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The N-15 NMR chemical shift in the characterization of weak halogen bonding in solution

Journal article
Authors Sebastiaan B. Hakkert
Jürgen Gräfenstein
Mate Erdelyi
Published in Faraday Discussions
Volume 203
Pages 333-346
ISSN 1359-6640
Publication year 2017
Published at Swedish NMR Centre at Göteborg University
Department of Chemistry and Molecular Biology
Pages 333-346
Language en
Links doi.org/10.1039/c7fd00107j
Subject categories Physical organic chemistry, Organic Chemistry, Chemical Sciences

Abstract

© The Royal Society of Chemistry. We have studied the applicability of 15 N NMR spectroscopy in the characterization of the very weak halogen bonds of nonfluorinated halogen bond donors with a nitrogenous Lewis base in solution. The ability of the technique to detect the relative strength of iodine-, bromine- and chlorine-centered halogen bonds, as well as solvent and substituent effects was evaluated. Whereas computations on the DFT level indicate that 15 N NMR chemical shifts reflect the diamagnetic deshielding associated with the formation of a weak halogen bond, the experimentally observed chemical shift differences were on the edge of detectability due to the low molar fraction of halogen-bonded complexes in solution. The formation of the analogous yet stronger hydrogen bond of phenols have induced approximately ten times larger chemical shift changes, and could be detected and correlated to the electronic properties of substituents of the hydrogen bond donors. Overall, 15 N NMR is shown to be a suitable tool for the characterization of comparably strong secondary interactions in solution, but not sufficiently accurate for the detection of the formation of thermodynamically labile, weak halogen bonded complexes.

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