Spin spin splitting (n+1 Rule) NMR spectroscopy

1. Presentation By:Humna Mehmood
Spin-Spin Splitting
(n+1 Rule)

2. Spin-Spin Splitting
The absorption signal due to a particular proton may split into double(two
peaks), triplet (three peaks) and so on. This phenomenon is commonly
known as spin-spin splitting.
Spin-spin splitting occurs only between nonequivalent protons on the same
carbon or adjacent carbons.
Spin–spin splitting can be explained empirically by the so-called n + 1
Rule.
“Each type of proton senses the number of equivalent protons (n) on the
carbon atom(s) next to the one to which it is bonded, and its resonance
peak is split into (n + 1) components”.

3. Multiplicity
• The no. of peaks into which the absorption signal split is called
multiplicity.

4. The Origin Of Spin–spin Splitting
• Spin–spin splitting arises because hydrogens on adjacent
carbon atoms can “sense” one another. The hydrogen on
carbon A can sense the spin direction of the hydrogen on
carbon B.
• The chemical shift of proton A is influenced by the direction
of the spin in proton B. Proton A is said to be coupled to
proton B(spin-spin coupling).
• The resonance of proton A is said to have been split by
proton B, and the general phenomenon is called spin–spin
splitting.

5. GENERAL RULES FOR SPIN-SPIN SPLITTING
Three general rules describe the splitting patterns commonly seen in the 1H NMR
spectra of organic compounds.
1.Equivalent protons do not split each other’s signals.
2.A set of n nonequivalent protons splits the signal of a nearby proton into n + 1
peaks.
3.Splitting is observed for nonequivalent protons on the same carbon or adjacent
carbons.

6. Proton spectrum of ethanol

7. Examine the case at hand, 1,1,2-trichloroethane, utilizing the n + 1 Rule.

8. 1,1,2-trichloroethane, utilizing the n + 1 Rule
• First the lone methine hydrogen is situated next to a carbon bearing two
methylene protons. According to the rule, it has two equivalent neighbors (n =
2) and is split into n + 1 = 3 peaks (a triplet).
• The methylene protons are situated next to a carbon bearing only one methine
hydrogen. According to the rule, these protons have one neighbor (n = 1) and
are split into n + 1 = 2 peaks (a doublet).

9. Consider the spectrum below:

10. How do the doublet due to the CH2 group on BrCH2CHBr2
occurs?
When placed in an applied electric field, (B0), the adjacent proton (CHBr2)
can be aligned with () or against () B0.
Thus, the absorbing CH2 protons feel two slightly different magnetic
fields—one slightly larger than B0, and one slightly smaller than B0.
Since the absorbing protons feel two different magnetic fields, they absorb
at two different frequencies in the NMR spectrum, thus splitting a single
absorption into a doublet.

11. How A Doublet Arises?

12. How do a triplet arises?
• When placed in an applied magnetic field (B0), the adjacent
protons Ha and Hb can each be aligned with () or against () B0.
• Thus, the absorbing proton feels three slightly different magnetic
fields—one slightly larger than B0, one slightly smaller than B0, and
one the same strength as B0.

13. How do a triplet arises?
Because the absorbing proton feels three different magnetic fields, it
absorbs at three different frequencies in the NMR spectrum, thus splitting a
single absorption into a triplet.
Because there are two different ways to align one proton with B0, and one
proton against B0—that is, ab and ab—the middle peak of the triplet is
twice as intense as the two outer peaks, making the ratio of the areas under
the three peaks 1:2:1.
Two adjacent protons split an NMR signal into a triplet.
When two protons split each other, they are said to be coupled.
The spacing between peaks in a split NMR signal, measured by the J
value, is equal for coupled protons.

14. How do a triplet arises?

15. Splitting is not generally observed between protons separated by more than
three  bonds.

16. Common splitting patterns

17. The H NMR spectrum of 2-bromopropane, (CH3)2CHBr

18. Spectrum of ethyl iodide

19. Spectrum of ethyl iodide
• Notice that the methylene protons are split into a quartet (four peaks), and
the methyl group is split into a triplet (three peaks). This is explained as
follows:

20. Spectrum of 2-nitropropane

21. Spectrum of 2-nitropropane
• In the case of 2-nitropropane there are two adjacent carbons that bear
hydrogens (two carbons, each with three hydrogens), and that all six
hydrogens as a group split the methine hydrogen into a septet.

22. Pascal’s Triangle
• We can easily verify that the intensity ratios of multiplets derived from the
n + 1 Rule follow the entries in the mathematical mnemonic device called
Pascal’s triangle.
• Each entry in the triangle is the sum of the two entries above it and to its
immediate left and right.
• Notice that the intensities of the outer peaks of a multiplet such as a septet
are so small compared to the inner peaks that they are often obscured in the
baseline of the spectrum.

23. Pascal’s Triangle