Coupling Constant and Factors Affecting Coupling Constant
Coupling Constant or Spin-Spin Coupling Constant(J)
Coupling constant is the strength of the spin-spin splitting interaction and the distance between centers of two adjacent peaks or lines in a multiplet. It is also called spin-spin coupling constant (J).In the above diagram, one is quartered (A) and other is triplet (B). Quartered means three more protons are present in the neighboring proton and triplet means two more protons are present in the neighboring proton. Quartered or triplet are a multiplet in which more than one peaks are present so the coupling constant is the distance 'J' between the two adjacent peaks in a multiplet.
The magnitude of 'J' is usually expressed in units of Hz or Cycle/sec and never in δ (ppm) values. The value of J remains constant and does not depend on magnetic field. Therefore, the separation between two peaks in a multiplet remains always constant but if this separation changes that means they represent different signals or does not belong to the same peak or multiplet. The value of 'J' is generally lies between 0 to 20Hz. The value of 'J' is positive when spin is paired while it is negative when spin is parallel.
Factors Affecting the Coupling Constant
The factor affecting the coupling constant are substituents, hybridization, ring stain, and dihedral angles. The coupling constant is independent of magnetic field strength means that the value of 'J' is not changed by changing the magnetic field strength.Substituent Effects
Electronegative substituents decreases the value of JH-H. For example-CH3CH2-Li, JH-H = 8.9 Hz
CH3CH2-+OR2, JH-H = 4.7 Hz
trans-CH2=CH-Li, JH-H = 23.9 Hz
trans-CH2=CH-F, JH-H = 12.8 Hz
Electronegative substituents increase the value of JC-H.
For example-
H-CH3, JC-H = 125 Hz
H-CH2F, JC-H = 149 Hz
H-CHF2, JC-H = 184 Hz
H-CF3, JC-H = 239 Hz for
Hybridization
JC-H increases as the amount of s-character in the C-H bond increases.sp3, JC-H = 125 Hz
sp2, JC-H = 167 Hz
sp, JC-H = 250 Hz
Thus, Order of JC-H = sp > sp2 > sp3
Ring Strain
Strained rings show unusually large C-H couplings. This is consistent with the idea that the C-H bonds in strained rings have a high degree of s- character. Typical values are 127 Hz for cyclohexane, 161 Hz for cyclopropane, and 228 Hz for the alkene C-H bond of cyclopropene.Dihedral Angles
When the dihedral angleis 0° (syn) or 180°(anti), JH-H is greatest (7 to 15 Hz). For a gauche conformation, it is less (2 to 5 Hz) and 0 or slightly negative for a 90° angle.The value of coupling constant can be approximately calculated from the dihedral angel θ-
J = 8.5 cos2θ − 0.28 (θ varies from 0 to 90°)
J = 9.5 cos2θ − 0.28 (θ varies from 90 to 180°)
Calculation of Coupling Constant
Coupling constant is the difference between two peaks and is measured in Hz, not ppm. The first thing to do is convert the peaks from ppm into hertz. To do this, just multiply the ppm values by the field strength in mHz.J Hz = ∆ ppm x instrument frequency
Suppose we have one peak at 4.260 ppm and another at 4.247 ppm. If we used a 500 mHz NMR machine(instrument), our peaks are at 2130 Hz and 2123.5 respectively.
Coupling constant (J) value is just the difference so in this case, J is 2130 - 2123.5 = 6.5 Hz.
