Understanding NMR Spectroscopy: Principles and Spectral Interpretation, Lecture notes of Nanotechnology

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Dr. Manale BITTAR

Dr. Mirella AZAR

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

NMR

1. Theory (^) Nuclear properties

A nucleus can be considered as a charged spherical particle

rotating around an axis.

 It therefore has a Kinetic momentum or angular momentum 𝑃

 According to quantum mechanics, 𝑃 ⃗ is quantified and can only take values

defined by the relation

I : the spin quantum number

The classical representation of spin consists of a rotational movement of the nucleus around an axis

passing through its center.

P=

I: the spin quantum number

Un noyau peut être étudié par RMN si son spin I est non nul

Mass (P + N)

Atomic weight

Charge (P)

Atomic number

Spin quantum number I Examples NMR phenomenon

Even Even 0

16 O,

12 C No

Even Odd 1 , 2 , …. (integer) I = 1

2 H,

14 N

I = 3

10 B

Yes

Odd Even or Odd (half integer) I =

1 H,

19 F,

13 C,

31 P

I =

11 B,

23 Na,

35 Cl,

37 Cl

I =

17 O,

27 Al

Yes

1. Theory Nuclear spin - Magnetic field interaction

Absence of static magnetic field H 0

magnetic moments randomly oriented

and have the same energy

Under static magnetic field H 0

these moments will be aligned

according to the direction of the

imposed field

According to the laws of quantum mechanics, only certain discrete orientations of these

vectors are authorized ( 2 I+ 1 ). Each orientation has an energy.

Two orientations are allowed for a proton(I= 1 / 2 ): E //

< E

anti//

the spin precesses around the axis of H

at an angular velocity ω 0

= γH 0

Precession accelerated with increasing H 0

Frequency of motion of the proton in rotation: Larmor frequency

The number of orientations that a nucleus can adopt in relation to an external

magnetic field is given by the formula 2 I+ 1. For

1 H 1

and

13 C 6

, I = 1 / 2 and 2 I+ 1 = 2

spin states: + 1 / 2 et - 1 / 2

H 0

same energy :

degenerescence

aligned :

more stable

opposite :

less stable

Resonance

1. Theory

It will be possible to make a transition between the two energy levels by supplying

the nucleus with electromagnetic energy. corresponding to

the Larmor frequency

When the transition takes place, we say that there is resonance of the nucleus.

same energy :

degenerescence

Precession 

NMR: Nuclear Magnetic Resonance

1. Theory

The principle of proton NMR (

1 H RMN) consists in :

(1) use a magnetic field H 0

to orient the nuclear "spins" of atoms,

( 2 ) excite these spins by a radio wave at the resonant frequency, which causes

certain spins to toggle

( 3 ) after excitation, the spins return to their initial state (relaxation).

2. Experimental techniques

Sample

For the solution study, the sample is dissolved in a solvent.

The amount of product required for proton NMR is 10 to 50 mg.

The sample is placed in a glass tube rotated in the center of a magnetic coil.

The solvent chosen must be free of hydrogen. In fact, the protons of the solvent must

not mask the protons of the sample examined.

Solvents used : CCl 4

, CDCl 3

; CD

3

COCD

3

; CD

3

OD; C

5

D

5

N; D

2

O ; DMSO-d 6 …

3. Main characteristics of the signal

1 H- NMR

The chemical shift

The position of the different lines of the

1 H-NMR spectrum is determined relative to a

reference. In the case of the proton, tetramethylsilane is used Si(CH 3

4

(TMS)

For convenience, we use a rating scale: the chemical shift noted 𝛿, in part per million

(ppm)

6

spectrometer

H TMS

H

( ) 10







 



The chemical shift of the nuclei depends on the chemical environmentn that

constitutes a magnetic screen to the exterior applied field B o

B effectif

= B 0

- B induit

= B o

(1 - σ)

Variations in the electron density around the nuclei

cause their shielding or deshielding from the

external magnetic field.

Shielding :

• decrease in the effective field felt by a nucleus, due

to a high local electronic density

• requires a stronger external field; or a higher

resonant frequency

B B

Be

B B = Bo

Be

B B

B : emitted field by the magnet

B : field felt by the nucleus

Deshielding:

• increase in the effective field felt by a nucleus, due to a

low local electronic density

• requires a weaker external field; or a lower resonant

frequency

shielded

nucleus

strong field

deshielded

nucleus

Weak field

This environment is mainly affected by :

a- The electron density b- the magnetic anistropy

c- the solvent d- The Hydrogen bonds

a) Effect of the electron density on the chemical shift

Since the increase in the screen effect on the nucleus is caused by the increase in electronic currents

due to the applied external magnetic field, it can be expected that the screen constant will increase

with the electronic density around the nucleus. The electronegativity of the substituents in the vicinity

of the nucleus and their inductive effect affect the electron density around the nucleus.

• Increase in electronegativity  decrease in the electon density around the nucleus  σ lower 

deshielding of the nucleus (δ higher).

• Decrease in electronegativity  increase in electron density around the nucleus  σ higher  shielding of

the nucleus (δ lower).

CH 3

F CH 3

Cl CH 3

Br CH 3

I CH 4

(CH 3

) 4

Si

Neighboring element F Cl Br I H Si

Electronegativity 4 3,1 2,8 2,5 2,1 1,

Chemical shift (ppm) 4,26 3,05 2,68 2,16 0,23 0

We can see that the resonance forms of a molecule show how the electron density can be

delocalized:

• Possibility of shielding and shielding

shielded by resonance

deshielded by inductive effect

b-Effect of magnetic anisotropy on chemical displacement

It often happens that it is not possible to explain the chemical displacement by the

simple reason of the electronegativity of the groups. This can be explained by what is

called magnetic anisotropy. In compounds with , the induced magnetic field associated

with the circulation of  is not symmetrical. The circulation of  electrons takes place

according to certain orientations. It is thus created around the multiple bonds shielding

zones or cones and deshielding zones. This effect is called the diamagnetic anisotropy of

multiple bonds. Thus a group or a substituent close to an unsaturation can be shielded or

deblinded according to its position in the molecule (if it falls in the shielding or deshielding

zone). Generally, all the groups which have  electrons in the vicinity of the proton studied

will generate magnetic anisotropy.

𝛿 𝐻 𝑎𝑙𝑑𝑒ℎ𝑦𝑑𝑒 : 9 − 10 ppm

𝛿 𝐻 𝐻 − 𝐶𝑠𝑝 : 1. 8 𝑝𝑝𝑚 (^) 𝛿 𝐻 = 7. 27 ppm

d- Effect of the Hydrogen bonds

Hydrogen bonds can also greatly modify the electronic environment of certain

protons (shielding or shielding), making it more difficult to predict chemical

shifts.

H bonds intermolecular  OH depends on concentration ,

temperature and the polarity of the solvent

H Bonds electron density around the OH proton, the proton peak is shifted

towards low field values or 

of concentration in a non polar solvent disrupts hydrogen bonds and the

peak appears at stronger fields = > 

Intramolecular H-bonds less affected by their environment than

intermolecular bonds

Absorption of β-diketones, for example, little affected by a change in

concentration or solvent effect, although it may be shifted by heating

The chemical shifts therefore give us indications on the chemical environment of the group to

which the proton considered belongs.

We can thus identify groups of protons from the value of δ.

Tables give the ranges of these displacements according to various environments.