24
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Why is crystallography still used in some cases for small molecule structure determination?
Our group uses both methods a lot so here are some examples why you would use X-ray, in addition to NMR, in organic synthesis.
Your compound isn't soluble enough:
Colleagues produce very large ...
- 5,692
18
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Why do hydroxyl groups only produce a singlet in proton NMR?
You are both right and wrong.
At low temperatures, the NMR spectrum will indeed be as you predicted: the $\ce{H}$ from $\ce{OH}$ will produce a quartet and likewise the $\ce{H}$ from $\ce{CH_3}$ will ...
- 1,222
17
votes
Why does Carbon-13 NMR not have splitting patterns?
In 1H NMR, spin-spin coupling leading to multiplet structure most commonly arises due to coupling to other 1H nuclei. This is most often taught in terms of neighbouring 1H nuclei having "up" and "down"...
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17
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Why has acetaminophen only two different NMR chemical shifts related to phenyl?
The 'amide bond being planar' refers to restricted rotation about the C(O)–N bond, not the N-aryl bond.
The restricted rotation about the C(O)–N bond does lead to inequivalence in NMR, depending on ...
- 70.8k
16
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Why don't labile protons such as -OH and -NH have a characteristic chemical shift?
Many $\ce{-OH}$ and $\ce{-NH2}$ protons do have characteristic shifts. However, their characteristic ranges tend to be much broader than those of $\ce{C-H}$ protons.
For example alcohol chemical ...
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16
votes
Why is crystallography still used in some cases for small molecule structure determination?
Coming from natural product chemistry, of course the structure elucidation by NMR is the most commonly used method, especially in isolation. NMR requires only a little substance dissolved in whichever ...
- 66.9k
15
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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
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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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15
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Multiplet shape in proton NMR of morpholines
It looks as if the NMR of morpholine is an AA′XX′ spectrum (the chemical shift difference is 0.80 ppm, or 320 Hz on your spectrometer, two orders of magnitude larger than the coupling constant).
...
- 17k
15
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Can non-deuterated solvents be used for 13C-NMR (and in fact other nuclei)?
If using a deuterium lock, then normally you would require, as minimum, about 10% deuterated solvent. Without any deuterated solvent, it is not possible to obtain a spectrometer lock (unless using a ...
- 11.9k
15
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NMR terminology: "Transients"
"Data acquisition parameters: 32 transients with 4k time domain points and a spectral width of 5618 Hz were accumulated with a relaxation delay of 500 ms."
Is transient just jargon for free ...
- 70.8k
14
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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14
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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
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Why do NMR samples require a solvent?
There is a minimal volume you need to fill into the NMR tube to be able to get a good shim, this is around 500 microliters with regular 5mm tubes. Not using a solvent would require a lot more of your ...
- 6,083
14
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Symmetry and NMR linewidths of quadrupolar nuclei
I know that you asked for an intuitive explanation, but I'm afraid that if one wants to go beyond ron's comment, the technical aspects are somewhat necessary. It isn't incredibly important to ...
- 70.8k
14
votes
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How to assign overlapping multiplets in 1H NMR spectra?
Referring to your comment on Buttonwood's answer:
Unfortunately, in this case I only have 1D HNMR and IR available to analyse the product
If this is really the case, then you are essentially out of ...
- 70.8k
13
votes
Why don't equivalent hydrogens cause splitting in NMR?
The first important point to note is that magnetically equivalent nuclei do in fact couple to each other, however no splitting is observed in the spectrum. The second point is that chemically ...
- 11.9k
13
votes
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Metal rod and NMR machine
A little bit about me - I manage a busy research NMR Facility at a leading international tertiary institution. I deal with questions like this not infrequently, and there are some very important ...
- 11.9k
13
votes
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NMR chemical shift range of different elements
The are a number of important factors that contribute to the shielding of a nucleus. Chemical shifts arise due to differences in the local magnetic field in the different environments within a ...
- 11.9k
13
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Is intensity in NMR spectra acquired at regular time intervals?
1D data
Generally, yes, data points in the FID are acquired uniformly. So, the sampling interval will simply be the acquisition time divided by the number of points.
As Buck Thorn pointed out in some (...
- 70.8k
13
votes
Reconstruction of NMR spectrum by entropy maximization (MaxEnt)
The entropy in "maximum entropy" refers to the information entropy concept as introduced by Shannon. The MaxEnt technique referred to by Hoch may be regarded as cosmetic because what it does ...
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12
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Do hydrogen atoms not bound to carbon also appear in 1H NMR spectra?
Yes, with a but.
First the yes: they do appear, albeit at slightly different chemical shifts as, of course, the environment around a proton in $\ce{-CH2\!\;\!-}$ is slightly different to the ...
- 485
11
votes
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Modern open-source tools for simulation of NMR spectra
In Org Biomol Chem 2016, 14, 3943, Goodman reports the replacement of some "tradition" (expensive) programs with free/open-source ones, as applied to their dp4 method.
Specifically:
Molecular ...
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11
votes
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What is the origin of 'tenting' or 'roofing' in NMR spectra?
Not just coupled, but strongly coupled; these roofing effects tend to be seen when the coupling constant $J$ is on the order of $\Delta \nu$ (the difference in resonance frequencies between the two ...
- 70.8k
11
votes
Qualitative explanation of how COSY works
In the 1D-proton experiment a hard 90° pulse is applied to the sample before the FID is acquired over some time, t. This set of events is known as the ‘pulse sequence’, and the resulting data is in ...
- 17k
11
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Nitrobenzene and other monosubstituted benzenes 1H vs 13C NMR assignments
As I wrote in the comments, there is not really a good way of arriving at these conclusions for a general aromatic molecule. The energy differences at play in these systems are so small that ...
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10
votes
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Calculated 13C NMR Shifts of brominated Carbons
The authors note in their review article* that such effects are expected as "heavy atom effects" due to electron-correlation effects and spin-orbit coupling: (references stripped for clarity)
This ...
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10
votes
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Strange 1H-NMR signals/signal ratio
The paper specifically addresses this issue and ascribes it to the tendency for boronic acids to exist as solvent and concentration dependent oligomers:
The dependence of the intensity ratio for ...
- 11.9k
10
votes
Why do NMR samples require a solvent?
Just to add some clarity to some already acceptable answers by others....
For running solution (liquid) state NMR, a solvent is not necessary. There are a number of applications where dilution with a ...
- 11.9k
10
votes
NMR chemical shift range of different elements
I only want to add some data and quotes to supplement long's answer. I am also indebted to long since the reference he provided was what helped me to find these pieces of information.
As already ...
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