Spectroscopy 3:Spectroscopy 3:Magnetic ResonanceMagnetic Resonance

CHAPTER 15CHAPTER 15

Conventional nuclear magnetic resonance Energies of nuclei in magnetic fields

Typical NMR spectrometer

The chemical shift (effect of nearby nuclei)

Fine structure (nuclear spin-spin coupling)

Pulsed techniques in FT-NMR

Fig 15.1 Interactions between ms states of an

electron and an external B field

precessionνL ≡ the Larmor freq

π

γν

2

BoeL mmss = +1/2 = +1/2

mmss = −1/2 = −1/2

where γe ≡ magnetogyric ratio

Bo ≡ applied magnetic field

Fig 15.3 Nuclear spin states of a spin-1/2 nucleus

(e.g., 1H or 13C) in a magnetic field

= hνradio

Typically:

• A 100 MHz NMR employs a 2.35 T field

• Resonance is achieved

when νradio = energy separation between levels

Fig 15.4 Layout of a typical NMR spectrometer

The Chemical Shift

• Nuclear magnetic moments interact with the local field

• In most cases, Bloc ≠ B0 due to electronic orbital ang momentum

• The Larmor frequency νL (frequency of precession)

differs for nuclei in different environments

• Resonance frequencies expressed as the chemical shift

21

20B

)(Bloc

L

610

TMS

Fig 15.5(a) Range of typical chemical shifts for 1H

TMS

Deshieldednuclei

(low field)

Shieldednuclei

(high field)

Fig 15.5(b) Range of typical chemical shifts for 13C

TMS

Deshieldednuclei

(low field)

Shieldednuclei

(high field)

Fig 15.6 The 1H-NMR spectrum of ethanol

Integrated signal

singlet

quartet

triplet

1

3

2

Fig 15.7 Variation of the chemical shift with electronegativity

Trend due to

magnetic anisotropy

Trend due to

electronegativity

P 523: Magnetic anisotropy shields proton

P 523: Magnetic anisotropy shields proton

P 523: Magnetic anisotropy shields proton

H

Bloc

Fig 15.9 Ring current deshields ring protons and

shields substituent protons

• Special case of neighboring group effect in aromatics

deshielded

shielded

Fig 15.6 The 1H-NMR spectrum of ethanol

Integrated signal

singlet

quartet

triplet

1

3

2

Fine structure

Fine Structure

• Each magnetic nucleus may contribute to the local field ofadjacent nuclei

• ∴ Resonance frequencies are modified

• Strength of interaction given by the coupling constant, J (Hz)

• J is independent of applied mag field, Bo

Margin pg 526 n equivalent nuclei split adjacent spin(s) into n+1 lines with intensity distribution given by Pascal’s triangle:

Fig 15.15 Origin of the 1:2:1 triplet in the proton

resonance of a –CH3 species

e.g., CH3CH2OH

⇇⇉⇆

⇄B0

Fig 15.16 Origin of the 1:3:3:1 quartet in the proton

resonance of a -CH2- species

e.g., CH3CH2OHB0