NMR Spectroscopy and Stereo Chemistry
Information from coupling constant
Distinction between cis and trans isomers
Trans coupling is greater than for cis. From the value the isomers can
be identified as in the following example.
For coupling the two hydrogens must have different
Trans coupling (range :11 to 19 Hz) is greater than
cis coupling (range: 5 to 14 Hz).
3JHH means coupling between
two hydrogens separated by a distance of three bonds (vicinal coupling).
If it is coupling between hydrogens at the same carbon then it is 2JHH (geminal coupling)
In some molecules the cis-trans, distinction is made even from the
position of the absorption, as in cis and trans stilbenes. In trans
case the hydrogens are deshielded by two aromatic rings while in the
cis case it is by one ring.
Variation of dihedral angle with coupling constant
Dihedral angle: Consider the following Newman and Sawhorse projections
- Looking from the front Hb is eclipsed by Ha the angle between the
plane containing Ha and Hb is zero (they are in the same plane).
- The second conformation has Hb at a different spatial position, they
are not in the same plane. The plane containing Hb is 600 to the right of the plane containing Ha. This is the dihedral angle, in the first case the dihedral angle is zero.
- In the third case though the two atoms are in the same plane Hb has
undergone a rotation of 1800 and that is the dihedral angle.
- The dihedral angle is used when two are separated by three bonds.
- Martin Karplus was the first to study the variation of coupling constant 3JHH with dihedral angle.
- The magnitude of the coupling constant between hydrogens of adjacent
carbon atoms depend directly on the dihedral angle.
- This magnitude is greatest when this is zero or 1800 and
smallest when it is 900.
- When it is zero side-side overlap of the two C-H bond orbitals will
be maximum, when it is 1800 back lobes overlap is maximum
leading to higher “J”. When it is 600 C-H bond
orbitals overlap is at a minimum since they are perpendicular with each
other leading to lower “J”.
Stereochemistry of cyclohexane derivatives
The information from the above can be used to assign the configuration
of the anomeric carbon in cyclic sugars. In the case of α- glucose
at the anomeric carbon the equatorial proton is has a chemical shift
of δ 5.2 which is down field by 0.5 for the corresponding axial
proton in β-glucose.