Coupling Constant.pptx

H-NMR Coupling Constant

1. Coupling Constant

2. Definition : The distance between the centres of the two adjacent peaks in a multiplet is usually constant and is called as coupling constant. The value of coupling constant is independent of the external magnetic field. It is measured in hertz (Hz) or in cycles per second (cps). It is denoted by the latter (J).

3. Triplet

4. From the value of J we can distinguish between two singlet and on doublet; two doublets from one quartet. This can be done by simply recording the spectrum at two different radio frequencies. The value of J genrally lies between 0 and 20 hertz.

5. Types of coupling Geminal coupling Substituent effects Vicinal coupling Long range coupling

6. Geminal Coupling The protons attached to same carbon but different environment known as geminal protons. J2=0.5-03 Hz Value of J depends on bond angle.

7. Increase by increasing bond angle. J increase by increasing electronegativity . 105 = -25 cps 109 = -12 cps 125 = 0 cps Greater than 125 = + cps

8. Substituent effects The increase in the electronegativity atoms or groups which withdraws sigma electrons that increase the value of coupling constant. Methyl chloride = -10.8 cps Methyl fluoride = -9.4 cps

9. Vicinal coupling: In vicinal coupling the protons separated by three bonds. Here the value of J varies with dihedral angle. Vicinal coupling for a few compounds Propane (gauche)=7.2 cps, (anti)=6.5 cps

10. Long range coupling No coupling observe about 4 bond distance or more than three bond in saturated molecules. J4 observed in Unsaturated molecules. Like Ethene Trans=12-18Hz , Cis =7-12Hz Benzene Ortho=6-9, Meta=1-3, Para=0-1

11. Peak Label δ  ( ppm) v (Hz) a 3.7805 226.83 b 3.6628 219.77 c 3.5452 212.71 d 3.4275 205.65 e 1.3646 81.88 f 1.2470 74.82 g 1.1293 67.76 To determine coupling constant for a Quartet the difference in frequency (ν) between each peak is calculated and the average of this value provides the coupling constant in Hz.

12. Using frequency data: Frequency of peak c - frequency of peak d = 212.71 Hz - 205.65 Hz = 7.06 Hz Frequency of peak b - frequency of peak c = 219.77 Hz – 212.71 Hz = 7.06 Hz Frequency of peak a - frequency of peak b = 226.83 Hz – 219.77 Hz = 7.06 Hz Average: 7.06 Hz ∴   J (H-H) = 7.06 Hz

13. Alternatively, using chemical shift data: Chemical shift of peak c - chemical shift of peak d = 3.5452 ppm – 3.4275 ppm = 0.1177 ppm Chemical shift of peak b - chemical shift of peak c = 3.6628 ppm – 3.5452 ppm = 0.1176 ppm Chemical shift of peak a - chemical shift of peak b = 3.7805 ppm – 3.6628 ppm = 0.1177 ppm Average: 0.1176 ppm Average difference in ppm x frequency of the NMR spectrometer = 0.1176 ppm x 60 MHz = 7.056 Hz ∴  J (H-H) = 7.06 Hz

14. Calculate the coupling constant for triplet. Using frequency data: Frequency of peak f - frequency of peak g = 74.82 Hz – 67.76 Hz = 7.06 Hz Frequency of peak e - frequency of peak f = 81.88 Hz – 74.82 Hz = 7.06 Hz Average = 7.06 Hz ∴  J (H-H) = 7.06 Hz

15. Alternatively, using chemical shift data: Chemical shift of peak f - chemical shift of peak g = 1.2470 ppm – 1.1293 ppm = 0.1177 ppm Chemical shift of peak e - chemical shift of peak f = 1.3646 ppm – 1.2470 ppm = 0.1176 ppm Average = 0.11765 ppm 0.11765 ppm x 60 MHz = 7.059 Hz ∴   J (H-H) = 7.06 Hz

16. The coupling constant, J (usually in frequency units, Hz) is  a measure of the interaction between a pair of protons .