Chapter 13 Nuclear Magnetic Resonance Spectroscopy

Chapter 13Nuclear Magnetic Resonance Spectroscopy

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Infrared Spectroscopy

Mass Spectrometry

Nuclear Magnetic Resonance Spectroscopy

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NMR Spectroscopy

• Maps the carbon-hydrogen framework• Probes molecular structure in greater detail

than IR or MS• A complete molecular structure can

oftentimes be determined by NMR alone• NMR has revolutionized organic chemistry

since its introduction in the 1950’s• Before the advent of NMR, structure

elucidation may have taken months (or years!)

413.1 An Overview of Proton NMR Spectroscopy

NMR Machine

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Sample + CDCl3 & (CH3)4Si

The NMR Spectrometer

713.11 The NMR Spectrometer

NMR Spectroscopy

• Nuclei with an odd # of protons and/or an odd # of neutrons exhibit magnetic properties– called “spin”

• Nuclei with even numbers of BOTH P’s and N’s = no magnetic phenomena

• 1H and 13C are the most important of the spin active nuclei

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• Nuclear spin causes the nuclei to behave like a tiny magnets

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Nuclear Spin• 1H (and 13C) nuclei can have one of two

spin states –quantum numbers +½ or -½

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Protons in a Magnetic Field

1113.2 Physical Basis of NMR Spectroscopy

Protons in a Magnetic Field

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• This absorption is nuclear magnetic resonance and is detected by an NMR spectrometer

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The 1H NMR Spectrum• Absorptions detect the protons in the molecule• Separate resonance for each chemically non-equivalent set of nuclei• The size (area) of a peak is proportional to the number of

contributing protons

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Chemical Shift

• The local magnetic field (Bp) “sensed” by a proton is different than the applied magnetic field (B0)

– This is due to the electrons around the proton which oppose the external field

– Bp = B0 – Be

• The reduction of the local field is called shielding

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Chemical Shift• Hence, the electronegativity of nearby atoms will affect the

shielding around a proton – Electronegative neighbors = deshielding• H in question “feels” external magnetic field more

– Electropositive neighbors = increase in shielding• H in question “feels” external magnetic field less

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Chemical Shift

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Chemical Shift and Structure

• Electronegativities of nearby groups is one of the most important factors in chemical shift

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Chemical Shift

• The amount of alkyl substitution will also affect chemical shift

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Chemical Shift

• A proton near more than one functional group will be affected by both

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