Q.1. (c) What is coupling constant? How it is calculated? Explain with suitable example.
Ans.1.(c)Coupling constant:The distance between the peaks in a given multiplet is a measure of the magnitude of splitting effect. , it is referred to as coupling constant and is denoted by the symbol J. Numerical value of J is expressed in Hz or cps.
Unlike the chemical shifts, the value of J is independent of the applied field strength and depends only upon the molecular structure.
For a pair of mutually coupled protons, the coupling constant due to splitting by one proton has the same value as the coupling constant due to the splitting by the second proton. In other words, mutually coupled protons show the same magnitude of the splitting of each other signals.
In general, the size of coupling constant is determined by the number and kind of intervening chemical bonds and the spatial relations between the protons as illustrated below as few cases.
(a) For protons attached to the same carbons atom (i.e., germinal protons), the values of J varies from 0-20 Hz depending upon the bond angle and overall structure of the molecule.
(b) For protons attached to adjacent carbon atoms (i.e., vicinal protons), J varies from 2-18 Hz depending upon the spatial positions and the structure of the molecules as a whole.
For example: In case of freely rotating groups, protons with anti- conformation have J= 5-12 Hz while protons with gauche conformation have J =2-4 Hz.
Calculation of Coupling constant:
For the simple case of a doublet, the coupling constant is the difference between two peaks. The trick is that J is measured in Hz, not ppm. The first thing to do is convert the peaks from ppm into hertz. Suppose we have one peak at 4.260 ppm and another at 4.247 ppm. To get Hz, just multiply these values by the field strength in mHz. If we used a 500 mHz NMR machine, our peaks are at 2130 Hz and 2123.5 respectively. The J value is just the difference. In this case it is 2130 - 2123.5 = 6.5 Hz. This can get more difficult if a proton is split by more than one another proton, especially if the protons are not identical.
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