9.8. KARPLUS CURVE – VARIATION OF COUPLING CONSTANTS WITH DIHEDRAL ANGLE
The Coupling constant, J varies with dihedral angle Figure 9.15. Staggered and eclipsed conformations
Vicinal Coupling (3J) For Vicinal protons value of coupling constant varies with dihedral angle. Vicinal protons are the protons separated by three bonds.
Dihedral angle between two axial hydrogen is 180° while the axial equatorial and equatorial equatorial dihedral angles are both 60°. The coupling constant reaches a maximum when the dihedral angle is 180° i.e when the hydrogen is anti-periplanar with the most effective overlap of orbitals. It is slightly smaller than maximum when the dihedral is 0° i.e. when the vicinal hydrogen is syncoplanar and reaches a minimum. When the dihedral angle between the hydrogen is 90° and orbitals are orthogonal.
One of the most important consequences of the karplus equation is on the order of magnitude of diaxial, axial- equatorial and diequatorial coupling constants (Jaa, Jac, and Jee resp) in a cyclohexane ring chair system.
The axial axial coupling constant in rigid cyclohexanes is normally large 8-15Hz, since the dihedral angle between two axial hydrogen is 180°.
The axial–equatorial and equatorial equatorial coupling constant are in the range 2-5 Hz since …show more content…
J can have any sign. This coupling ranges from -20 to 40 Hz. 2JHH depends on hybridization of carbon atom and the bond angle and the substituent such as electronegative atoms. When S-character is increased, Geminal coupling constant is increased : 2Jsp1> 2Jsp2> 2Jsp3 The bond angle(HCH) gives rise to change 2JHH value and depend on the strain of the ring in the cyclic systems. Geminal coupling constant determines ring size. When bond angle is decreased, ring size is decreased so that geminal coupling constant is more positive. If a atom is replace to an electronegative atom, Geminal coupling constant move to positive