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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 question intriguing nonetheless.

My basic knowledge of the history of NMR spectroscopy is that in the very old, almost prehistoric days, Bloch and Purcell performed some experiments that applied radiofrequency to a sample and by chance got some radiofrequency signal returned. Then I have a big black box and arrive at modern spectrometers that tune themselves into hydrogen spectra, carbon spectra and see beautiful Fourier-transformed spectra on my PC screen (unless it’s carbon, in which case the baseline is, of course, fuzzy).

I imagine that there had to be a lot of research invested until it was realised that each signal in the Fourier-transformed spectrum corresponds to a hydrogen atom, that they are shifted around depending on how electron-rich or -poor they are, how couplings work and so on. I can imagine that most of this stemmed from trial and error: a certain combination of magnetic field and radiofrequency gives a signal, Fourier transformation seems to happen anywhere, and once you have rationalised that ‘signals’ correspond to ‘hydrogens’ in a certain type of spectrum (likely the one that gave most resonance anyway) you’re half done and the rest is a walk in the park.

But there is one step that seem very unintuitive:
Realising that ‘signals’ correspond to hydrogens.

There is another important step, namely figuring out that solutes can be analysed if hydrogen-free ($\ce{CCl4}$) or at least protium free (deuterated) solvents are used. Now either it was figured out first that hydrogen is the cause for the signals, in which case choosing the correct solvents is a non-issue, but capturing signals from the solute may be. Or it was figured out that using certain solvents gave ‘cleaner’ spectra, in which case it might also be interesting to know how the solvents were chosen.

I realise that there may be a good deal of speculation, retrofitting, inflationary use of the term serendipitously, anecdotical evidence, storytelling and bad memories involved, but is it known how the discoverers of NMR spectroscopy realised that a ‘signal’ is a hydrogen atom in the molecule observed?

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  • $\begingroup$ You are seriously underestimating physics, and the capability of physicists plan experiments and predict their outcome. $\endgroup$ – Karl May 3 '16 at 1:43
  • $\begingroup$ The Fourier Tranformation was a later addition to the NMR spectrometers, the earlier spectrometers used Continuous Wave NMR. Pulsed FT NMR is a much later development $\endgroup$ – Mad Scientist May 3 '16 at 5:54
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The NMR experiment is a classic example of experimental results supporting theoretical prediction. The first reports of NMR signals of bulk matter (water or paraffin) were confidently reported as signals from the $\ce{^1H}$ nucleus because that was what was expected from theoretical prediction.

When Bloch, Hansen and Packard carried out their Nuclear Induction experiment published in 1946 (Phys. Rev. 1946, 70, 474), they knew the signal they were observing came from the $\ce{^1H}$ nucleus because of the basic equation for the observing the NMR phenomenon (Larmor equation):

$$\omega = \gamma B_0$$

That is, the resonance condition will be met when a superimposed oscillating magnetic field (what we call excitation Rf) is applied to matter in a magnetic field $B_0$, for a nucleus with gyromagnetic ratio $\gamma$.

$\gamma$, the ratio of nuclear magnetic moment to angular momentum, is characteristic of a particular type of nucleus, and measurements by Stern and Rabi had provided quite precise values for $\ce{^1H}$ by the time Bloch carried out his experiment. However, it should be pointed out that field homogeneity was so poor in initial experiments that the error in observed frequency (actually the magnetic field) actually still created some doubt that $\gamma$ was constant for a given nucleus. Improvements in field stability and homogeneity finally showed that this was true. Observation of chemical shift some years later lead to the notion of a $B_\mathrm{eff}$; an effective magnetic field attenuated by local shielding.

It should be noted that there were other attempts to observe Nuclear Magnetic Resonance prior to Bloch and Purcell, and these were done on other nuclei. Gorter although unnsuccessful and little known, made an attempt to observe $\ce{^7Li}$ signals at very low temperatures. Rabi first observed Nuclear Magnetic Resonance on $\ce{LiCl}$.

Of course, as we dig deeper and remind ourselves that all great science stands on the shoulders of those that have gone before, this understanding of resonance frequencies and magnetic fields all stems from the famous Larmor equation, published way back in 1897, long before the notion of NMR was born and even while the structure of the atom was still being refined. The relationship, though, holds true for any charged particle with angular momentum, and originally $\gamma$ was known as Larmor's constant - a measurement of the particle charge:mass ratio. Knowing this, then, once the $\gamma$ for $\ce{^1H}$ had been calculated, scientists could safely assume that at a given magnetic field, the signal observed when matter was irradiated with the correct Rf field was indeed $\ce{^1H}$.

And of course FT NMR as is widely used today was a late-comer to the table also; introduced commercially in the 1960's. Prior to this, CW spectrometers operated and NMR was largely an absorbance technique rather than the emission detection technique it predominantly is now.

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    $\begingroup$ + 1 Bloch's theoretical paper adds the footnote: "The first purely magnetic experiment to find an effect due to nuclear moments by measuring the susceptibility of liquid hydrogen was published by B. G. Lasarew and L. W. Schubnikow, Sov. Phys. ll, 445 (1937)" $\endgroup$ – DavePhD May 3 '16 at 16:54
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TL;DR: Scientists knew that electrons had a spin, and thought, "wouldn't it be nice if atomic nuclei did too?" When they passed molecular hydrogen through a homogenous magnetic field (in a modified version of the Stern–Gerlach experiment) and subjected it radio waves, they found energy was absorbed at precise frequencies. This set the stage for the usefulness of $^1\ce{H}$ NMR in the distant future.

Taken from A Brief History of Nuclear Magnetic Resonance:

In the mid-1920s it became apparent that many features in atomic spectra could be accounted for only if certain atomic nuclei likewise possessed spin and a magnetic moment. Refinements of the Stern-Gerlach experiment verified this concept by 1933.

In 1939 Rabi et al. made a major improvement in beam techniques by sending a stream of hydrogen molecules through not only the inhomogeneous magnetic field required for deflection, but also through a homogeneous magnetic field, where they were subjected to radio frequency electromagnetic energy. Energy was absorbed by the molecules at a sharply defined frequency, and the absorption caused a small but measurable deflection of the beam. This was the first observation of NMR, and Rabi received the Nobel Prize in 1944. However, such studies were limited to nuclei in small molecules under very high vacuum in a molecular beam, the deflection of which served to detect the resonance.

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NMR reflects the difference of energy levels upon absorbance of a wave length precessing close to the Larmor frequency (Zeeman interaction). You must remember that NMR also obeys exclusion rules as any other spectroscopic methods, that is that only certain changes in energy of the spin of nuclei are observed, in NMR +1 and -1, which sometimes we call single quantum coherences. Now with respect to why certain solvents e.g. deuterated are NMR invisible is not because was a trial and error, it was because of the foundation of NMR is based also on 1/2 spins like 1H that obeys the exclusion rules of NMR, other nuclei like 2H or deuterated proton do not have spin of 1/2. This led us to immediately take advantage of this nice property and make a variety of experiments to analyse a variety of samples. NMR unlike any other spectroscopy, the observer is the nucleus spin and not the experimentalist, as often it is the case with IR or UV spectroscopy.

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    $\begingroup$ Deuterated molecules are not NMR invisible; 2H has spin=1, and can be observed just as most other nuclei that do not have spin=0 can be. I don't understand what you mean by the foundation of NMR is based on 1/2 spins. Gorter's first attempts to observe NMR signal were made on 7Li - spin 3/2. How do you explain how we observe nuclei like 2H, 6Li, 7Li, 11B, 14N, 17O, 51V.......? $\endgroup$ – long Sep 15 '16 at 14:09
  • $\begingroup$ You are right when you say that it is possible to observe spins bigger than 1/2. Deuterium is considered NMR friendly with spin of 1, but most of these are technically challenging and considered NMR silent. since their spin will have electric energy terms (Quadropolar terms) that depend on the orientation or internal structure of the nucleus. This means that spins>1/2 integers will interact the electric fild gradient $\endgroup$ – Aaron Kloudpaper Sep 15 '16 at 15:13

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