March 23 2011 NMR Spectroscopy Chemical Shift and Diamagnetic Anisotropy SpinSpin Coupling This For copyright notice see final page of this file Components of Effective Magnetic Field ID: 623028
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Slide1
Chemistry 125: Lecture 60March 23, 2011 NMR SpectroscopyChemical Shift and Diamagnetic Anisotropy, Spin-Spin Coupling
This
For copyright notice see final page of this fileSlide2
Components ofEffective Magnetic Field.Applied Field
Molecular Field:Net electron orbiting - “Chemical Shift”
(Range ~12
ppm for 1H, ~ 200
ppm
for
13
C)
Nearby magnetic nuclei - “Spin-Spin Splitting”
(In solution JHH 0-30 Hz ; JCH 0-250 Hz)
B
effective
B
molecular
(diamagnetic)
B
appliedSlide3
The Chemical Shift:Electron Orbiting andDiamagnetic AnisotropySlide4
Chemical Shift and Shieldinghigh
electrondensityshielded
upfield
high e
-
density
low chemical shift
low frequency
deshielded
downfield
low e
-
density
high chemical shift
high frequency
CH3C C-
H
! ???
TMS
B
effective
B
molecular
(diamagnetic)
B
applied
Note: Electron orbiting to give
B
is driven by
B
; so
B
B.
d
(ppm)
0
1
2
3
4
5
6
7
8
9
10
11
Alkyl
R-
H
H
C C
H
C
H
X
X = O, Hal, N
RC
C
H
O
RC
H
O
RC
O
H
O
R-O
H
(depends on conc, T)
d
+
d
-Slide5
ZERO!
Suppose molecule
in fluid undergoes rotational averaging.
net from average over sphere
net from average around circle
1/r
3
Electrons Orbiting
Other Nuclei
Diamagnetism from Orbiting
Electrons
Ignore electrons on other atoms!
B
applied
PPM
Suppose the
studied nucleus
is fixed relative to the
other
nucleus
by
bond(s)
.Slide6
ZERO!
net from average over sphere
Electrons Orbiting
Other Nuclei
Unless orbiting depends on molecular orientation
B
applied
Diamagnetic
“Anisotropy”
(depends on orientation)
NOT
suppose less
orbiting
for this molecular orientation
reinforces
B
appliedSlide7
B
0
Diamagnetic Anisotropy
Benzene “Ring Current”
B
0
can only drive circulation about a path to which it is perpendicular.
If the ring rotates so
that it is no longer perpendicular to
B
0
,
the ring current stops.
Net deshielding
of aromatic
protons;
shifted downfieldSlide8
0
1
2
3
4
5
6
7
8
Aromaticity: PMR Chemical Shift Criterion
HCCl
3
TMS
-4.23
14
electrons
(4
3) + 2
DIAMAGNETIC ANISOTROPY!
?
DIAMAGNETIC ANISOTROPY
8 H
2 H
TMS
10
electrons
(distorted – less overlap & ring current)
9
-1
-2
-3
-4
d
(ppm)
Boekelheide (1969)Slide9
9
0
1
2
3
4
5
6
7
8
HCCl
3
-1
-2
-3
-4
TMS
Aromaticity: PMR Chemical Shift Criterion
-4.23
14
electrons
(4
3) + 2
DIAMAGNETIC ANISOTROPY!
DIAMAGNETIC ANISOTROPY
4
6
8
10
12
14
16
18
20
22
2
0
-2
-4
Metallic K adds 2
electrons
to give 16
(4n)
-2
CH
3
signals shift downfield by 26 ppm despite addition of “shielding” electrons.
“Anti-Aromatic” Dianion
d
(ppm)
Shrink Scale
Boekelheide (1969)
THF
solventSlide10
Diamagnetic Anisotropy
Acetylene “Ring Current”
H
H
H
H
The H nuclei of benzene lie
beside
the orbital path
when there is ring current. (
B
0
at H reinforced; signal shifts downfield).
The H nuclei of acetylene lie
above
the orbiting path when there is ring current. (
B
0
at H diminshed; signal shifts upfield).
H
H
Warning!
This handy picture of diamagnetic anisotropy due to ring current may well be nonsense!
(Prof. Wiberg showed it
/ /
to be nonsense for
13
C.)Slide11
Spin-Spin SplittingSlide12
d
(ppm)
0
1
2
3
4
5
6
7
8
CH
3
C
OCH
2
CH
3
O
Triplet
(1:2:1)
C
.
H
H
Four (2
2
) sets of molecules that differ in spins of adjacent H nuclei
“Spin Isomers”
so similar in energy that equilibrium keeps them
equally abundant
Chem 220
NMR Problem 1
(of 40)Slide13
CH3COCH
2CH3
O
C
.
H
H
H
d
(ppm)
0
1
2
3
4
5
6
7
8
Quartet
(1:3:3:1)
7.3
7.3
7.3
Triplet
(1:2:1)
Eight (2
3
) sets of molecules that differ in spins of
adjacent
H nuclei
7.3
7.3
Influence of CH
2
on CH
3
must be the
same
as that of CH
3
on CH
2
and
independent of B
o
J in Hz
vs
.
Chemical Shift in
(Orbiting driven by B
o
)
Chem 220
NMR Problem 1
(of 40)
binomial
coefficients
1
1
1
6
4
1
4
1
1
2
2:
1
3
3
1
3:
4:
1:
Slide14
DMSO-d
5
CD
3
SCD
2
H
O
HO-CH
2
-CH
3
7.2 Hz
5.1 Hz
Doublet
of
Quartets
1.8 Hz
7.2
5.1
?
7.2
124 Hz
13
CH
3
1:4:6:4:1
Quintet?
?
Dd
0.018 ppm
× 400 MHz
J
= 7.2 Hz
1.1% of C
D is a weaker magnet than H.
?
H
2
O
1:2:3:2:1
Quintet
Subtle
Asymmetry
d
(ppm)
1.070
1.052
d
D can be oriented 3 ways in B
o
.Slide15
What determinesthe Strength of Spin-Spin Splitting?Slide16
Isotropic JH-H is mediated by bonding electrons
(the anisotropic through-space part is averaged to zero by tumbling)Slide17
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 is
available 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 Hz
J
= 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. Slide18
good p
-p
good s-s
bad
p
-
p
bad
s
-
s
2 bad
s-
p
good 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!Slide19
C Overlap
1.0
0.8
0.6
0.4
0.2
0.0
Overlap Integral
1.2
1.3
1.4
1.5
Å
s-p
s
s-s
C
C
C
C
C
C
p-p
sSlide20
H
2-13 Hz, depends on conformation
(overlap)
13 Hz
2 Hz
H
gauche ~7 Hz
11 Hz
(approximate way to measure a rigid torsional angle!)
10-
20
No “handle” for rf if same chem shift
(see Frame 26 below)
invisibleSlide21
End of Lecture 60March 23, 2011
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