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Nuclear magnetic resonance (NMR)

Ilona Heckler

Nuclear magnetic resonance (NMR) spectroscopy is a very important and useful method to determine the detailed chemical structure of organic compounds. In the field of polymer science NMR spectroscopy is mainly used for characterization of the monomer molecules and their precursors. The method is based on magnetic properties of the nuclei of atoms which can yield in the chemical information. Subatomic particles (electrons, neutrons, protons) have spins. In atoms like $^{12}C$ the spins are paired and give a total spin value of zero. Other atoms like $^1H$ and $^{13}C$ as well as $^{15}N$, $^{19}F$ and $^{31}P$ have a total spin different than zero. Applying a magnetic field of $B_0$ will orientate the magnetic moment of the nuclei (where the spin has a magnetic quantum number I) in 2 I + 1 possible spin states. So for a proton with a spin of ½ the possible states are +½ (parallel) and -½(anti-parallel). The rotational axis cannot be oriented exactly parallel. The rotation process, a motion of magnetic moments, induces an electric field with the frequency $\omega_0$ which is characteristic for each nucleus. Thereby a change of the spin state of each nucleus to a higher or lower energy yields in this magnetic resonance which can be detected. This takes place by the absorption of an amount of energy applied as electromagnetic radiation with a specific frequency depending on the nucleus. As an example, in a sample of the proton nucleus, the nuclei will have a distribution through the two spin states (+½ and -½). Since the energy difference between the two states is very small the population distribution is almost equal in both energy states. The consequence is an insensitive technique since the population difference is proportional to the amount of signal intensity. So NMR occurs when a nucleus with a spin different than zero experiences an electromagnetic pulse which changes the spin state. The resulting high undetectable resonance frequency is mixed with a lower frequency to result in an interferogram which is digitalized and called Free Induction Decay (FID). Fourier transformation of the FID yields in the spectrum.J.C. Edwards. “Principles of NMR”. Process NMR Associates. Retrieved 2009-02-23, pp. 1-10 F. A. Bovey, P. A. Mirau, NMR of Polymers, Elsevier Inc., 1996, pp. 155-169. High-Resolution NMR Techniques in Organic Chemistry 2nd Edition, Timothy D. W. Claridge, Elsevier Ltd, Oxford, 2009 pp. 11-17.

$^1H$ and $^{13}C$ are mostly used in NMR spectroscopy for organic material characterization due to the fact that most atoms in the conjugated polymer chain are carbons and protons. Atoms in a molecule can feel the presence of other atoms and/or chemical groups in their chemical environment. These different chemical environments that each atom feels have an influence on the appearance and the shape of the signals observed in the NMR spectrum. Based on this it is to a high degree possible to determine the chemical structure of even highly complicated compounds. NMR spectroscopy can furthermore be applied in the characterization of conjugated polymers. Polymers with similar monomer structure often have different electrical, mechanical, optical or thermal properties as a consequence of a different microstructure. One such example is P3HT that can be synthesized in different ways to yield either a regiorandom or a regioregular polymer. The regioregular polymer consists of head-to-tail coupling, while the regiorandom material contains also head-to-head coupling. These features can be detected by NMR spectroscopy. DOI:10.1021/ja00051a066

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