energy curve for dissociation of cation of carbon dioxide

Can anyone suggest me some literature where I can find the potential energy curve or the electronic states for singly ionized $\ce{CO2}$ ($\ce{CO2+}$) dissociating into singly charged oxygen ($\ce{O+}$...
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2answers
72 views

High content of potassium reported by spectrometer in silverware items

We tried to analyze a few silver items (800/1000 fineness) using a hand-held ED-XRF spectrometer, which is normally used for analyzing industrial alloys etc. The operating element range of the device ...
2
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2answers
194 views

Do microwaves heat polar molecules or molecules with polar bonds?

IR is only absorbed my molecules with polar bonds regardless of the overall polarity of the molecule , what about Microwaves ?
2
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2answers
11k 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}{...
7
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2answers
4k views

Why can't carbon and nitrogen be detected in ICP-MS and ICP-OES techniques?

Why can't carbon and nitrogen be detected in ICP-MS and ICP-OES techniques? My book says this technique is efficient for detecting multiple elements, with exception of halogens and carbon. I found ...
3
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2answers
89 views

Restrictions on transitions in Spectroscopy

Just for the sake of an example, let's assume a transition from one energy state to a higher energy state in atomic absorption spectrum If the energy difference between the ground state and first ...
10
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1answer
566 views

Hydrogen's confusing purple glow

While reading about hydrogen on Wikipedia, I read that hydrogen has a purple glow in its plasma state. This confused me because although hydrogen is present in its plasma state on the sun, the sun ...
4
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1answer
258 views

The carbon-13 NMR for 1,1,1-trichloroethane

I was working the following spectroscopy problem from Clayden 2e that asks us to identify the structure of a compound from information about its carbon-13 NMR peaks. Problem 3.5: The solvent ...
12
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2answers
6k 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 ($\pu{253 nm}$...
2
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1answer
29 views

Can nuclear shielding affect relaxation time?

Can nuclear shielding affect the transverse relaxation time, i.e extend the relaxation time the more the nuclei is shielded?
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0answers
26 views

Does conjugation increase λmax of n to π* transitions as well as π to π* transitions?

I would presume yes if the lone pair is on an atom involved in the conjugated system, but no if the lone pair is not on an atom involved in the conjugated system.
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0answers
47 views

Molar extinction coefficient a measure of the probability of a photon of the associated wavelength being absorbed?

Obviously the molar extinction coefficient is not equal to the probability of a photon of the associated wavelength being absorbed, but is it a measure of the probability i.e. the higher the value of ...
5
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2answers
438 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 ...
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0answers
50 views

Are the molar extinction coefficient and cross-section of an absorption related to the probability of absorption?

I have struggled to find a formal definition of cross-section of absorption; from what I've gathered, its best defined as 'the intensity of absorption'. Wikipedia's formal definition of the molar ...
1
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1answer
32 views

Are we using an assumption in calculating the ratio of first state excited to unexcited chemical species using the Boltmann distribution?

In calculating the ratio of first state excited to unexcited chemical species using the Boltzmann distribution, are we assuming that we only have species in the unexcited and first state excited ...
1
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1answer
52 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 ...
2
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1answer
68 views

Is a time domain spectrum obtainable from a frequency domain spectrum? [closed]

I know for a fact that a frequency domain spectrum can be obtained from a time domain spectrum using a Fourier transform - but can you do the reverse? Also what are the advantages and disadvantages ...
0
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1answer
45 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 ...
9
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1answer
254 views

Does light absorption result in a momentarily weaker bond?

I may have just misunderstood the topic, but light absorption by a molecule is a result of a promotion of an electron into an empty antibonding orbital; hence, does that mean that the strength of the ...
4
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1answer
104 views

Is there a finite number to the possible state of electron in an atom?

Okay, I know you may say, there are infinite numbers of possible state of an electron between its ground state and ionic state, since energy levels get continuous as electron gets farther away from ...
4
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3answers
5k views

Uv vis and fluorescence spectroscopy: sensitivity

Is there any reason why fluorescence spectroscopy is said to be more sensitive than Uv-Vis?
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0answers
151 views

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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0answers
279 views

Determining absorption spectrum, magnitude of Δo and spin for Iron Oxalate

Iron Oxalate ions form an emerald green solution when dissolved. If the ions in solution appears emerald green to our eyes, what color of visible light is it mostly absorbing? What does that tell you ...
7
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1answer
137 views

Do molecules change shape when they become vibrationally excited?

In quantum mechanics we learn the the classical picture of a vibrational excitation is wrong. The atoms will 'vibrate' only while the molecule is in a coherent superposition of two vibrational states, ...
5
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0answers
166 views

Why is spin-contamination undesirable?

Why is spin-contamination, introduced by e.g. Unrestricted Hartree Fock, undesirable, and to which problems further on can it lead? The only problem I currently see myself is that spin-contamined ...

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