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

Symmetry

关键词: diastereotopic groups;    polyphenylated rings;    hexaalkylbenzenes;    mixed metal clusters;    tripodal rotations;    alkene rotations;    corannulenes;    indenylsilanes;    sigmatropic and haptotropic shifts;    triptycenes;    molecular brakes;   

DOI  :  10.3390/sym6030622

来源: mdpi

【 摘 要 】

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.

【 授权许可】

CC BY   
© 2014 by the authors; licensee MDPI, Basel, Switzerland