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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

J. Chem. Educ., 2000, 77 (7), p 905

DOI: 10.1021/ed077p905

Publication Date (Web): July 1, 2000

Section:

History, Education, and Documentation

Abstract

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 n_y-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.

Keywords (Audience):

Upper-Division Undergraduate

Keywords (Domain):

Physical Chemistry

Keywords (Subject):

NMR Spectroscopy

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Citing Articles

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This article has been cited by 3 ACS Journal articles (3 most recent appear below).

Changes in the Isotropic Shielding of the ¹⁷O Nucleus upon Torsion in Terminal Oxygen Systems: A Computational Study on Their Origin
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The Journal of Physical Chemistry A2008 112 (38), 9118-9127
- Changes in the Isotropic Shielding of the ¹⁷O Nucleus upon Torsion in Terminal Oxygen Systems: A Computational Study on Their Origin
  Heidi M. Muchall
  The Journal of Physical Chemistry A2008 112 (38), 9118-9127
  We are presenting a computational study on the isotropic shielding, charge, and orbital contributions to the shielding of oxygen in benzaldehydes (Ar−CHO), nitrobenzenes (Ar−NO2), phenyl isocyanates (Ar−NCO), anilides (Ar−NHCOCH3), and N-sulfinylamines (...
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Acetylenic Carbon-13 Chemical Shift Tensors for Diphenylacetylene and (η²-Diphenylacetylene)Pt(PPh₃)₂: A Solid-State NMR and Theoretical Study
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  Inorganic Chemistry2006 45 (6), 2461-2473
  The title systems are investigated with solid-state NMR to provide a detailed picture of the changes in the chemical shifts of alkynyl carbons when they undergo bonding to a late transition metal. DFT calculations of magnetic shielding are performed to ...
  Abstract | Full Text HTML | Hi-Res PDF
Are Halocarboranes Suitable for Substitution Reactions? The Case for 3-I-1,2-closo-C₂B₁₀H₁₁: Molecular Orbital Calculations, Aryldehalogenation Reactions, ¹¹B NMR Interpretation of closo-Carboranes, and Molecular Structures of 1-Ph-3-Br-1,2-closo-C₂B₁₀H₁₀ and 3-Ph-1,2-closo-C₂B₁₀H₁₁
Clara Viñas, Gemma Barberà, Josep M. Oliva, Francesc Teixidor, Alan J. Welch, and Georgina M. Rosair
Inorganic Chemistry2001 40 (26), 6555-6562
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  Inorganic Chemistry2001 40 (26), 6555-6562
  The synthesis of 3-X-1-R-o-carborane (X = I, Br and R = Me, Ph) derivatives is reported, and aryldehalogenation at the B3 position is reported for the first time. The molecular and crystal structures of 1-phenyl-3-bromo-1,2-dicarba-closo-dodecaborane and ...
  Abstract | Full Text HTML | Hi-Res PDF

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Understanding Chemical Shifts in pi-Systems: 13C, 15N, 17O NMR