NMR Relaxometry Applied to Chemical Studies of Paramagnetic Metal Cation Complexes: Fundamentals and Applications

Abstract

Low field time-domain nuclear magnetic resonance (TD-NMR) relaxometry of paramagnetic metal cations (PMC) has been widely used to study and improve contrast agents for magnetic resonance imaging (MRI). However, despite its remarkable potential, TD-NMR is rarely used to study PMC complexes, and coordination compounds in non-biomedical application. Therefore, this review aimed to provide comprehensive information not only to non-nuclear magnetic resonance (NMR) relaxation specialists, but also to scientists from distinct levels and expertise, as a way to pave the path for modern analytical and research of PMC complexes. Some fundamental concepts about NMR, paramagnetic relaxation mechanism, as well as traditional and modern methods to measure the longitudinal (T1) and transverse (T2) relaxation times constants are addressed. Here, we address some applications in analytical, electrochemical, and inorganic chemistry, electrodeposition reactions, as well as studies on PMC complexes and coordination compounds in solution.

Nuclear magnetic resonance relaxometry to monitor chromium (VI) reduction by hydrogen peroxide, ascorbic acid, and aluminum powder

“…Second, from the proton NMR relaxation point of view, a solution of diamagnetic ions is similar to pure water with long relaxation times (⁓2 s), while a solution of paramagnetic ions will present shorter relaxation times, because of the coupling of water protons magnetic moments with the paramagnetic ion magnetic moment, which causes relaxation. [18][19][20] It is the same mechanism that is used in Gd 3+ -based contrast agents for magnetic resonance imaging. This "relaxation boost" should be, in our case, proportional to the Cr(III) concentration.…”

Earth's field NMR relaxation of pre-polarised water protons for real-time detection of free-radical formation

“…8 NMR can be used to detect organic free radicals and paramagnetic metal ions. 9 It is worth mentioning that both intermediate and stable organic free radicals are increasingly used at low fields as sources of polarisation for NMR in avatars of dynamic nuclear polarisation (DNP): 10 chemically-induced DNP (CIDNP), solid-state DNP (ss-DNP) and dissolution-DNP for the liquid state (d-DNP). When sensitivity at the detection site is not an issue, the effect of unpaired electrons on nuclear magnetism can be detected via relaxation effects exclusively.…”