# NMR definition and meaning [closed]

honestly, I have seen a lot of problems with "NMR" spectrums, I looked for this title in Charles Mortimer's general chemistry book, but I didn't see any sort of data, can anyone unfold what does NMR spectrum consist of? thank you very much

When a radiation like the light is sent on some colored substance, part of the light goes through, and the other radiation (other colors) are absorbed. The energy of the absorbed radiation is used to push orbital electrons to a more excited orbital, may be at a greater distance from the nucleus.

The visible light is characterized by a wave length situated between $$400$$ and $$700$$ nanometers. Radiations having shorter wavelength exist but they are not visible with our eye : it is ultra-violet light. Radiations having longer wavelength exist too, but they are also invisible : they are called infra-red. Unfortunately they don't have enough energy to excite electrons. Infra-red rays are just sufficient to make the chemical bonds vibrate more strongly. Their wavelength are about $$10 - 100$$ micrometers. At bigger wavelength ($$0.1 - 10$$ cm), the energy of the radiations is too weak to excite vibrations. They are called micro-wave and they are just sufficient to promote rotations.

At still longer wavelengths (about $$0.3$$ to $$10$$ meters), the energy is too weak to promote vibrations or rotations. The radiations are called radio waves and all they can do is to change the spin of the protons in the center of the Hydrogen atom (and of some other atoms). But as half of the H atoms have spin up and half spin down, they have first to be all (or nearly all) alined in the same spin state for the radiation to produce a change of this spin orientation. This alinement is done by a strong magnetic field. The substance is then placed between the North and the South pole of magnet. And a radiation is sent on the sample, whose energy (or wavelength) can be changed at will, as the color is chosen with the visible light. The intensity of the absorbed radiation is measured versus the wavelength or the energy the incoming radiation. This gives a spectrum, called nuclear magnetic resonance, because the absorption is due to the nucleus, and that it is due to the interaction of a magnetic field.

Sorry for the specialist reader : the previous explanation has been over-simplified. May be too much ! Sorry in advance !

There are atom nuclei with a non-zero magnetic spin, for example $$\ce{^1H}$$ (the most frequently seen isotope of hydrogen), but not $$\ce{^2H}$$ (deuterium). You may think about these atoms like small magnets. The orientation of these little magnets in space does not matter until these are brought into a strong magnetic field of the NMR spectrometer; here, they either orient parallel or antiparallel in respect to the magnetic field applied externally to the sample. Thus the «N» in «NMR».

The «MR» for magnetic resonance is derived from the possibility excite these little magnets by radio waves; these «atomic magnets» then may start to wiggle, and eventually, a detectable portion of these magnets' orientation flips from «up» ($$\uparrow$$) to «down» ($$\downarrow$$) in respect to the outer magnetic field at will. Thus, you record the number of these atomic nuclei (magnets) participating in these transitions by their decay signal, when relaxing to the initial state; again, this is in the frequency range of radio waves. (NMR is not about radioactivity, neither bonds are broken / newly formed, nor new elements generated.)

For one, the energy required to yield these magnetic transitions depend on the experimental setup; thus -- by convention -- the abscissa traces a dimensionless number (ppm) accounting e.g., for the outer magnetic field strength the spectrometer may apply to the sample. (But note, the scale has direction, this real number may be either positive, or negative.) For two, and thus so useful to elucidating structures (both organic, as well as inorganic samples), the transition energies depends both on the nature of the nuclei examined (e.g., $$\ce{^1H}$$, $$\ce{^{13}C}$$, $$\ce{^{19}F}$$, $$\ce{^{31}P}$$), and their chemical environment.

• These little magnets experience interaction with nuclei of the same sort (e.g., coupling between $$\ce{^1H}$$ and $$\ce{^1H}$$ in proximity), which may yield typical coupling pattern of diagnostic value.

• Atom nuclei are surrounded by an electron cloud, which shields the atom nuclei from the spectrometer's magnetic field. Atoms nearby the one examined may increase or decrease this electron density, relatively to a neuter, unbound and isolated atom. (Possibly you heard about electron donating / electron withdrawing groups, or how difference in electronegativity yields polarized molecular bonds). This equally influences the energy required to yield such a flip, thus shifts the signal of said atom in the spectrum, too. Thus you get to know, e.g., protons bound to a benzene are typically recorded around $$\pu{7.26 ppm}$$, while those bound, e.g., to an alkine around $$\pu{1.7 ppm}$$.

Beside the «typical suspects» in Wikipedia (English, simple English), the ChemistryLibreTexts offer a textbook like approach to the large topic.