nuclear magnetic resonance (nmr) for beginners. overview nmr is a sensitive, non-destructive method...
TRANSCRIPT
![Page 1: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/1.jpg)
Nuclear Magnetic Nuclear Magnetic Resonance (NMR)Resonance (NMR)Nuclear Magnetic Nuclear Magnetic Resonance (NMR)Resonance (NMR)
for beginnersfor beginners
![Page 2: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/2.jpg)
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
![Page 3: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/3.jpg)
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
![Page 4: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/4.jpg)
Spin States• In the absence of a magnetic field
the spin states are degenerate• The “spinning” nucleus generates
its own magnetic field
![Page 5: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/5.jpg)
Spin States• In a magnetic field the states have
different energies
Bo
B’B’
![Page 6: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/6.jpg)
Spin states in a magnetic field
• Energy difference linearly depends on field strength
= magnetic moment of H (2.7927N or +14.106067x10-27J/T)
![Page 7: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/7.jpg)
Spin states in a magnetic field
• Even in a very large field (1-20T) the energy difference is small (~0.1cal/mol)
![Page 8: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/8.jpg)
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)
![Page 9: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/9.jpg)
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
![Page 10: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/10.jpg)
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
![Page 11: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/11.jpg)
Making NMR work
![Page 12: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/12.jpg)
Sample must be spun to average out magnetic field inhomogeneity
![Page 13: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/13.jpg)
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
![Page 14: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/14.jpg)
NMR data collection
CW NMR of isopropanol
![Page 15: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/15.jpg)
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
![Page 16: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/16.jpg)
Simple FT FID and spectrum
![Page 17: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/17.jpg)
More complex FT FID and spectrum
![Page 18: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/18.jpg)
Even more complex FT FID
![Page 19: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/19.jpg)
FT NMR Spectrum
![Page 20: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/20.jpg)
Pulsed FT NMR of isopropanol
![Page 21: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/21.jpg)
Chemical shift• Protons in different environments
absorb at different field strengths (for the same frequency)
• Different environment = different electron density around the H
![Page 22: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/22.jpg)
Chemical shift positions
High field, shielded
Low field, deshielded
Reference (tetramethylsilane)
PPM of applied field () from reference
![Page 23: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/23.jpg)
Chemical shift positions
![Page 24: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/24.jpg)
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
![Page 25: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/25.jpg)
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
![Page 26: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/26.jpg)
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
![Page 27: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/27.jpg)
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
![Page 28: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/28.jpg)
Anisotropy• “Anisotropy”: any characteristic
that varies with direction (asymmetric)
• Applied to the shielding/deshielding characteristics of electrons in some systems
![Page 29: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/29.jpg)
Anisotropy• Aromatic hydrogens are in the
deshielding region of the magnetic field generated by circulating electrons
![Page 30: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/30.jpg)
Typical chemical shifts
![Page 31: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/31.jpg)
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
![Page 32: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/32.jpg)
NMR spectrum of dichloroacetaldehyde
![Page 33: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/33.jpg)
Coupling constants• Separation between peaks is the
“coupling constant” • Symbol: J• Measured in Hz• It is the same for both coupled
protons
![Page 34: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/34.jpg)
Spin-spin coupling• Case 2: Single proton next to a pair• Single proton splits the pair into a
doublet• Spin state possibilities for pair:
Bo
Equal energy
Integration ratio: 1:2:1
![Page 35: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/35.jpg)
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)
![Page 36: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/36.jpg)
Spin coupling example• Chloroethane CH3CH2Cl
![Page 37: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/37.jpg)
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
![Page 38: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/38.jpg)
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
![Page 39: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/39.jpg)
Phenolic Protons and Concentration
![Page 40: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/40.jpg)
Alcoholic protons and coupling
1H NMR spectrum of methanol at various temperatures
![Page 41: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/41.jpg)
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
![Page 42: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/42.jpg)
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
![Page 43: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/43.jpg)
Chemical Shift Differences and
Coupling
![Page 44: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/44.jpg)
AX system with some distortion
![Page 45: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/45.jpg)
Ternary systems• AaMmXx systems exhibit simple
splitting with two coupling constants
![Page 46: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/46.jpg)
Ternary Systems
![Page 47: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/47.jpg)
Ternary systems
![Page 48: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/48.jpg)
Chemical and magnetic equivalence
![Page 49: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/49.jpg)
Chemical and magnetic equivalence
![Page 50: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/50.jpg)
Chemical and magnetic equivalence
NMR spectrum of butane
![Page 51: Nuclear Magnetic Resonance (NMR) for beginners. Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules](https://reader035.vdocuments.net/reader035/viewer/2022062421/56649cce5503460f94999fe4/html5/thumbnails/51.jpg)
Chemical Shift Differences and
Coupling• AaBbXx systems are approximately
first order (simple splitting)
• AaBbCc systems are complex