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PHYSICOCHEMICAL PROPERTIES OF MATTER

Anisotropy and NMR spectroscopy Francesca Nardelli1,3   · Silvia Borsacchi2,3   · Lucia Calucci2,3   · Elisa Carignani1,3 · Francesca Martini1,3   · Marco Geppi1,3  Received: 1 June 2020 / Accepted: 1 August 2020 © The Author(s) 2020

Abstract  In this paper, different aspects concerning anisotropy in Nuclear Magnetic Resonance (NMR) spectroscopy have been reviewed. In particular, the relevant theory has been presented, showing how anisotropy stems from the dependence of internal nuclear spin interactions on the molecular orientation with respect to the external magnetic field direction. The consequences of anisotropy in the use of NMR spectroscopy have been critically discussed: on one side, the availability of very detailed structural and dynamic information, and on the other side, the loss of spectral resolution. The experiments used to measure the anisotropic properties in solid and soft materials, where, in contrast to liquids, such properties are not averaged out by the molecular tumbling, have been described. Such experiments can be based either on static low-resolution techniques or on one- and two-dimensional pulse sequences exploiting Magic Angle Spinning (MAS). Examples of applications of NMR spectroscopy have been shown, which exploit anisotropy to obtain important physico-chemical information on several categories of systems, including pharmaceuticals, inorganic materials, polymers, liquid crystals, and selfassembling amphiphiles in water. Solid-state NMR spectroscopy can be considered, nowadays, one of the most powerful characterization techniques for all kinds of solid, either amorphous or crystalline, and semi-solid systems for the obtainment of both structural and dynamic properties on a molecular and supra-molecular scale. Graphic abstract

Keywords  Solid-state NMR · Chemical shift · Dipolar interaction · Quadrupolar interaction · MAS · HPD · Soft matter · Liquid crystals · Line broadening · Line shape analysis

This paper is the peer-reviewed version of a contribution presented at the Conference on Anisotropic Properties of Matter, organized by Giovanni Ferraris and held at Accademia Nazionale dei Lincei in Rome, October 16–17, 2019. Extended author information available on the last page of the article

1 Introduction Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful techniques used for the determination of molecular structure and dynamics (Levitt 2008; Keeler

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2010). One of the most important features that makes this technique so popular is its broad applicability, due to the fact that every material contains chemical elements with NMR-active nuclei, i.e, nuclei with non-null nuclear spin I, such as, for example, 1H, 13C, 31P, 29Si, and 27Al. Indeed, NMR spectroscopy exploits the interaction between suitable radio waves, typically applied for very short times (r.f. pulses) and such nuclei, consisting in stimulated transitions between different spin levels, the degeneration of which is preliminarily removed b