Nuclear Magnetic Resonance (NMR) Spectroscopy: Principles and Applications, Summaries of Construction law

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Unit-2 NMR Spectroscopy

Applied Chemistry-B CYCI-102, CSE

NIT Jalandhar

2021

Dr Singh

Radio waves have the lowest energy of the various kinds of electromagnetic radiation. We use them for radio and television

communication, digital imaging, remote control devices, and wireless linkages for computers. Radio waves are also used in

NMR spectroscopy and in magnetic resonance imaging (MRI).

Image and notes from Organic chemistry, P Y Bruice

2

Consider simple example of Hydrogen atom , as it consist of one electron and one

proton

a. So one electron circulating around hydrogen (Figure 1)

b. And the hydrogen has a spin as shown in the figure (Figure 2)

c. We can consider hydrogen as a bar magnet (Figure 3), so hydrogen behaves a

magnet

Many atomic nuclei have a property called spin: the nuclei behave as if they were spinning.

In fact, any atomic nucleus that possesses either odd mass, odd atomic number, or both has a quantized spin

angular momentum and a magnetic moment

(Figure 1)

(Figure 2) (Figure 3)

All molecules with non-zero spin have a magnetic moment

=  l

Here  is magnetic moment,  gyromagnetic ratio and l is the angular momentum

Because of magnetic moment of the nucleus, forces nucleus to behave as a tiny bar magnet

Nucleus Spin  gyromagnetic ratio Natural Abundance

1

H ½ 42.57 99.9985%

13

C ½ 10.71 1.07%

S N

Representing the magnet

What happens when we bring two magnets together

Repulsion and attraction

Magnetic field and orientation of the other magnet

S N S N

Magnetic field B in this direction and influences the orientation of the other magnet

B

NMR machine ON

Our magnet align with

the spin

MAGNET

NMR machine

MAGNET

Applied magnetic field

B

NMR machine ON

MAGNET

8

The number of allowed spin state of a spinning nucleus can be determined from spin quantum number I

Where allowed spin states are 2I+1 with integral difference ranging from + I to – I

For proton I = ½ so allowed transitions will be ====== 2I+1 so 2 *1/2 + 1 = 2, i.e. +1/2 to - 1/

In the absence of applied magnetic field all spin states of a given nucleus are of equivalent energy (degenerate)

Hydrogen atom (proton) in different environment

e

e

e

e

e

e

e

MAGNET

MAGNET

A

B

A proton in high energy opposed orientation can come back to

lower (aligned state) by releasing energy

The transition from one state to another state is called spin

flipping

 E = hv

The energy required to bring out transition or flip depends upon

the strength of external magnetic field.

14

1. The energy difference ( E ) between the  - and β - spin states depends on the strength of the applied magnetic

field ( B o): the greater the strength of the applied magnetic field, the greater the  E

Image and notes from Organic chemistry, P Y Bruice

2. The energy difference ( E ) also

depends upon the type of nucleus

involved (which depends upon the

gyromagnetic ration), and chemical

environment

THE MECHANISM OF ABSORPTION (RESONANCE)

Proton is considered as a child’s spinning top

Proton: like a spinning top

The phenomenon of precession is similar to that of a spinning top. Owing to the influence of the earth’s gravitational

field, the top begins to “wobble,” or precess, about its axis. A spinning nucleus behaves in a similar fashion under the

influence of an applied magnetic field (as shown in next slide).

17

A spinning nucleus “wobble,” under the influence of an applied

magnetic field

The precession of a spinning nucleus resulting

from the influence of an applied magnetic field

A top precessing in the earth’s gravitational field

ClCH

2

CH

3

a b

a b

SHIELDING OR DESHIELDING

In a magnetic field, the electrons circulate about the nuclei and induce a local magnetic field that acts in opposition to the

applied magnetic field and, therefore, subtracts from it. As a result, the effective magnetic field —the amount

of magnetic field that the nuclei actually “sense” through the surrounding electrons—is somewhat smaller than the applied

magnetic field:

Diamagnetic shielding

The protons in electron-rich environments sense a smaller

effective magnetic field.

Therefore, they require a lower frequency to come into

resonance—that is, flip their spin—because E is smaller (

Figure). Protons in electron-poor environments sense

a larger effective magnetic field and so require a higher

frequency to come into resonance

because E is larger.

ClCH

2

CH

3

a b