FT-NMR

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FT-NMR
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Fundamentals

• Nuclear spin
• Spin quantum number ½
• Nuclei with spin state ½ are like little bar
  magnets and align with a B field.
• Can align with () or against (-) B
• Small energy gap between and spin alignment
  (NMR insensitive/Boltzman dist)
• Can probe difference with RW

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(NMR insensitive/Boltzman dist)

• Small population difference between 1/2 and -1/2
  state
• It is the small excess of nuclei in the -1/2 that
  produce NMR signal

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Common NMR nuclei

• Protons, 1H
• 13C
• 15N
• 19F
• 31P
• Sensitivity depends on natural isotopic abundance
  and g
• DE g?B0 , bigger magnet, greater sensitivity

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Precession of nuclear dipoles
z
1/2
M0 net magnetic moment From small excess
of Nuclei in 1/2 state
y
M0
B0 from magnet
x
-1/2
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FT pulse

• Radiofrequency generator
• A short, intense pulse generates a magnetic field
  in the x-y plane (excites all nuclei)
• M0 of the nuclei interacts with the magnetic
  field produced by the pulse.
• Tips M0 off axis
• T gB1tp
• tp length of pulse, 90? pulse

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Vector Illustration of the pulse
1/2
M0
RF pulse
B0 from magnet
RF coil
-1/2
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Relaxation

• T1 spin-lattice (relaxing back to precessing
  about the z axis)
• T2 spin-spin (fanning out)

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Induced current in coil

• After pulse, nuclei begin to precess in phase in
  the x-y plane
• Packet of nuclei induce current in RF coil
• Relaxation is measured by monitoring the induced
  coil
• ? FID (? FT) NMR spectrum

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FID
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Noise reduction and increasing resolution

• Apodization Multiply the free-induction decay
  (FID) by a decreasing exponential function which
  mathematically suppresses the noise at long
  times. Other forms of apodization functions can
  be used to improve resolution or lineshape.
• Zero filling

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Chemical Shift

• Shielding
• Electrons have spin, produce local B environments
• Protons in different electronic environments
  experience different B, different precessing
  frequencies, DE hu
• Chemical shift proportional to size of magnet
• ppm (s-s0)/s0106

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Spin-Spin Coupling

• Adjacent nuclei have a 50/50 chance of being spin
  up (1/2) or spin down (-1/2)
• Each produce a small magnetic field that is
  either with or against B0
• 1 adjacent proton CHOCH3
• CH3 is a doublet at frequencies
• u0-ua, u0ua (equal intensity), 11
• CH is a quadruplet
• 1331

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Splitting Patterns

• J values
• Quadruplet ? ?????? ? ?? ? ? ? ? ? ? ?
  ? ? ?? ? ? ? ? ? ? ? ? ? ?
• Triplet ? ?? ? ? ? ? ? ?
• Multiplets
• 1 3 3 1
• 1 2 1
• ¼ ½ ¼ ¾ 1 ½ ¾ ¾ 1 ½ ¾ ¼ ½
  ¼
• 1 2 1 3 6 3 3 6 3 1 2 1

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13C NMR

• 13C frequency
• Different tuning folk
• Broadband Decoupling of 1H
• No spin-spin coupling
• NOE effect
• Assignments based on chemical shift
• Wider frequency range

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Obtaining a 13C NMR Spectrum

• 1H Broadband decoupling
• Gives singlet 13C peaks, provided no F, P, or D
  present in the molecule)
• Continuous sequence of pulses at the 1H frequency
  causes a rapid reversal of spin orientation
  relative to the B0, causing coupling to 13C to
  disappear

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Broadband Decoupling
1H channel
13C channel
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H3C4-C3HC2H-C1OOH
solvent
C-4
C-2
C-3
C-1
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180
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13C Chemical Shifts

• Reference is TMS, sets 0 ppm
• A range of 200 ppm
• Chemical shifts can be predicted
• Empirical correlations
• Ex. Alkanes

di -2.3 9.1na 9.4nb 2.5ng 0.3nd 0.1ne
Sij
2-methylbutane
di -2.3 9.11 9.42 2.51 - 1.1 22.0
(22.3)
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Signal averaging

• 13C experiment generally take longer than 1H
  experiments because many more FIDs need to be
  acquired and averaged to obtain adequate
  sensitivity.
• NOE effect (enhancement/reduction in signal as a
  result of decoupling)

N4
N4
13C
W2
1H
N2
N2
N3
N3
W1
1H
13C
N1
N1
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NOE effect

• W2 (Enhancement) dominates in small molecules
• Relevant for all decoupling experiments

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Other more complex 1D Experiments

• 1H NOE experiment
• Inversion Recovery Experiment Determination of
  T1
• J modulated Spin Echo
• INEPT Experiment
• DEPT Experiment

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Targeted 1H Spin Decoupling

• Continuous irradiation at a frequency (n2) that
  corresponds to a specific proton in the molecule
  during the 1H NMR experiment
• All coupling associated with the protons
  corresponding to n2 disappears from the spectrum

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1H targeted decoupling (NOE)
n2 channel
1H channel
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1
3
2
TMS
n2
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NOE- nuclear Overhauser effect

• Saturation of one spin system changes the
  equilibrium populations of another spin system
• NOE effect can be positive or negative. In small
  molecules it is usually positive

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Selective Heteronuclear Decoupling

• Saturate at a specific frequency
• Multiplets collapse reveal connectivity

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More Complex NMR Pulse Sequences

• J-Modulated Spin Echo experiment
• Cq and CH2 down and CH3 and CH up
• DEPT experiment
• Q 45?, 90?, 135?
• CH3 DEPT(90), CH2 DEPT(45)-DEPT(135), CH
  DEPT(45)DEPT(135)-0.707DEPT(90)
• 2D-NMR
• Het. 2D J resolved/Homo 2D J resolved
• 1H-1H COSY
• 1H/13C HETCOR

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J-Modulated Spin Echo

• 13C channel 90?x t 180?x t(echo)
• 1H channel _____________BBBBBBB
• t 1/J(C-H)
• CH CH3 (up)
• C(q) CH2 down

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Neuraminic acid
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CH and CH3
Cq and CH2
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DEPT

• 13C ch 90?xt 180?xtFID
• 1H ch 90?xt180?xt fy t-BBBBBB
• t 1/2J(C-H)
• fy 90?, 45?, 135?
• CH DEPT(90)
• CH2 DEPT(45)- DEPT(135)
• CH3 DEPT(45) DEPT(135) - 0.707DEPT(90)

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DEPT
DEPT(90) CH3
DEPT(45) DEPT(135) CH2
DEPT(45)DEPT(135)- 0.707DEPT(90) CH
13C decoupled spectra
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HET 2D J Resolved

• 13C ch 90?x t 180?y t - FID
• 1H ch BBBBBB__________BBBBBBB
• t 1/J(C-H)
• Gives J(H-C) values as a function of 13C chemical
  shift

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Homo 2D J Resolved

• 1H ch 90?x t 180?x t - FID
• t 1/J(H-H)
• Gives J(H-H) values as a function of 1H chemical
  shift

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H-H COSY

• 1H ch 90?x t1 f?- FID (t2)
• t 1/J(H-H)
• Coupled protons give cross correlation peaks off
  the diagonal

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HOOC(1)-C(2)H(NH2)-C(3)H2-C(4)H2-C(5)OOH
CH2(3)
CH2(4)
CH (2)
CH2(3)
CH2(4)
CH (2)
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HETCOR

• Plots 13C chemical shifts as a function of 1H
  chemical shifts of the connected carbon/protons
  pairs.

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F2 (13C NMR decoupled Spectrum)
H(3)
H(4)
H(2)
C(2)
C(4)
C(3)
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Practical Aspects to Running a sample

• Deuterated solvent
• Air drop, sample height
• Lock-in the deuterated peak (B drift)
• Shimming the magnet parallel magnetic field
  lines for limiting broadening of line width.
• Setting the parameter nuclei, spectral range,
  FID time, number of scans, and apodization, ect.