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FT-NMR

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Nuclei with spin state are like little bar magnets and ... Precession of nuclear dipoles. M0; net. magnetic moment. From small excess of. Nuclei in 1/2 state ... – PowerPoint PPT presentation

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Title: FT-NMR


1
FT-NMR
2
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

3
(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

4
Common NMR nuclei
  • Protons, 1H
  • 13C
  • 15N
  • 19F
  • 31P
  • Sensitivity depends on natural isotopic abundance
    and g
  • DE g?B0 , bigger magnet, greater sensitivity

5
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
6
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

7
Vector Illustration of the pulse
1/2
M0
RF pulse
B0 from magnet
RF coil
-1/2
8
Relaxation
  • T1 spin-lattice (relaxing back to precessing
    about the z axis)
  • T2 spin-spin (fanning out)

9
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

10
FID
11
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

12
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

13
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

14
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

15
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

16
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

17
Broadband Decoupling
1H channel
13C channel
18
H3C4-C3HC2H-C1OOH
solvent
C-4
C-2
C-3
C-1
10
180
19
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)
20
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
21
NOE effect
  • W2 (Enhancement) dominates in small molecules
  • Relevant for all decoupling experiments

22
Other more complex 1D Experiments
  • 1H NOE experiment
  • Inversion Recovery Experiment Determination of
    T1
  • J modulated Spin Echo
  • INEPT Experiment
  • DEPT Experiment

23
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

24
1H targeted decoupling (NOE)
n2 channel
1H channel
25
1
3
2
TMS
n2
26
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

27
Selective Heteronuclear Decoupling
  • Saturate at a specific frequency
  • Multiplets collapse reveal connectivity

28
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

29
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

30
Neuraminic acid
31
CH and CH3
Cq and CH2
32
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)

33
DEPT
DEPT(90) CH3
DEPT(45) DEPT(135) CH2
DEPT(45)DEPT(135)- 0.707DEPT(90) CH
13C decoupled spectra
34
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

35
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36
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

37
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38
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

39
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)
40
HETCOR
  • Plots 13C chemical shifts as a function of 1H
    chemical shifts of the connected carbon/protons
    pairs.

41
F2 (13C NMR decoupled Spectrum)
H(3)
H(4)
H(2)
C(2)
C(4)
C(3)
42
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.
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