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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4
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1answer
293 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 ...
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2answers
7k 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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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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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
52 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 ...
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2answers
464 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
55 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 ...
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1answer
33 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 ...
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1answer
53 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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1answer
69 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 ...
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1answer
50 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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1answer
279 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 ...
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1answer
109 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 ...
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3answers
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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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156 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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290 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 ...
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1answer
143 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, ...
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0answers
172 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 ...
4
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1answer
7k views

Why are square planar coordination compounds with four different ligands optically inactive?

Why do square planar coordination compounds of type $\ce{[Mabcd]}$ not show optical activity, although they contain 4 different ligands (i.e. chiral central metal atom)?
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1answer
994 views

What is the difference between X-ray photoelectron spectroscopy and X-ray absorption spectroscopy?

XPS is used for surface chemical state identification. But other than that, is XAS just an inverse of XPS?
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0answers
154 views

Vibrational Self-Consistent Field

There are a few things about VSCF that I don't understand: Which coordinates do you work in? If you work in normal mode coordinates, how do you evaluate the effect of the average of one normal mode ...
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0answers
140 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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3answers
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Why is filter paper fluorescent?

Typical filter paper used in chemistry lab (e.g. Whatman #1) exhibits autofluorescence in near UV with excitation maximum at 365 nm and emission maximum at 440 nm. It was reported in literature1 that ...
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1answer
1k views

Is the photoelectric effect possible for semiconductors?

Is the photoelectric effect possible for semiconductors? I imagine that this might be, if possible, a two-photon process: excitation of an electron from a filled to conductance band, and then on to ...
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1answer
81 views

What is the relationship between emission and fluorescence?

A molecule exposed to a photon with some energy is put into an excited state, and emits a photon of some energy when it returns to ground state. The photon that provided the energy for excitation ...
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2answers
1k views

What splitting pattern is this?

I am trying to find the structure of an unknown molecule. What is the splitting pattern of these sets of lines? I think my unknown molecule is butanol and the hydrogens this splitting pattern is ...
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0answers
92 views

Differences between LIBS over LA-ICP-MS? [closed]

Which differences should I consider between the techniques Laser-Induced Breakdown Spectroscopy and Laser Ablation Inductively Coupled Plasma Mass Spectrometry for a geochemistry fingerprint focused ...
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0answers
93 views

Absence of anti-Stokes line in Raman spectrum

If we have a molecule 1H-25X which shows a band at $3000~\mathrm{cm}^{-1}$ in the IR spectrum and you want to record a Stokes and anti-Stokes Raman spectrum with a laser of 1000 nm, why is the anti-...
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0answers
166 views

Why aren't lasers used as light sources for atomic absorption spectroscopy?

Atomic absorption spectroscopy yields extremely narrow signals. It is difficult to separate white light with sufficient precision using monochromators, and hollow cathode lamps are often used as light ...
14
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1answer
535 views

Dipole-quadrupole polarizability term for induced dipole

For describing an induced dipole, I have usually seen the following equation, $$ P_{i} = \alpha_{ij}E_{j} + \frac{1}{2}\beta_{ijk}E_{j}E_{k} $$ where $P_{i}$ is the $i^{\text{th}}$ component of the ...
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1answer
522 views

Maximum (Z) to (E) ratio in the photoisomerization of azobenzene?

Considering the photoswitching of azobenzene, I was told the following argument: Considering that the photoexcited state of azobenzene has a about 50% to 50% chance of yielding either the (E)- or (Z)-...
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1answer
2k views

Can visible light or infrared radiation excite electrons?

So the limit of ionizing radiation seems to be UV light. I'm wondering if IR light or Visible light can excite the electrons in an atom. It has to do with the difference in energy level right? So ...
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1answer
92 views

How to interpret detection limits

I am learning about different analytical methods such as Atomic Absorption Spectrscopy and Atomic Emission Spectroscopy and I'm having a difficult time understand how to interpret the detection limits ...
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1answer
94 views

Detection limits

I am given this question: A study showed that over 2000 children in China have been poisoned by lead. Blood samples have been taken and show that there is 10 times as much as the acceptable amount of ...
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0answers
129 views

How does activated carbon catalyse oxidation?

Context: I am doing an experiment on the quantitative analysis of ascorbic acid in a blackcurrent syrup using fluorescence spectroscopy. In order to do so, ascorbic acid was first oxidised into ...
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1answer
2k views

Why do carboxylic acid derivatives show a more upfield shift than carbonyls in 13C-NMR?

The prototypical 13C NMR shifts of carbonyl carbons is in the range 190–200 ppm, however, carboxylic acid derivatives have a 13C NMR shift in the range 160–180 ppm instead. What structural factors ...
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0answers
21 views

The Salt Effect stops working in extreme cases?

I and three friends did an experiment where we investigate the salt effect, also known as the uncommon ion effect. The specifics are that we made a $\pu{1L}$ solution saturated with $\ce{CaCO3}$ and $...
5
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1answer
22k views

What is number of 1H NMR signals for toluene?

I am searching the web for the number of 1H NMR signals of toluene and I find two different results. Some say there are two signals while others say there are four (According to me, it should be 4). ...
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0answers
1k views

How to draw a photoelectron spectrum with ionization energies?

I understand the basic concepts of the photoelectron spectrum. However, say I am instructed to draw the PES graph for Silicon. I know how to figure out the relative peaks based on its configuration. ...
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0answers
111 views

Raman database of polymer spectra for analyses in R/python

Can anyone reccomend a raman database of polymer spectra that is open source or freely available? I'm analyzing spectra of a number of plastics and looking for reference spectra for identification. ...
7
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1answer
262 views

Determining overtones bands of degenerate modes

I have a question of a more mathematical nature on the mathSE (Symmetric Direct Product Distributive?) that received a good answer, but I think an answer more oriented to chemists would be a useful ...
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0answers
100 views

Theoretical limits of (NIR) spectroscopy for quantitative analysis

Edit (months later): In hindsight, I was asking about the theoretical limits of multivariate curve resolution (see Lawton & Sylvestre: Self-modeling curve resolution) in chemometric applications. ...
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1answer
580 views

Stretching frequency of nitro and nitrito

I got the values of the stretching frequency of nitro and nitrito ligands from this webpage. I understand why stretching frequencies are close in nitro and well separated in nitrito. How to explain ...
8
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1answer
218 views

What is the range of photon energies can be effectively used to split a diatomic molecule like hydrogen?

I saw photo-ignition of hydrogen-chlorine mixture using UV light on YouTube. Chlorine molecule bond dissociation energy is $\pu{2.51 eV}$ (bluish colour). Bromine molecule bond dissociation energy is ...
5
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1answer
4k views

Assignment of the IR spectrum of 2,4-pentanedione (acetylacetone)

The IR spectrum is shown here: My assignments so far: The 1728 and 1708 stretches correspond to $\ce{C=O}$ stretches in the keto form - the symmetric/asymmetric stretches respectively. The 3003 ...
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1answer
570 views

Comparison of chemical shifts among alkanes in NMR spectroscopy

I was reading through the factors influencing chemical shifts in proton NMR. I found the order of chemical shifts in the image above. Why is this order supposed to be as it is? I thought it would be ...
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1answer
5k views

What is the role of a monochromator in AAS?

What would happen if we didn't use a monochromator; i.e. what is its role in Atomic Absorption Spectroscopy?
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1answer
87 views

I'm trying to make an optical dissolved oxygen sensor. I need help finding an appropriate oxygen sensitive dye

I have been trying to make a dissolved oxygen sensor, the basics are described on this page. I have a good understanding of the electronics behind this measurement, but I cannot find a suitable ...
5
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1answer
364 views

NMR spectrum of 3-hexylthiophene: why is the methyl group not a triplet?

I have an example spectrum, and it shows which peaks correspond to which H, but I don't understand why the spectrum looks like that e.g. I would expect the peak at 0.88 to be a triplet based on the (...
4
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1answer
441 views

What is the approximate lifetime of an SN2 transition state complex?

Imagine some SN2 reaction involving an aliphatic sp3 carbon with a halide leaving group (e.g. iodide or bromide). Given the rise of femtosecond spectroscopy (e.g. using a mode-locked Ti:sapphire laser)...

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