chemistry 125: lecture 60 march 23, 2011 nmr spectroscopy chemical shift and diamagnetic anisotropy,...
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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
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
The Chemical Shift:Electron Orbiting and
Diamagnetic Anisotropy
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-
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).
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
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
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)
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
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.)
Spin-Spin Splitting
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)
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:
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.
What determinesthe Strength of
Spin-Spin Splitting?
Isotropic JH-H is mediated by
bonding electrons(the anisotropic through-space part
is averaged to zero by tumbling)
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.
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!
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
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
End of Lecture 60March 23, 2011
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