CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 23 - Professor J. Michael McBride
Diamagnetic Anisotropy and Spin-Spin Splitting
Through-space interaction between magnets of fixed strength and
orientation averages to zero during random molecular tumbling,
suggesting that the local field about a proton should be sensitive only
to electrons that orbit about itself. The chemical shift can be
sensitive to electrons orbiting elsewhere if the amount of orbiting
varies with molecular orientation. This "diamagnetic anisotropy" is
commonly used to rationalize the unusual chemical shifts of protons in
acetylene and in aromatic and antiaromatic compounds. The other source
of a proton's local field is nearby magnetic nuclei, which can be
counted by the splitting multiplicity. Unlike chemical shift, which is
measured in fractional units because it depends on the strength of the
applied field, this spin-spin splitting (J), measured in Hz, is dependent only on molecular structure. J
depends not on spatial proximity, but on orbital overlap, which,
remarkably, is larger for anti- than for eclipsed conformations.
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