Spectroscopy 3: Magnetic Resonance CHAPTER 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 m s states of an electron and an external B field precession ν L ≡ the Larmor freq m s = +1/2 m s = −1/2 where γ e ≡ magnetogyric ratio B o ≡ applied magnetic field

Fig 15.3 Nuclear spin states of a spin-1/2 nucleus (e.g., 1 H or 13 C) in a magnetic field = h ν radio Typically: A 100 MHz NMR employs a 2.35 T field Resonance is achieved when ν radio = 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, B loc ≠ B 0 due to electronic orbital ang momentum The Larmor frequency ν L (frequency of precession) differs for nuclei in different environments Resonance frequencies expressed as the chemical shift TMS

Fig 15.5(a) Range of typical chemical shifts for 1 H TMS Deshielded nuclei (low field) Shielded nuclei (high field)

Fig 15.5(b) Range of typical chemical shifts for 13 C TMS Deshielded nuclei (low field) Shielded nuclei (high field )

Fig 15.6 The 1 H-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

H B loc

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 1 H-NMR spectrum of ethanol Integrated signal singlet quartet triplet Fine structure

Fine Structure Each magnetic nucleus may contribute to the local field of adjacent nuclei ∴ Resonance frequencies are modified Strength of interaction given by the coupling constant, J (Hz) J is independent of applied mag field, B o

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

Fig Origin of the 1:2:1 triplet in the proton resonance of a –CH 3 species e.g., CH 3 CH 2 OH ⇇⇉ ⇆ ⇄ B0B0

Fig Origin of the 1:3:3:1 quartet in the proton resonance of a -CH 2 - species e.g., CH 3 CH 2 OHB0B0