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User:Benjah-bmm27/degree/2/CJA

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NMR, CJA

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"Introduction to NMR Spectroscopy"

Overview

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Fundamentals

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Chemical shift

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NMR active nuclei

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Coupling

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Typical coupling constants

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One bond coupling

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

Two bond (geminal) coupling

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

Three bond (vicinal) coupling

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

Benzene rings

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  • 3JHH, ortho ≈ 8 Hz
  • 4JHH, meta ≈ 2 Hz
  • 5JHH, para ≈ 0 Hz

Alkenes

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  • 3JHH, trans ≈ 16 Hz
  • 3JHH, cis ≈ 8 Hz
  • 2JHH, gem ≈ 5 Hz

Equivalence

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Fluxionality

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Technical details

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Beyond the scope of the course

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