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Symmetry 2014, 6(3), 622-654; doi:10.3390/sym6030622

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

 http://www.mdpi.com/2073-8994/6/3/622
 

Michael J. McGlinchey

School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland; E-Mail: michael.mcglinchey@ucd.ie; Tel.: +353-1-716-2165; Fax: +353-1-716-1178

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 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.

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 Multiple Cope rearrangements equilibrate all ten CH positions in bullvalene.


 
Interconversion of distal and proximal ethyls, combined with rapid tripodal rotation, generates effective C_6v symmetry.