Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Use this tag for questions about spectroscopic methods in practice and in theory, especially where more specific tags for the individual method (such as [tag:nmr-spectroscopy] or [tag:ir-spectroscopy]) are not available.

1
vote
0answers
17 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.
1
vote
0answers
10 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 ...
1
vote
0answers
12 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
vote
1answer
19 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
votes
1answer
26 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 ...
1
vote
1answer
26 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
vote
1answer
30 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 ...
0
votes
0answers
15 views

What's causing this artifact in FCS data?

This is a fluorescence autocorrelation curve from a fluorescence correlation spectroscopy experiment. I am very inexperienced with FCS and the setup I used has never been used for FCS specifically (we ...
0
votes
0answers
21 views

How to use Hund third rule for cases like Nb, Ru, Rh, Pt?

How do I use Hund third rule to determine the value of $J$ for ground state for cases like Nb, Ru, Rh, Pt where there is 1 electron in s orbital and then an incomplete p orbital, and besides, S nor L ...
2
votes
2answers
38 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 ...
2
votes
0answers
31 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-...
1
vote
0answers
50 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 ...
4
votes
2answers
89 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 ...
0
votes
0answers
29 views

PLQY for FIrpic, Rhodamine 6G, Ir(ppy)3, and 4CzIPN?

I'm looking for PLQY values for FIrpic, Rhodamine 6G, Ir(ppy)3, and 4CzIPN in solution, or in thin film (preferrably). Is there anyone who knows those values from reliable source?
2
votes
1answer
55 views

Franck-Condon factors in photoelectron spectroscopy

Franck-Condon factors have been used to predict / assign the vibrational structure of photo-electron spectra. $FCF=|\langle\psi''_{vib}|\psi'_{vib}\rangle|^2 = |\langle\psi''_{vib,1}\psi''_{vib,2},\...
5
votes
0answers
73 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 ...
7
votes
1answer
114 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, ...
0
votes
0answers
25 views

What is the lambda max of formaldehyde?

I am working with a remote sensing spectroscopy unit and attempting to measure formaldehyde vertical column densities in the atmosphere. The instrument's fitting window needs to be set to the lambda ...
5
votes
0answers
95 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 ...
4
votes
0answers
47 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 ...
1
vote
1answer
50 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 ...
2
votes
0answers
49 views

Where can I find example MS, IR, NMR spectra for teaching molecular spectroscopy?

I am teaching a course on analytical chemistry, and I would like to give my students MS, IR and NMR spectra to practice with. It would be ideal if they could analyze spectra in a more "realistic" way ...
1
vote
0answers
33 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-...
1
vote
1answer
88 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?
3
votes
0answers
33 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 ...
1
vote
0answers
39 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 ...
2
votes
1answer
240 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 ...
2
votes
0answers
21 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 ...
0
votes
0answers
29 views

Blank in Fluorescence spectroscopy

Quantum dots were dissolved in Millipore water and used as a sensor for the determination of an analyte by fluorescence spectroscopy. I need to know what blank is in this system? Millipore water, or ...
4
votes
1answer
89 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 ...
1
vote
0answers
19 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
votes
1answer
2k 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). ...
0
votes
0answers
29 views

Effect of interaction between lactic acid and glucose monohydrate on optical rotation

Would anyone know why I get double the expected optical rotation (OR) result for a solution containing D(+)-glucose and L(+)-lactic acid in comparison to a solution of D(+)-glucose on its own? When I ...
7
votes
1answer
1k 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 ($\pu{253 nm}$...
1
vote
0answers
36 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. ...
-1
votes
1answer
39 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 ...
1
vote
1answer
73 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 ...
0
votes
1answer
116 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 ...
3
votes
1answer
113 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)-...
0
votes
1answer
73 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 ...
4
votes
1answer
51 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
votes
1answer
133 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 (...
2
votes
2answers
112 views

Need help explaining OH-peaks in IR

I have synthesized (1S, 2S)-1,2-diphenylethene-1,2-diol, and I'm wondering why the $\ce{OH}$-peaks in the IR-spectrum looks the way it does. I would expect a broad signal (because the solution wasn't ...
-1
votes
1answer
36 views

Why do spectra of elements never overlap?

I understand the concept behind how spectra is formed, through exciting the electron beyond it's ground state, however my question is why do the spectra of the elements never overlap? From my ...
7
votes
1answer
119 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 ...
2
votes
2answers
171 views

Matching the NMR to a compound

I was given a molecule that I had to identify the NMR of. The molecule is 2-methylpent-1-ene (shown below): The correct NMR spectra for this molecule is: My question is, why is this the right ...
1
vote
0answers
76 views

Why must the ground state of hydrogen molecule be gerade?

As we know, the electronic ground state of hydrogen molecule is $\Sigma_g^+$. My question is, how do we know it from the theoretical perspective? The nuclear potential in the hamiltonian of hydrogen ...
8
votes
0answers
158 views

How are polarizabilities measured experimentally?

Most common spectroscopies that produce either a full spectrum, a tensor, or a scalar value have a specific instrument associated with them that is relatively self-contained and not custom. For ...
0
votes
0answers
32 views

Why don’t fluorescent compounds glow in the Sun?

Fluorescent compounds exhibit a glow because of their exposure to UV. Since sunlight is 10% of UV, why don’t fluorescent compounds simply glow in the Sun? Is it because the UV there is just not enough?...
0
votes
1answer
40 views

Why is it necessary to measure the NMR frequency relative to a standard such as TMS?

The horizontal axis of an NMR spectrum is "chemical shift". A standard environment such as tetramethylsilane is chosen as the zero of chemical shift. The frequency at which resonance occurs for other ...