chem 125 lecture 63 preliminary 4/1/08 projected material this material is for the exclusive use of...
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Chem 125 Lecture 63Preliminary
4/1/08Projected material
This material is for the exclusive use of Chem 125 students at Yale and may not
be copied or distributed further.
It is not readily understood without reference to notes from the lecture.
A 90° pulse makesspinning nuclei (1H, 13C) “broadcast” a frequency that tells their
local magnetic field.
MRI:locating protons
within body using non-uniform field
MRI
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
How to find Crickets,if you can’t see them:
Establish a temperature gradientand listen with a stopwatch.
X-Ray Tomographywww.colorado.edu/physics/2000/tomography/final_rib_cage.html
MRI: find protons in body
z-gradient
Main Field (z)
x-gradient
y-gradient
radio antenna
(e.g. fluid H2O)
Functional MRI:locating protons
whose signal strength is being fiddled with
BOLD Imaging
http://www.csc.mrc.ac.uk/d/file/LargeFigs/Bell/fMRI.jpg
Cell activity increasesblood oxygen supplyincreases relaxation
Functional MRI (fMRI)e.g. Blood Oxygen-Level
Dependent (BOLD) ImagingSpatial Resolution ~1 mmTemporal Resolution 2 sec
NMR:locating protonswithin molecules
using uniform field?
HO-CH2-CH3http://www.wooster.edu/chemistry/is/brubaker/nmr
Oscilliscope Trace(1951)
The “Chemical” Shift
2.48 ppmFractional
difference in applied field 0.00000248 ! Requires very high
uniformity of field
Dr. Lauterbur became interested in possible biological applications of nuclear magnetic resonance after reading a paper in 1971 by Raymond V. Damadian, who described how some cancerous tissues responded differently to the magnetic fields than normal tissue.
Until then, most scientists placed the samples in a uniform magnetic field, and the radio signals emanated from the entire sample. Dr. Lauterbur realized that if a non-uniform magnetic field were used, then the radio signals would come from just one slice of the sample, allowing a two-dimensional image to be created.
The nuclear magnetic resonance machine at SUNY was shared among the chemistry professors, and the other professors needed to perform their measurements in a uniform magnetic field. Dr. Lauterbur had to conduct his work at night, returning the machine to its original settings each morning.
i.e. one particular frequency
Some of theMagnetic Resonance
Spectrometersin Yale's
Chemistry Departmenthave put classical structure proof
by chemical transformation (and even IR!) out of business.
One “natural products” organic chemist turned to quantum theory,
another to photography.
500 MHz
500 MHz
600 MHz
600 MHz
800 MHz
~83 = 512times assensitive
as 100 MHz
(not to mentionchemical shift
advantage)
EPR (Electron Paramagnetic Resonance)
(for Free Radicals with SOMOs)e magnet is 660x H+!
EPR (Electron Paramagnetic Resonance)9 GHz
~3000 Gauss(0.3 Tesla)
NHFML - Florida State University VarianAssociates
New 900 MHz (21 Tesla) NMR spectrometers
NHFML now has a pulsed field NMR at 45 Tesla(there is no charge for use, but you have to have a great experiment
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?
http://www.wooster.edu/chemistry/is/brubaker/nmr
2.9 1
1955
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
http://www.wooster.edu/chemistry/is/brubaker/nmr
Advantage of “similarity” of protons(unlike IR where various modes have very different strengths)
Higher Resolution Shows Splitting
1959
Ethyl Acetate
Averages field inhomogeneities
1959
A 90° pulse makesspinning nuclei (1H, 13C) “broadcast” a frequency
that tells theirLOCAL magnetic field.
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
Chemical Shift and ShieldingCH3
SiCH3H3C
H3C
highelectrondensity
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
N.B. The orbiting to give B is driven by B; so B B.
ZERO!
average over
sphere
average around circle
1/r3 Electrons Orbiting
Other Nuclei
Diamagnetism from Orbiting
Electrons
Bapplied
PPM
ZERO!
average over
sphere
Electrons Orbiting
Other Nuclei
Unless orbiting depends on molecular orientation
Bapplied
Diamagnetic“Anisotropy”
(depends on direction)
NOT
Diamagnetic AnisotropyBenzene “Ring Current”
B0 can only drive circulation about a path to which it is perpendicular.
If the ring rotates so that it is no longer perpendicular to B0,
the ring current stops.
Diamagnetic AnisotropyAcetylene “Ring Current”
Warning!This handy picture of
diamagnetic anisotropy due to ring current
may well be nonsense!
(Prof. Wiberg showed it to be nonsense for 13C.)
In acetylene C-H is parallel to B0 when there is ring current (B0 diminshed;
shifted upfield).
In benzene they are perpendicular
(B0 augmented; shifted downfield).
End of Lecture 63April 1, 2009