2D NMR SPECTRA INTERPRETATIONS–How To Read COSY Spectra

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Two-dimensional nuclear magnetic resonance spectroscopy (2D NMR) is a set of nuclear magnetic resonance spectroscopy(NMR) methods which give data plotted in a space defined by two frequency axes rather than one. Types of 2D NMR includecorrelation spectroscopy (COSY), J-spectroscopy, exchange spectroscopy (EXSY), and Nuclear Overhauser effect spectroscopy (NOESY). Two-dimensional NMR spectra provide more information about a molecule than one-dimensional NMR spectra and are especially useful in determining the structure of a molecule, particularly for molecules that are too complicated to work with using one-dimensional NMR.

The first two-dimensional experiment, COSY, was proposed by Jean Jeener, a professor at the Université Libre de Bruxelles, in 1971. This experiment was later implemented by Walter P. Aue, Enrico Bartholdi and Richard R. Ernst, who published their work in 1976.

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http://faculty.sdmiramar.edu/fgarces/LabMatters/Instruments/NMR/Spectra_Interp/Reading2D_Spectrum.htm

 

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SN1 REACTION, STEREOCHEMISTRY

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Stereochemistry

The carbocation intermediate formed in the reaction’s rate limiting step is an sp² hybridized carbon with trigonal planar molecular geometry. This allows two different avenues for the nucleophilic attack, one on either side of the planar molecule. If neither avenue is preferentially favored, these two avenues occur equally, yielding a racemic mix of enantiomers if the reaction takes place at a stereocenter.This is illustrated below in the S_N1 reaction of S-3-chloro-3-methylhexane with an iodide ion, which yields a racemic mixture of 3-iodo-3-methylhexane:

However, an excess of one stereoisomer can be observed, as the leaving group can remain in proximity to the carbocation intermediate for a short time and block nucleophilic attack. This stands in contrast to the S_N2 mechanism, which is a stereospecific mechanism where stereochemistry is always inverted.

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