Questions tagged [nmr-spectroscopy]

This tag should be applied to all questions about nuclear magnetic resonance, including the underpinning theory and the measurement or interpretation of spectra.

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Splitting of multiplets in ¹H NMR spectra?

I'm currently taking VCE (Victorian Certificate of Education) Chemistry classes, and we're currently studying the interpretation of spectra produced by Hydrogen NMR (Nuclear Magnetic Resonance) ...
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What benefits do higher-rated NMR tubes provide?

You can buy NMR tubes in a huge variety of qualities, with an equally huge difference in price between the cheapest and the most expensive NMR tubes. They are usually rated for a specific spectrometer ...
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Why don't equivalent hydrogens cause splitting in NMR?

When doing NMR spectroscopy, it is an observed fact that equivalent hydrogens do not split one another. Why don't equivalent hydrogens split each other's signals? For example, why is the NMR spectrum ...
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What is the isomer distribution in monosubstituted fluorobullvalene?

Bullvalene (tricyclo[3.3.2.02,8]deca-3,6,9-triene) is a fluxional molecule able to interconvert any two carbon atoms through a series of degenerate Cope rearrangements (for more information, see the ...
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Modern open-source tools for simulation of NMR spectra

I am interested in predicting the NMR spectrum of small organic compounds. It doesn't matter to me if the prediction is very accurate. I'll eventually be comparing the prediction to experimental ...
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1answer
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What are the differences between the HMQC and HSQC experiments?

This afternoon, I was having some trouble with an HSQC (Heteronuclear Single Quantum Coherence Spectroscopy) spectrum I was trying to acquire, and the spectroscopist recommended I try HMQC (...
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1answer
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Multiplet shape in proton NMR of morpholines

I recently carried out a Buchwald–Hartwig reaction to attach a morpholin-4-yl group to an aromatic ring: In the proton NMR of the (columned) product, I found the peaks corresponding to the morpholine ...
18
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2answers
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What factors are important for quantitative analysis of a proton 1D-NMR spectrum?

Besides elucidating or verifying a chemical structure, NMR can also be used e.g. for quantifying a mixture of different chemicals. Depending on the quality of the spectrum and the specific substance,...
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1answer
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Why does the transverse magnetization in Fourier Transform NMR consist of Ix+iIy ($\hat{I}_{\!x}+\mathrm{i}\hat{I}_{\!y}$)?

More specifically, the expression for the average expectation value of the transverse magnetization has the following form: $$M^+(t)=N\gamma \hbar\,\text{Tr}(\hat{\sigma}(t)\hat{F}^+)$$ with $\hat{\...
17
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1answer
290 views

Directionality of ring current-induced magnetic fields in purines

What would be the directions of ring current-induced magnetic fields in purine-like heterocycles in a ​strong magnetic field? Would be the antiparallel state be a default state for these substances in ...
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Why is crystallography still used in some cases for small molecule structure determination?

I was wondering for some time why in some cases x-ray crystallography and not NMR is used for structure determination of small molecules? Does crystallization maybe require a smaller amount of the ...
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Correlation between 1H and 13C shifts - coincidence or not?

A quick glance at any table of typical chemical shifts will reveal that the chemical shifts of protons $(\delta_\mathrm H)$ tend to correlate pretty well with the chemical shifts of the carbons to ...
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What is deuterium-depleted water actually used for?

I noted, flipping through the paperback Alfa Aesar catalogue today, that they sell deuterium-depleted water. Under the usage category, they list 'NMR'. I'm trying to work out what specific use this ...
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Where does magnetic inequivalence come from?

As the question says, I'm having trouble understanding where does magnetic inequivalence in $\ce{^1H}$ NMR come from. All the sources I've found say that two protons are magnetically equivalent ...
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1answer
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Why do aromatic hydrogen atoms have a higher chemical shift value compared to aliphatic hydrogen atoms?

In Nuclear magnetic resonance (H-NMR) spectroscopy, the chemical shift of aliphatic hydrogen atoms are very much closer to the 1.0 ppm than that of aromatic hydrogen atoms. Aromatic hydrogen atoms ...
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1answer
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Calculated 13C NMR Shifts of brominated Carbons

Since I calculate NMR spectra for a while by simply using one of recommendations by CHEmical SHIft REpository: mPW1PW91/6-311+G(2d,p)-SCRF//B3LYP/6-31+G(d,p) ...
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Why do hydroxyl groups only produce a singlet in proton NMR?

Take, for example, methanol ($\ce{CH3OH}$). The hydroxyl proton ($\ce{-O$\color{red}{\ce{H}}$}$) is three bonds away from three more protons ($\ce{-C$\color{blue}{\ce{H}}$_3}$), which aren't ...
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Sources for raw 1H-NMR data?

Where can one find examples of raw data for an 1H-NMR spectrum? This may sound as an odd request, so I'll give some context. I have found lots of images of spectra such as this: where you can see ...
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NMR chemical shift range of different elements

A typical $\ce{^1H}$ NMR runs from approximately 0 to 10 ppm, give or take a bit. $\ce{^13C}$ NMR runs from 0 to 200. And $\ce{^59Co}$ NMR runs from -5000 to 15000 ppm! There seems to be some ...
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Qualitative explanation of how COSY works

I think I understand 1D Proton NMR with the Fourier Transform, qualitatively. However, I am struggling to understand 2D COSY NMR. I am stuck on a few things; In 1D Proton FT-NMR, the "Pulse" I ...
13
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2answers
572 views

Can the amount of shielding in proton NMR caused by ring currents be predicted?

My textbook (Organic Chemistry, 2nd Ed. Hornback, Joseph) states that if a hydrogen is located near the center of an aromatic ring it will experience an (often quite sizable) upfield shift, as seen in ...
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1answer
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Is there a real detection limit in NMR spectroscopy?

From what I've read, my impression is that with a sufficient number of scans etc., one can get spectra from even very dilute samples (S:N ratio ~ $\sqrt{\text{NS}}$). Obviously, this is not always ...
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Comparison of COSY and TOCSY for detection of small couplings between protons

I routinely run COSY spectra to help with assignment of my proton NMR. The experiment is fast to run and generally useful, however some of the intermediates I've recently been making have had ...
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Why is tetramethylsilane (TMS) used as an internal standard in NMR spectroscopy?

I understand that TMS has a large proton number, giving a strong peak in the NMR spectrum. But apart from that is there any other reason why just TMS is used and not any other molecule?
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Can non-deuterated solvents be used for 13C-NMR (and in fact other nuclei)?

Basically, my question is can you used a non-deuterated solvent for 13C NMR? I'm thinking that there might be some problems with locking?
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Determining structure using HNMR and IR

I have a homework problem where I'm to draw the structure of a compound of molecular formula $\ce{C10H12O2}$ using the following IR data and $^1$H NMR spectrum: IR absorption at 1718 cm$^{-1}$ NMR ...
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Why is proton coupling through bonds and how does this affect identical protons?

I had the opportunity to work with a NMR spectrometer and decided to learn some of the theory behind it. After finishing Clayden's chapter on $^{1}\textrm{H}$ NMR (Ch. 11) I'm still left with some ...
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1answer
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19F NMR of hexafluorobromine(VII) cation

$\ce{[BrF6]+}$ is an octahedral species, and should have only one fluorine environment due to the symmetry. $\ce{Br}$ has nuclear spin $I=3/2$ and therefore is unlikely to cause splitting due to rapid ...
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1answer
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Nitrobenzene and other monosubstituted benzenes 1H vs 13C NMR assignments

The reported 1H NMR spectrum for nitrobenzene has three signals corresponding to H2/H6, H3/H5 and H4, respectively. The signals (ppm) appear at 8.25 (H2/H6), 7.71 (H4), and 7.56 (H3/H5). This is, the ...
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1answer
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Why do electronegative atoms depress coupling constants?

Related to this previous question about detection of small couplings between protons. In a recent conversation with our labs NMR technician, it was stated that coupling constants decrease in the ...
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How to assign overlapping multiplets in 1H NMR spectra?

In the above $\ce{^1H}$ NMR spectrum ($\pu{400 MHz}$, $\ce{CDCl3}$) the methyl, alkene, and $\ce{CCl3CH2}$ protons are fairly easy to assign however, the protons in the cyclohexene ring show overlap ...
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Why are all the phenyl protons in benzyl alcohol equivalent in the ¹H-NMR spectrum?

Below is a $\ce{^1H}$ Nuclear Magnetic Resonance (NMR) spectrum for benzyl alcohol. The hydrogen in the alcohol group gives rise to a unique signal. The two hydrogens in $\ce{CH2}$ give rise to a ...
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Why do NMR samples require a solvent?

I am running a proton NMR on a liquid organic product. I understand that we wouldn't want to use anything that contains hydrogen atoms as a solvent, but I'm not sure why we need a solvent at all.
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Historically, how was the connection between NMR peaks and hydrogen atoms made?

A large part of the basis of this question may be due to the student who didn’t pay attention in the history section of the NMR course (and not during the technical details bit either) but I find the ...
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1answer
872 views

Symmetry and NMR linewidths of quadrupolar nuclei

The linewidth of peaks when taking NMR spectra of quadrupolar nuclei (11B or 14N for example) is often very broad, making their interpretation challenging. Recently, when acquiring 11B-NMR spectra ...
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2answers
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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
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Why is gadolinium specifically used in MRI contrast agents?

Gadolinium(III) chelate complexes are routinely used as contrast agents in magnetic resonance imaging (MRI);1 the usual explanation is that paramagnetic species contain unpaired electrons, which cause ...
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Radiofrequency transmitter in an NMR experiment: Is there an involvement of (electromagnetic) radio wave?

A student in Chemistry StackExchange asked the effect of radio waves on matter, which led to an interesting set of arguments by some users. It was pointed out that in a proton nuclear magnetic ...
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How do I figure out how many hydrogens my compound actually has using a mass and NMR spectrum?

Question 3: It said $m/z = 122,$ and $m/z = 124$ is in a $3:1$ ratio, so I figured that meant that chlorine is present. Then I thought $m/z$ was the actual compound's molecular mass. So I used ...
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1answer
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Why does Carbon-13 NMR not have splitting patterns?

I read about splitting patterns in proton NMR and found that this is due to spin-spin coupling, where reference to Pascal's triangle can explain the splitting patterns found in doublet, triplet and ...
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Difference between S and I in the context of magnetic resonance experiments

In the lecture our professor told us that nuclei with $S>1/2$ have a quadrupole and are therefore detected with NQR instead of NMR. The majority of all elements have nuclei with $S>1/2$. I ...
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1H (proton) NMR spectra for alkanes

I do not fully understand the idea of coupling and why chemically equivalent hydrogens do not couple. I wanted to work with the alkanes, so let's take propane first: There would be a triplet signal ...
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13C NMR spectrum only showing solvent

This is my first time taking a $\ce{{}^{13}C-NMR}$ spectrum and while I'm aware the signal to noise ratio is much lower compared to $\ce{{}^{1}H-NMR}$, I didn't realize it was quite this bad. I took a ...
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1answer
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What is the origin of 'tenting' or 'roofing' in NMR spectra?

Given an NMR spectrum with two coupled nuclei one can often observe so-called 'roofing' – an asymmetry in the multiplet. For example, here is a 300 MHz proton spectrum of 1-bromo-2-methoxyethane, ...
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Can a NOESY NMR experiment be halted partway through and give meaningful results?

Some NMR experiments can be halted partway through the acquisition and still give perfectly valid results: The traditional 1H experiment for example. Others, like a Saturation Transfer Difference ...
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Multiple-quantum coherences in NMR

Why are multiple-quantum coherences (coherences with changes in magnetic quantum numbers, $|\Delta m| \neq 1$) unobservable in regular NMR experiments?
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NMR calculations with NWChem quantum chemistry program

I tried experimenting with some ab initio NMR calculations using the NWChem program. I used quinuclidine as a simple test case. The input file was: ...
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1answer
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Why is an alcohol proton so shielded?

Considering how electronegative oxygen is, why is the chemical shift of alcohol protons (1H-NMR) not so high? The chemical shifts of the protons on 3-propanol are about 1.20 for the carbon-1 hydrogens,...
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H-NMR spectroscopy of [18]annulene

Why does [18]annulene show only one peak in its proton NMR at $\pu{100^\circ C}$? Does any structural change happen such that all the hydrogens are alike?
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Do hydrogen atoms not bound to carbon also appear in 1H NMR spectra?

I would like to know if hydrogens that are not bound to a carbon (such as in groups $\ce{-OH}$ and $\ce{-NH2}$) would lead to the presence of a singlet peaks in the NMR spectra or not, and if we have ...

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