Nuclear Magnetic Resonance Spectroscopy: Theory and Applications
487
The multiplets are named as singlet, doublet, triplet, quartet, quintet, sextet, septet,
octet, nonet etc. (Fig. 17).
Protons that are equivalent by symmetry do not couple (split) each other.
C
H
C
H
Cl
Cl
No splitting
(n + 1) rules can be applied on protons attached to aliphatic chains not on double
bonds or benzene rings.
H 3 C
H 2
C
H 2
C
H 2
C CH 3
CH 3
H
H
H
CH 3
H
CH 3
splitting
splitting
No splitting
No splitting
Intensities of multiplet sub-peaks depends upon number as well as nature of nearby
atoms.
Let us consider the case of ethyl bromide (CH 3 –CH 2 –Br) to understand number
of sub-peaks in multiplets [17], The spin of two protons of (–CH 2 –) can couple with
adjacent –CH 3 protons in the following ways:
(1) Reinforcing (↑↑) (2) Not effecting (↑↓ ↓↑) and (3) Opposing (↓↓).
Thus triplet peak (n + 1 = 3) of CH 3 protons will originate with intensity ratio
1:2:1. Similarly, intensity ratio of CH 2 multiplets can be calculated by observing
spin-patterns of nuclei observed (1:3:3:1).
↑↑↑
↑↑↓ ↑↓↑ ↓↑↑
↓↓↑ ↓↑↓ ↑↓↓
↓↓↓
Strongly reinforcing
Weakly reinforcing
Weakly opposing
Strongly opposing
The relative intensities of the individual lines of multiplets correspond to the
numerical coefficient of the lines in the binomial expressions—(1 + x)
n .
For n = 1, (1 + x)
1
= 1 + x; n = 2, (1 + x)
2
= 1 + 2x + x
2 ; n = 3, (1 + x)
3
= 1
+ 3x
2
+ 3x + x
3 .
The relative intensities for n = 1, 2 and 3 will be 1, 1:2:1 and 1:3:3:1 respectively
i.e. numerical coefficient of their respective equations.
487
The multiplets are named as singlet, doublet, triplet, quartet, quintet, sextet, septet,
octet, nonet etc. (Fig. 17).
Protons that are equivalent by symmetry do not couple (split) each other.
C
H
C
H
Cl
Cl
No splitting
(n + 1) rules can be applied on protons attached to aliphatic chains not on double
bonds or benzene rings.
H 3 C
H 2
C
H 2
C
H 2
C CH 3
CH 3
H
H
H
CH 3
H
CH 3
splitting
splitting
No splitting
No splitting
Intensities of multiplet sub-peaks depends upon number as well as nature of nearby
atoms.
Let us consider the case of ethyl bromide (CH 3 –CH 2 –Br) to understand number
of sub-peaks in multiplets [17], The spin of two protons of (–CH 2 –) can couple with
adjacent –CH 3 protons in the following ways:
(1) Reinforcing (↑↑) (2) Not effecting (↑↓ ↓↑) and (3) Opposing (↓↓).
Thus triplet peak (n + 1 = 3) of CH 3 protons will originate with intensity ratio
1:2:1. Similarly, intensity ratio of CH 2 multiplets can be calculated by observing
spin-patterns of nuclei observed (1:3:3:1).
↑↑↑
↑↑↓ ↑↓↑ ↓↑↑
↓↓↑ ↓↑↓ ↑↓↓
↓↓↓
Strongly reinforcing
Weakly reinforcing
Weakly opposing
Strongly opposing
The relative intensities of the individual lines of multiplets correspond to the
numerical coefficient of the lines in the binomial expressions—(1 + x)
n .
For n = 1, (1 + x)
1
= 1 + x; n = 2, (1 + x)
2
= 1 + 2x + x
2 ; n = 3, (1 + x)
3
= 1
+ 3x
2
+ 3x + x
3 .
The relative intensities for n = 1, 2 and 3 will be 1, 1:2:1 and 1:3:3:1 respectively
i.e. numerical coefficient of their respective equations.