Relativistic Heavy-Neighbor-Atom Effects on NMR Shifts: Concepts and Trends Across the Periodic Table

Abstract

Chemical shifts present crucial information about an NMR spectrum. They show the influence of the chemical environment on the nuclei being probed. Relativistic effects caused by the presence of an atom of a heavy element in a compound can appreciably, even drastically, alter the NMR shifts of the nearby nuclei. A fundamental understanding of such relativistic effects on NMR shifts is important in many branches of chemical and physical science. This review provides a comprehensive overview of the tools, concepts, and periodic trends pertaining to the shielding effects by a neighboring heavy atom in diamagnetic systems, with particular emphasis on the "spin-orbit heavy-atom effect on the light-atom" NMR shift (SO-HALA effect). The analyses and tools described in this review provide guidelines to help NMR spectroscopists and computational chemists estimate the ranges of the NMR shifts for an unknown compound, identify intermediates in catalytic and other processes, analyze conformational aspects and intermolecular interactions, and predict trends in series of compounds throughout the Periodic Table . The present review provides a current snapshot of this important subfield of NMR spectroscopy and a basis and framework for including future findings in the field.

Synthesis and Hydrogenation of Heavy Homologues of Rhodium Carbynes: [(Me3P)2(Ph3P)Rh≡E‐Ar*] (E=Sn, Pb)

“…exhibiting signals at 280.6, 279.1, 278.7 ppm [3l,m] . The shift of the 13 C NMR signals to high frequencies can be rationalized by the influence of the heavy atom lead on the chemical shift of the neighbouring atom [16] …”

Tetryl‐Tetrylene Addition to Phenylacetylene

“…Very remarkable is the 1 H NMR chemical shift, 13.10 ppm, of the vinyl proton Ge−C H =CPh−Pb next to the Ge atom. The influence of relativistic effects of a heavy atom on the chemical shift of a neighboring light atom, in particular caused by spin–orbit coupling (the spin–orbit‐induced heavy‐atom on light‐atom shielding, SO‐HALA effect), received substantial interest over the past decades [12] . In compound 5 the CH unit is not directly bonded to the lead atom and so this unusual shift is primarily due to a three bond SO‐HALA effect [13] .…”

“…In the 1 H and 13 C NMR spectra of Z ‐ 8 the signals of the Pb− CH=C unit were found in the 1 H NMR spectrum at 11.50 ppm and in the 13 C{ 1 H} NMR spectrum at 284.7 ppm. To rationalize these high frequency signals relativistic DFT calculations of compound Z ‐ 8 were performed [12] . Based on the molecular structure determined in the solid state the structure of Z ‐ 8 was optimized at DFT level with scalar relativistic effective core potentials.…”

Synthese und Hydrierung schwerer Homologe eines Rhodium‐Carbins: [(Me3P)2(Ph3P)Rh≡E‐Ar*] (E=Sn, Pb)

“…Diese in 12 gefundene erhebliche Hochfrequenz‐NMR‐Verschiebung ist höchstwahrscheinlich auf den Einfluss des Plumbylenliganden zurückzuführen. Dies ist ein weiteres Beispiel für den für Schweratome diskutierten HALA‐Effekt aufgrund von Spin‐Bahn‐Kopplung [16] . Im Gegensatz dazu ist der Effekt des Plumbylens auf das direkt gebundene Rhodiumatom, wiedergegeben durch die chemische Verschiebung des 103 Rh‐Kerns, relativ gering (vergleiche 103 Rh‐NMR‐Daten für Verbindungen 3 , 7 und 12 , 13 in Tabelle 1).…”

“…Die Hochfrequenzverschiebung des Rh‐H‐Signals des Bleiderivates 7 kann mit dem Einfluss des schweren Atoms Blei auf die chemische Verschiebung von Leichtatomen erklärt werden. Die relativistischen Effekte von Spin‐Bahn‐Kopplungen auf NMR chemische Verschiebungen wurden mit quantenchemischen Methoden untersucht [16] . Die bei 1728 ppm gefundene 119 Sn‐NMR‐Resonanz von 3 deutet auf ein Aryl‐Rhodostannylen hin.…”