Chemistry 125: Lecture 59 March 21, 2011 Precession and MRI NMR Spectroscopy Chemical Shift This For...
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Chemistry 125: Lecture 59March 21, 2011
Precession and MRINMR Spectroscopy
Chemical Shift This
For copyright notice see final page of this file
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90° RF Pulse and the “Rotating Frame”
Applied Magnetic Field
Precessing proton gives riseto constant vertical field
Will rotating horizontal field generate 100 MHz RF signal?No, because there are many
precessing protons with all possible phases.
Consider a “rotating frame” in which the observer orbits at 100 MHz - protons seem to
stand still as if no applied field.
(just long enough to rotate all nuclear spin axes by 90°).
Fast precession(~100 MHz)
Slow precession
(~0.1 MHz)
Horizontal fields cancel.
Subsequent precession generates100 MHz RF signal in lab frame.
100 MHz RFin lab frame
Until “relaxation”reestablishes equilibrium.
and rotating horizontal field.
Pulse a very weak magnetic field fixed in this rotating frame
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A 90° pulse makesspinning nuclei (1H, 13C) “broadcast” a frequency
that reports theirlocal magnetic field.
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MRI:locating protons
within body using non-uniform field
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X-Ray Tomographywww.colorado.edu/physics/2000/tomography/final_rib_cage.html
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MRI: find protons in body(e.g. fluid H2O) Bz
wrap in several miles of special wire at 4K
~1.5 Tesla(15,000 Gauss)
Superconducting Solenoidprotons precess at 63 MHz
So there are protons in the body, but where?
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How to locate Crickets,if you can’t see them:
Establish a temperature gradientand listen with a stopwatch.
P. LeMone(2007)
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subtract from Bz on left
MRI: find protons in body(e.g. fluid H2O)
add to Bz near head
const Hz
63.10 MHz
63.05 MHzconst B
z
subtract from Bz
near feet
dB z/dz
Bz
add to Bz on right
dBz/dx
~1.5 Tesla(15,000 Gauss)
protons precess at 63 MHz40 mT/mm
(~30 ppm/mm)
63.00 MHz
Superconducting Solenoid
Four analogous top/bottom coils
establish
These three gradients allow slicing in all directions to construct a 3Dtomograph.
dBz/dySo there are protons in the body,
but where?
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Functional MRI:locating protons
whose signal strength is being fiddled with
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BOLD Imaging
SubjectFasting
Functional MRI (fMRI)e.g. Blood Oxygen-Level
Dependent (BOLD) ImagingSpatial Resolution ~1 mmTemporal Resolution 2 sec
Subjectrecently
fed
DifferenceMap
wit
h pe
rmis
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of
Dr.
Tony
Gol
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mpe
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minus
Cell activity increasesblood oxygen supply,
speeds relaxation.
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NMR:locating protonswithin molecules
using uniform field?
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HO-CH2-CH3http://www3.wooster.edu/chemistry/is/brubaker/nmr/nmr_spectrum.html
Oscilliscope Trace(1951)
The “Chemical” Shift
2.48 ppm
Fractional difference in applied field 0.00000248 !
Requires very high uniformity of field
to avoid “MRI”
Bo
Listen at fixed frequency.Tune Bo to “hear” precession.
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In the late 1950s chemistry departments began buying NMR spectrometers with fields homogeneous enough to determine molecular structures from chemical shifts (and spin-spin
splittings). With multi-user equipment, it was a challenge to keep the fields sufficiently homogeneous to obtain sharp lines.
At SUNY-Stony Brook in 1972 physical chemist Paul Lauterbur would take over the departmental machine nightly and destroy the field inhomogeneity.
By establishing gradients in different directions he located two 1 mm tubes of H2O within a 5 mm tube of D2O, and published this “zeugmatogram” in Nature in 1973. 30 years later he shared the Nobel Prize in Physiology or Medicine for inventing MRI.
Do Not Touch These Gradient Knobs!!! or this one!
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have put classical structure proof by chemical transformation
(and even IR!) out of business.One Yale “natural products” organic professor, whose research used chemical transformations to puzzle out molecular structures, abandoned organic chemistry to take up fundamental research on quantum theory
(and later became a professional studio photographer).
Magnetic Resonance Spectrometers
(and X-ray Diffractometers)
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Some of theMagnetic Resonance
Spectrometersin Yale's
Chemistry Department
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500 MHz
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500 MHz
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600 MHz
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600 MHz
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800 MHz
~83 = 512times assensitive
as 100 MHz(not to mentionthe chemical
shift advantage discussed below)
*
1) Boltzmann factor2) Energy quantum3) Electronics sensitivity
*
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EPR (Electron Paramagnetic Resonance)
(for Free Radicals with SOMOs)e magnet is 660x H+!
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EPR (Electron Paramagnetic Resonance)9 GHz
~3000 Gauss(0.3 Tesla)
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New 1000 MHz (23.5 Tesla) NMR Spectrometer
NHFML - Florida State Universitynow has a pulsed field NMR at 45 Tesla
(there is no charge for use, but you have to have a great experiment
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HO-CH2-CH3http://www.wooster.edu/chemistry/is/brubaker/nmr
Oscilliscope Trace(1951)
1 2 3
Area(integral)
Which peak is which set of
protons? number of protons, because
they are so similar
(not like IR)
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http://www.wooster.edu/chemistry/is/brubaker/nmr
2.9 1
1955Advertisement
1) O3 2) H2O2
C-OHHO-COO
cis-caronic acid
1:1
Structural proof by chemical degradation
(venerable)
3:1
?
?
OO O
O
O
O O
O
H CC
H
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d (ppm) 012345678
2 3 3
in CDCl3 solventat 5.9T (250 MHz)
CH3COCH2CH3
O
0.029 ppm× 250 MHz
7.3 Hz
CHCl3
“Low” Resolution(~0.3 ppm)
High Resolution(~3 ppb, sample spun)
?
Triplet(1:2:1)
Quartet(1:3:3:1)
Peak Width~3 ppb
7.3
7.37.37.3
Chem 220 NMR problem 7
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A 90° pulse makesspinning nuclei (1H, 13C) “broadcast” a frequency
that tells theirLOCAL magnetic field.
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Components ofEffective Magnetic Field.
Inhomogeneous ~ 30,000 G for MRI CAT scan. (4 G/cm for humans, 50 G/cm for small animals)
Applied Field:
Homogeneous for Chemical NMR Spectroscopy (spin sample)
Molecular Field:Net electron orbiting - “Chemical Shift” (Range ~12 ppm for 1H, ~ 200 ppm for 13C)
Nearby magnetic nuclei - “Spin-Spin Splitting” (In solution JHH 0-30 Hz ; JCH 0-250 Hz)
Beffective
Bmolecular (diamagnetic)
Bapplied
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The Chemical Shift
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Chemical Shift and Shieldinghighelectrondensity
shielded
upfield
high e- densitylow chemical shift
low frequency
deshielded
downfield
low e- densityhigh chemical shift
high frequency
CH3C C-H ? ! ???
TMS
Beffective
Bmolecular (diamagnetic)
Bapplied
Note: Electron orbiting to give B is driven by B; so B B.
d (ppm) 01234567891011
AlkylR-H H
C CH CH X
X = O, Hal, NRC
CH
O
RC H
ORC
OH
O
R-OH(depends on conc, T)
d+
d-
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Diamagnetism from Orbiting
Electrons
Bapplied
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End of Lecture 59March 21, 2011
Copyright © J. M. McBride 2011. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).
Use of this content constitutes your acceptance of the noted license and the terms and conditions of use.
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Third party materials may be subject to additional intellectual property notices, information, or restrictions.
The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0