Post on 11-Jan-2016
Chemistry 125: Lecture 59March 22, 2010
NMR Spectroscopy Chemical Shift and Spin-Spin Coupling
This
For copyright notice see final page of this file
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 ppm
Fractional difference in applied field 0.00000248 !
Requires very high uniformity of field
to avoid “MRI”
H
Listen at fixed frequency.Tune H to “hear” precession.
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 Yale “natural products” organic professor, whose research used chemical transformations to determine molecular structures, abandoned organic chemistry to take up fundamental research on quantum theory
(and later became a professional studio photographer).
500 MHz
500 MHz
600 MHz
600 MHz
800 MHz
~83 = 512times assensitive
as 100 MHz(not to mentionthe chemical
shift advantage discussed below)
*
1) Boltzmann factor2) Energy quantum3) Electronics sensitivity
*
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? number of protons, because
they are so similar
(not like IR)
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
http://www.wooster.edu/chemistry/is/brubaker/nmr
Advantage of “similarity” of protons(unlike IR where various modes have very different
changes in dipole moment, and thus very different signal 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
Note: Electron orbiting to give B is driven by B; so B B.
Cf. Table 15.4 p. 720
ZERO!Suppose molecule
undergoes rotational averaging.
average over
sphere
average around circle
1/r3 Electrons Orbiting
Other Nuclei
Diamagnetism from Orbiting
Electrons
Bapplied
PPM
Ignore them!
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.15.30
Aromaticity: PMR Chemical Shift Criterion
HCCl3
TMS
-4.23
14 electrons(43 + 2)
DIAMAGNETIC ANISOTROPY!
?
DIAMAGNETIC ANISOTROPY
8 H 2 H
TMS10 electrons
(distorted)
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.)
The H nuclei of benzene lie outside the orbital path
when there is ring current. (B0 augmented; signal shifts downfield).
The H nuclei of acetylene lie on the orbital axis
when there is ring current. (B0 diminshed;
signal shifts upfield).
15.32
Spin-Spin Splitting
15.36.jpg
Three peaks from four different sets of
molecules in the sample.
~1:2:1 Triplet
15.37.jpg
~1:3:3:1 Quartet
Isotropic JH-H is mediated by
bonding electrons(through-space part is anisotropic,
averaged to zero by tumbling)
End of Lecture 59March 22, 2010
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