NMR Spectroscopy - Spin spin Coupling-Splitting ,Multiplicity, (n+1) rule, Pascal Triangle, Coupling Constant.pdf

This is all about NMR spectroscopy,spin-spin splitting ,multiplicity,n+1 rule etc

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