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Chemistry 125: Lecture 60March 23, 2011
NMR SpectroscopyChemical Shift and
Diamagnetic Anisotropy, Spin-Spin Coupling This
For copyright notice see final page of this
file
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Components ofEffective Magnetic Field.
Applied Field
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:Electron Orbiting and
Diamagnetic Anisotropy
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Chemical Shift and Shieldinghighelectrondensity
shielded
upfield
high e- density
low chemical shift
low frequency
deshielded
downfield
low e- density
high 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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ZERO!Suppose molecule
in fluid undergoes rotational averaging.
net from average
over sphere
net from average around circle
1/r3 Electrons Orbiting Other Nuclei
Diamagnetism from Orbiting
Electrons
Ignore electrons on other atoms!
Bapplied
PPM
Suppose thestudied nucleus is fixed
relative to the other nucleus
by bond(s).
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ZERO!
net from average
over sphere
Electrons Orbiting Other Nuclei
Unless orbiting depends on molecular orientation
Bapplied
Diamagnetic“Anisotropy”
(depends on orientation)
NOT
suppose less orbiting for this molecular
orientation
reinforces Bapplied
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B0
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.
Net deshieldingof aromatic
protons;shifted downfield
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012345678
Aromaticity: PMR Chemical Shift Criterion
HCCl3
TMS
-4.23
14 electrons(43) + 2
DIAMAGNETIC ANISOTROPY!
?
DIAMAGNETIC ANISOTROPY
8 H 2 H
TMS10 electrons
(distorted – less overlap & ring current)
9 -1 -2 -3 -4
d (ppm)Boekelheide (1969)
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9 012345678
HCCl3
-1 -2 -3 -4
TMS
Aromaticity: PMR Chemical Shift Criterion
-4.23
14 electrons(43) + 2
DIAMAGNETIC ANISOTROPY!
DIAMAGNETIC ANISOTROPY
468101214161820 22 2 0 -2 -4
Metallic K adds 2 electronsto give 16
(4n)
-2CH3 signals shift downfield by 26 ppm despite addition
of “shielding” electrons.
“Anti-Aromatic” Dianion
d (ppm)
Shrink Scale
Boekelheide (1969)
THF solvent
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Diamagnetic AnisotropyAcetylene “Ring Current”
H
H
HH
The H nuclei of benzene lie beside the orbital path when there is ring current. (B0 at
H reinforced; signal shifts downfield).
The H nuclei of acetylene lie above the orbiting path when there is ring current.
(B0 at H diminshed; signal shifts upfield).
HH
Warning!This handy picture of diamagnetic
anisotropy due to ring current may well be nonsense!
(Prof. Wiberg showed it / /to be nonsense for 13C.)
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Spin-Spin Splitting
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d (ppm) 012345678
CH3COCH2CH3
O
Triplet(1:2:1)
C. H
H Four (22) sets of molecules that
differ in spins of adjacent H nuclei“Spin Isomers”
so similar in energy that equilibrium
keeps them equally abundant
Chem 220NMR Problem 1
(of 40)
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CH3COCH2CH3
O
C. H
H H
d (ppm) 012345678
Quartet(1:3:3:1)
7.37.37.3
Triplet(1:2:1)
Eight (23) sets of molecules that
differ in spins of adjacent H nuclei 7.3 7.3
Influence of CH2 on CH3 must be the same as that of CH3 on CH2
and independent of Bo
J in Hz
vs. Chemical Shift in (Orbiting driven by Bo)
Chem 220NMR Problem 1
(of 40)
binomialcoefficients
1 1
1 6 4 1 4
1 1 2 2: 1 3 3 1 3:
4:
1:
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DMSO-d5
CD3SCD2H
O
HO-CH2-CH3
7.2 Hz5.1 Hz
Doublet of Quartets
1.8 Hz
7.2
5.1
?
7.2
124 Hz
13CH31:4:6:4:1Quintet?
?
Dd 0.018 ppm× 400 MHzJ = 7.2 Hz
1.1% of C
D is a weaker magnet than H.
?H2O1:2:3:2:1Quintet
SubtleAsymmetry
d (ppm)
1.070
1.052
d
D can be oriented 3 ways in Bo.
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What determinesthe Strength of
Spin-Spin Splitting?
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Isotropic JH-H is mediated by
bonding electrons(the anisotropic through-space part
is averaged to zero by tumbling)
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Not spatial proximity!
Might overlap be greater for anti C-H bonds ??
HOMO-3
When the “up” electron of this MO is on Nucleus A
only its “down” electron isavailable to be on Nucleus B
In tumbling molecules, nuclear spins communicate not through space, but
through paired electrons on the nuclei.
Through-space interaction of dipoles averages to zero on tumbling.
J = 0-3 Hz J = 12-18 HzJ = 6-12 Hz
J = 6-8 Hz J = 1-3 Hz J = 0-1 Hz
3.07 Å
1.85 Å
2.38 Å
J depends on the s-orbital content of molecular orbitals.
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good p-p
good s-s
bad p-p
bad s-s
2 bad s-pgood s-p; good p-s + +
+ +
Examine the overlap of the components.
Which gives better overlap?
s-p > s-s or p-p(See Lecture 12)
Backside overlap is counterintuitive.
Better Overlap!
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C Overlap
1.0
0.8
0.6
0.4
0.2
0.0
Ove
rlap
Inte
gra
l
1.2 1.3 1.4 1.5 Å
s-pss-s
C C C C C C
p-ps
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H
2-13 Hz, depends on conformation (overlap)
13 Hz2 Hz Hgauche ~7 Hz
11 Hz
(approximate way to measure a rigid torsional angle!)
10-20No “handle” for rf if same chem shift
(see Frame 26 below)invisible
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End of Lecture 60March 23, 2011
Copyright © J. M. McBride 2010. 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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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