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Symmetry Breaking in NMR Spectroscopy: The Elucidation of Hidden Molecular Rearrangement Processes

School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland
Symmetry 2014, 6(3), 622-654; https://doi.org/10.3390/sym6030622
Received: 4 July 2014 / Revised: 28 July 2014 / Accepted: 31 July 2014 / Published: 4 August 2014
(This article belongs to the Special Issue Chemical Applications of Symmetry)
Variable-temperature NMR spectroscopy is probably the most convenient and sensitive technique to monitor changes in molecular structure in solution. Rearrangements that are rapid on the NMR time-scale exhibit simplified spectra, whereby non-equivalent nuclear environments yield time-averaged resonances. At lower temperatures, when the rate of exchange is sufficiently reduced, these degeneracies are split and the underlying “static” molecular symmetry, as seen by X-ray crystallography, becomes apparent. Frequently, however, such rearrangement processes are hidden, even when they become slow on the NMR time-scale, because the molecular point group remains unchanged. Judicious symmetry breaking, such as by substitution of a molecular fragment by a similar, but not identical moiety, or by the incorporation of potentially diastereotopic (chemically non-equivalent) nuclei, allows the elucidation of the kinetics and energetics of such processes. Examples are chosen that include a wide range of rotations, migrations and other rearrangements in organic, inorganic and organometallic chemistry. View Full-Text
Keywords: diastereotopic groups; polyphenylated rings; hexaalkylbenzenes; mixed metal clusters; tripodal rotations; alkene rotations; corannulenes; indenylsilanes; sigmatropic and haptotropic shifts; triptycenes; molecular brakes diastereotopic groups; polyphenylated rings; hexaalkylbenzenes; mixed metal clusters; tripodal rotations; alkene rotations; corannulenes; indenylsilanes; sigmatropic and haptotropic shifts; triptycenes; molecular brakes
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McGlinchey, M.J. Symmetry Breaking in NMR Spectroscopy: The Elucidation of Hidden Molecular Rearrangement Processes. Symmetry 2014, 6, 622-654.

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