User:Benjah-bmm27/degree/2/CJA

NMR, CJA

"Introduction to NMR Spectroscopy"

Overview

• Nuclear magnetic resonance
• NMR spectroscopy

Fundamentals

• Gyromagnetic ratio

Chemical shift

• Chemical shift

NMR active nuclei

• Proton NMR
• Carbon-13 NMR
• Fluorine-19 NMR
• Phosphorus-31 NMR
• Platinum-195 NMR

Coupling

• J-coupling

Typical coupling constants

One bond coupling

• e.g. ¹³C—¹H in CHCl₃:
• ¹J_CH ≈ 120 Hz

Two bond (geminal) coupling

• e.g. bridgehead CH₂ in norbornene:
• ²J_HH ≈ 10–18 Hz

Three bond (vicinal) coupling

• if rotation about C–C is free (e.g. e.g. H–C–C–H in ethyl iodide), ³J_HH ≈ 8 Hz
• if rotation is restricted, can predict ³J_HH from dihedral angle φ with the Karplus equation: ³J_HH = α cos²(φ)
• ³J_HH is maximal (i.e. ³J_HH = α) where φ = 0° or 180°,

Benzene rings

• ³J_{HH, ortho} ≈ 8 Hz
• ⁴J_{HH, meta} ≈ 2 Hz
• ⁵J_{HH, para} ≈ 0 Hz

Alkenes

• ³J_{HH, trans} ≈ 16 Hz
• ³J_{HH, cis} ≈ 8 Hz
• ²J_{HH, gem} ≈ 5 Hz

Equivalence

• Chemical equivalence
• Magnetic equivalence

Fluxionality

• Fluxionality
• Berry pseudorotation: PF₅ animation in Jmol

Technical details

• Relaxation (NMR)

Beyond the scope of the course

• Larmor precession
• Keeler lectures
• Understanding NMR Spectroscopy by James Keeler