spectroscopy 3: magnetic resonance chapter 15. conventional nuclear magnetic resonance energies of...

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

The 1H-NMR low-resolution spectrum of ethanol

Fig 15.6 The 1H-NMR high-res 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 group from coupling with –CH2 protons

e.g., CH3CH2OH

⇇⇉⇆

⇄B0

⇉ ⇇⇆

⇄

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

resonance of a -CH2 from coupling with -CH3 protons

e.g., CH3CH2OHB0

Spectrum 1 Empiricial formula C4H8O

Spectrum 2 Empiricial formula C4H8O2

Spectrum 3 Contains C, H, O MW = 72