Nuclear Magnetic Resonance (NMR)
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Transcript of Nuclear Magnetic Resonance (NMR)
Nuclear Magnetic Nuclear Magnetic Resonance (NMR)Resonance (NMR)Nuclear Magnetic Nuclear Magnetic Resonance (NMR)Resonance (NMR)
for beginnersfor beginners
Overview• NMR is a sensitive, non-destructive
method for elucidating the structure of organic molecules
• Information can be gained from the hydrogens (proton NMR, the most common), carbons (13C NMR) or (rarely) other elements
Spin States• All nuclei have a spin state (I )• Hydrogen nuclei have a spin of I =
±½ (like electrons)• Spin number gives number of
ways a particle can be oriented in a magnetic field: 2I + 1
Spin States• In the absence of a magnetic field
the spin states are degenerate• The “spinning” nucleus generates
its own magnetic field
Spin States• In a magnetic field the states have
different energies
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B’B’
Spin states in a magnetic field
• Energy difference linearly depends on field strength
= magnetic moment of H (2.7927N or +14.106067x10-27J/T)
Spin states in a magnetic field
• Even in a very large field (1-20T) the energy difference is small (~0.1cal/mol)
Spin states in a magnetic field
• A small excess of protons will be in the lower energy state
• These can be promoted to the higher state by zapping them with EM radiation of the proper wavelength
• Wavelength falls in the radio/TV band (frequency of 60-500MHz)
Spin states in a magnetic field
• Stronger magnetic field necessitates shorter wavelength (higher frequency)
• After low energy protons are promoted to the higher energy state they relax back to the lower state
Making NMR work• Not all protons absorb at the same
field values• Either magnetic field strength or
radio frequency must be varied • Frequency/field strength at which
the proton absorbs tells something about the proton’s surroundings
Making NMR work
Sample must be spun to average out magnetic field inhomogeneity
NMR data collection• Continuous wave data collection
(CW): – Magnetic field value is varied– Intensity of emitted RF compared to
RF at detector– Absorption is plotted on graph
NMR data collection
CW NMR of isopropanol
NMR data collection• Pulsed Fourier transform data
collection:– Short bursts of RF energy are shot at
sample– Produces a decay pattern– FT done by computer produces
spectrum
Simple FT FID and spectrum
More complex FT FID and spectrum
Even more complex FT FID
FT NMR Spectrum
Pulsed FT NMR of isopropanol
Chemical shift• Protons in different environments
absorb at different field strengths (for the same frequency)
• Different environment = different electron density around the H
Chemical shift positions
High field, shielded
Low field, deshielded
Reference (tetramethylsilane)
PPM of applied field () from reference
Chemical shift positions
NMR reference• Tetramethylsilane ((CH3)4Si)
• Advantages:– Makes one peak– 12 equivalent H, so little is needed– Volatile, inert, soluble in organic solvents– Absorbs upfield of hydrogens in most
organic compounds
Shielding/deshielding• Electron density affects chemical
shift • Electrons generate a magnetic
field opposed to the applied field• H in high electron density absorbs
upfield (toward TMS, 0ppm) from H in low electron density
Shielding/deshielding• Effect of electronegativity:
electronegative atom nearby removes electron density and causes deshielding
• TMS protons are extremely shielded because Si is electropositive compared to C
Shielding/deshielding• Few protons absorb upfield of TMS• Alkyl groups are electron donating,
so alkanes absorb around 0-2ppm ()
• Hydrogens near electronegative atoms are deshielded
• Absorption is around 3-4
Anisotropy• “Anisotropy”: any characteristic
that varies with direction (asymmetric)
• Applied to the shielding/deshielding characteristics of electrons in some systems
Anisotropy• Aromatic hydrogens are in the
deshielding region of the magnetic field generated by circulating electrons
Typical chemical shifts
Spin-spin coupling• Magnetic field felt by a proton is
affected by the spin states of nearby protons – either shielding or deshielding
• Case 1: neighboring single protons• These H can either be the same or
opposite spins – equal probability• Makes doublets of two equal peaks at
both absorptions
NMR spectrum of dichloroacetaldehyde
Coupling constants• Separation between peaks is the
“coupling constant” • Symbol: J• Measured in Hz• It is the same for both coupled
protons
Spin-spin coupling• Case 2: Single proton next to a pair• Single proton splits the pair into a
doublet• Spin state possibilities for pair:
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Equal energy
Integration ratio: 1:2:1
Spin-spin coupling• Single proton is split into a triplet• Any group of n protons will split its
neighbors into n + 1 peaks• Intensity follows Pascal’s triangle
(Fibonacci series)
Spin coupling example• Chloroethane CH3CH2Cl
Protons on Heteroatoms
• Protons on N or O often give broad uncoupled peaks of uncertain chemical shift
• Protons on nitrogen are broad due to coupling with nitrogen nucleus (spin # = 1)
• Chemical shift can depend on concentration
• Peaks will be sharp and coupled if there is no acid or water present
Protons on heteroatoms
Proton on nitrogen: broad due to interaction with nitrogen (spin number = 1)
Split into doublet by NH – reciprocal splitting is not seen
Phenolic Protons and Concentration
Alcoholic protons and coupling
1H NMR spectrum of methanol at various temperatures
Chemical Shift Differences and
Coupling• Equivalent protons do not split each
other• Adjacent protons (“vicinal”) exhibit
simple coupling if their chemical shifts are very different (/J >10)
• Designated an “AaXx” system (“AaMmXx” for three widely separated sets)
• Subscripts designate the number of protons involved
Chemical Shift Differences and
Coupling
• Sets of protons close to each other are “AaBb” or “AaBbCc”
• The closer two sets are the more the peaks are distorted AX system becoming
an AB system
Chemical Shift Differences and
Coupling
AX system with some distortion
Ternary systems• AaMmXx systems exhibit simple
splitting with two coupling constants
Ternary Systems
Ternary systems
Chemical and magnetic equivalence
Chemical and magnetic equivalence
Chemical and magnetic equivalence
NMR spectrum of butane
Chemical Shift Differences and
Coupling• AaBbXx systems are approximately
first order (simple splitting)
• AaBbCc systems are complex