24 votes

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 ...
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19 votes
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What is the isomer distribution in monosubstituted fluorobullvalene?

Extensive NMR studies of substituted bullvalenes were done in the 1960's and 70's, by Oth et al. Much of their original work was published in German. Thankfully, some of these topics were revisited (...
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18 votes
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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 ...
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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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16 votes

What is the isomer distribution in monosubstituted fluorobullvalene?

Let me begin by thanking Martin and long for supplying the key reference (J. Am. Chem. Soc. 1996, 118 (34), 7995–8005). Importantly, long also provided the isomer ratio I was asking for; according to ...
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16 votes

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 ...
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15 votes

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 votes
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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 ...
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15 votes
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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 ...
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14 votes
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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 votes
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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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14 votes
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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). ...
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14 votes
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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 ...
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14 votes
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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 ...
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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 ...
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13 votes
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Can the amount of shielding in proton NMR caused by ring currents be predicted?

Johnson and Bovey first proposed a theory describing the ring current effect,[1] and in 1972 Haigh and Mallion published their often-cited paper on ring current shielding.[2] A later review by these ...
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13 votes
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19F NMR of hexafluorobromine(VII) cation

$\ce{BrF6^+}$ does have an octahedral structure consequently all of the fluorines are equivalent. As you point out, bromine is a spin = 3/2 nucleus, therefore, it is also a quadrupolar nucleus (all ...
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13 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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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 ...
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12 votes

What is the isomer distribution in monosubstituted fluorobullvalene?

I had some free time and resources at hand and did some highly accurate quantum chemical calculations for this system. I took it as a chance to compute some benchmarks and play a little with density ...
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12 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 ...
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12 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 ...
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12 votes
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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 ...
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11 votes
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How do I find out what benzene containing compound this is with IR and NMR?

This is another example of how important it is to work with good quality data, which can only come from preparing good quality samples. Sample preparation comes from good technique and lots of ...
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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 ...
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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 ...
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11 votes
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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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Proton NMR signals and rings

Here is the proton nmr spectrum for pyridine. Here is the proton nmr spectrum for furan For comparison, benzene has a single proton nmr signal at 7.27 ppm. In both pyridine and furan there are ...
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