This document discusses spin-spin coupling in NMR spectroscopy. It describes how the magnetic moments of adjacent nuclei interact, resulting in the splitting of NMR signals into multiplets such as doublets and triplets. The number of peaks in a multiplet, known as the multiplicity, reflects the number of neighboring nuclei. The distance between peaks, called the coupling constant J, provides important structural information. Different types of coupling are discussed, including homonuclear, heteronuclear, geminal, vicinal, and long-range coupling. Examples and diagrams are provided to illustrate these concepts.

1. NMR Spectroscopy
Lecture 3
Prepared by Ashar

2. Outline :
1.1 Spin Spin Coupling
1.2 Spin spin Splitting
1.3 Multiplicity
1.4 (n+1) Rule
1.5 Pascal Triangle
1.6 Coupling Constant and its Types

3. 1.1 Spin Spin Coupling:
● Interactions between different types of protons present in the molecule
● Cause a single peak on an NMR spectrum to split into doublet, triplet, or
multiplet.
●
● Nuclear spins Couple to Give Splitting of Resonance Lines
● Equivalent Proton show no Coupling among themselves
● Phenomenon by which the spins of resonating protons cause the peaks on
the NMR spectrum to multiply is known as peak splitting.

4. 1.2 Spin spin splitting
● Interaction of magnetic fields between two or more nuclei.
● Occurs through connecting bonds and space.
● Involves the magnetic moments of adjacent nuclei.
● Results in the splitting of NMR signals.
● Signals appear as multiplets.
● Examples include doublets, triplets, etc.
● Causes PMR signals to split.
● Each split component represents a different magnetic environment.

5. 1.3 Multiplicity:
● Refers to the number of split peaks in a signal.
● Reflects the number of neighboring nuclei.
● Tells how many hydrogen atoms are immediately next door to the hydrogens
producing that peak
● The simplest signal consists of one line and is called a singlet, followed by the
doublet, triplet, etc.
● A signal with more than seven lines is referred to as a multiplet.

6. Examples:

7. 1.4 (n+1) Rule:
● Describes the number of peaks in a multiplet.
● The signal is split into (n + 1) peaks, where n is the number of equivalent
protons.
Example;
CH3—CH2—Cl
● CH3 protons experience spin-spin coupling with equivalent protons in
—CH2—.
● The signal for CH3 protons appears as a triplet.
● This follows the (n + 1) rule, where n is the number of equivalent
protons.e.g 2+1 = 3 ( triple) in Above Example

9. 1.5 Pascal Triangle:
● It is the Intensities Ratios of multiplets follow the entries in the Mathematical
Expression.
● This is a number pattern invented by a famous French mathematician, Blaise
Pascal
● Each entry in triangle is the sum of two Entries above it and to its immediate
left and right

11. 1.6 Coupling Constant (J):
● The distance between the centres of the two adjacent peaks in a multiplet is
usually constant and is called the coupling constant.
● The value of the coupling constant is independent of the external field.
● It is denoted by the letter J.
● The value of ‘J’ generally lies between 0 and 20 Hz.
● It is measured in Hertz (Hz) or in cps (cycles per second).

12. ● From the value of coupling constant, one can distinguish between the two
singlets and one doublet and also a quartet from two doublets.
● It measure the Effectiveness of Coupling i.e Bond Distance
● Magnitude of Coupling Constant Give important structural information.

13. Example
● In this compound two signals are expected in the nmr spectrum
● Under the influence of two equivalent protons ‘a’, the signal for proton ‘b’ will
appear as a triplet.
● The distance between any two adjacent peaks in a multiplet will be exactly the
same
● The triplet formed due to spin-spin coupling is shown as;
a)

14. ● In the nmr spectrum of this compound, proton ‘b’ is under the influence of
three equivalent protons ‘a’.
● Due to spin-spin coupling, the signal for proton ‘b’ will appear as a quartet with
intensity ratio 1 : 3 : 3 : 1.
● The quartet for ‘b’ can be represented as;
b)

15. ● In the nmr spectrum of 1, 2, 2– trichloroethane two multiplets are
observed.
● The value J in each of its multiplets is found to be constant & shown
as:
c)

16. Types of Coupling
Homonuclear
Coupling
Heteronuclear
Coupling
Geminal Coupling: Vicinal coupling Long-range coupling

17. 1.6.1 Coupling Types:
● Homonuclear coupling occurs between nuclei of the same type, such as
1H-1H coupling.
● Predominantly observed in proton NMR spectroscopy.
● Results in multiplets with patterns like doublets, triplets, and quartets.
● Common in organic molecules with multiple protons
● Provides details about the local environment and connectivity of equivalent
protons.
a) Homonuclear Coupling :

18. ● In heteronuclear coupling, the interacting nuclei are of different isotopes,
such as 1H-13C coupling or 1H-15N coupling,1H-19F Coupling.
● Commonly observed in heteronuclear NMR spectroscopy, such as
proton-coupled carbon-13 NMR.
● Typically results in doublets in the proton-coupled spectra of heteronuclear
nuclei.
● Observed in molecules containing both proton and heteronuclear nuclei.
● Gives information about the local environment of heteronuclear atoms (e.g.,
carbon) and their coupling with protons.
b) Heteronuclear Coupling :

19. c) Geminal Coupling:
● Refers to protons attached to the same carbon atom but with different
chemical environments.
● The value of J (coupling constant) in geminal protons depends on the bond
angle
● J can have any sign, and its magnitude varies with the bond angle.
● Geminal protons are separated by two bonds in a saturated compound.
● If geminal protons are in different chemical environments, the coupling is
usually strong.

20. ● At a bond angle of 105°, J is approximately –25 cps (negative sign indicates
anti-phase coupling).
● As the bond angle increases to 109°, J becomes nearly –12 cps.
● With a further widened bond angle of 125°, the value of J increases to zero.
● Bond angle influences the strength and nature of coupling between geminal
protons.
● Variations in J values provide information about the molecular geometry and
conformation.

21. Graph :
A plot showing the relationship between the values of J versus the bond angle is
shown as under :

22. Example :
The values of geminal coupling constants for some compounds are as follows:
Compound Jgem
Methane – 12.4 cps
Methyl chloride – 10.8 cps
Methyl fluoride – 9.4 cps
Ethene + 2.5 cps
Formaldehyde + 41.0 cps

23. d) Vicinal coupling :
● For vicinal protons, the value of coupling constant varies with the dihedral
angle.
● Vicinal protons are the protons which are separated by three bonds.
● A plot for the values of J versus the dihedral angle for vicinal protons is
shown as under :

24. The Graph shows :
● When the dihedral angle is 0° or 180°, we observe largest values for the
coupling constants.
● The value of ‘J’ is slightly negative when the dihedral angle is 90°

25. Example :
The values of vicinal coupling constants for a few compounds are listed below :
● (i) Propane (Gauche) = 7.2 cps
● (ii) Propane (Anti) = 6.5 cps
● (iii) Dibromoethane = 3.4 cps (Gauche) = 15.1 cps (Anti)

26. e) Long-range coupling:
● Long-range coupling refers to observable coupling when the nuclei are more
than three bonds apart.
● Long-range coupling refers to the indirect interaction between nuclear spins
that are not directly bonded to each other.
● It arises from a mechanism involving virtual transitions of electrons through
the molecular framework.

27. Example :
The values of long range coupling constants in compounds which involve
coupling are as follows:
● (i) CH3—CH = CH2 = – 1.7 cps
● (ii) H2C = C—CH2—CH3 = – 2.3 cps
● (iii) CH3—C = C(CH3)—COOH = – 1.2 cp