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2000
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Research: Science and Education
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Understanding Chemical Shifts in pi-Systems: 13C, 15N, 17O NMR
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H. Dahn
Institut de Chimie Organique, University of Lausanne, BCH, CH-1015 Lausanne, Switzerland
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July 2000
Vol. 77 No. 7
p. 905
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| Abstract |
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In the NMR spectra of nuclei other than 1H, pi-systems show
strong shielding or deshielding effects that are not explained by the
simple consideration of electron densities. This is mainly because, in
the presence of a magnetic field, the low-lying unoccupied pi*
orbital allows combinations with energy-rich occupied orbitals of
appropriate spatial orientation. The underlying principles can be
understood using simple qualitative MO discussions, which follow, in
principle, the same reasoning as the very successful ab initio
calculations. The typical argument is developed for three examples
taken from the literature. First, in phenyllithium the ipso carbon atom
is strongly deshielded in a direction in the ring plane perpendicular to
the C-Li bond; this is due to the n-pi* orbital
combination. Second, in carbonyl compounds R-CO-X the influence of X is
systematically felt on the chemical shift of 17O,
but not on the chemical shift of 13C;
this is attributed to the ny-pi* orbital
combination, magnetically acting at O in the direction of the C-O bond,
but lacking at C. Third, in linear pi-systems (acetylene, CO2,
isocyanate ion, etc.) all atoms are strongly shielded with respect to a
nonlinear comparison; this is because, for reasons of orbital symmetry,
a deshielding contribution in the direction of the molecular axis has
vanished.
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| More Information |
 Citation |
Dahn, H. J. Chem. Educ. 2000 77 905.
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Keywords |
Physical Chemistry; Organic Chemistry; NMR Spectrometry; MO Theory
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History |
Created:
Last Updated: |
June 2, 2000
April 15, 2005
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