1. Diamagnetic Anisotropy, Spin-Spin Coupling
Chemistry 125: Lecture 60 March 23, 2011
NMR Spectroscopy
Chemical Shift and
Diamagnetic Anisotropy, Spin-Spin Coupling
This
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2. Components of Effective Magnetic Field.
Bapplied
Bmolecular (diamagnetic)
Beffective
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 Hz)

3. Diamagnetic Anisotropy
The Chemical Shift:
Electron Orbiting and
Diamagnetic Anisotropy

4. Chemical Shift and Shielding
high
electron
density
Chemical Shift and Shielding
Note: Electron orbiting to give B is driven by B; so B  B. d+ d-
Beffective
Bmolecular (diamagnetic)
Bapplied
TMS
R-OH
(depends on conc, T) CH X
X = O, Hal, N RC CH O RC OH O RC H O H
C C
Alkyl
R-H H 1 2 3 4 5 6 7 8 9 10 11
d (ppm)
! ???
CH3C C-H
deshielded
shielded
downfield
upfield
low e- density
high e- density
high chemical shift
low chemical shift
high frequency
low frequency

5. Electrons Orbiting Other Nuclei  1/r3
Bapplied
 1/r3
Suppose the
studied nucleus is fixed relative to the other nucleus
by bond(s).
net from average around circle
net from average over sphere
PPM
Suppose molecule
in fluid undergoes rotational averaging.
ZERO!
Ignore electrons on other atoms!
Diamagnetism from Orbiting
Electrons

6. Diamagnetic “Anisotropy” Electrons Orbiting Other Nuclei
Bapplied
NOT
net from average over sphere
reinforces Bapplied
suppose less orbiting for this molecular orientation
ZERO!
Unless orbiting depends on molecular orientation
Diamagnetic
“Anisotropy”
(depends on orientation)

7. Diamagnetic Anisotropy Benzene “Ring Current”
B0 can only drive circulation about a path to which it is perpendicular. B0
Net deshielding
of aromatic
protons;
shifted downfield
If the ring rotates so
that it is no longer perpendicular to B0,
the ring current stops.

8. Aromaticity: PMR Chemical Shift Criterion
14  electrons
(43) + 2 ? 1 2 3 4 5 6 7 8
8 H
2 H
TMS
TMS
Vogel, Aromaticity, 1967, p. 119
DIAMAGNETIC ANISOTROPY
DIAMAGNETIC ANISOTROPY!
10  electrons
(distorted – less overlap & ring current)
HCCl3
 -4.23 9 -1 -2 -3 -4
d (ppm)
Boekelheide (1969)

9. Aromaticity: PMR Chemical Shift Criterion
“Anti-Aromatic” Dianion
Metallic K adds 2  electrons
to give 16
(4n) -2
CH3 signals shift downfield by 26 ppm despite addition of “shielding” electrons.
14  electrons
(43) + 2 9 1 2 3 4 5 6 7 8
HCCl3 -1 -2 -3 -4
TMS
Vogel, Aromaticity, 1967, p. 119
DIAMAGNETIC ANISOTROPY
DIAMAGNETIC ANISOTROPY!
THF solvent
Shrink Scale
 -4.23 4 6 8 10 12 14 16 18 20 22 2 -2 -4
d (ppm)
Boekelheide (1969)

10. Diamagnetic Anisotropy Acetylene “Ring Current”H
The H nuclei of benzene lie beside the orbital path when there is ring current (B0 at H reinforced; signal shifts downfield). H
Warning!
This handy picture of diamagnetic anisotropy due to ring current may well be nonsense!
(Prof. Wiberg showed it / /to be nonsense for 13C.) H
The H nuclei of acetylene lie above the orbiting path when there is ring current (B0 at H diminshed; signal shifts upfield).

11. Spin-Spin Splitting

12. Chem 220
NMR Problem 1
(of 40)
CH3C
OCH2CH3 O C. H
Triplet
(1:2:1)
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
d (ppm) 1 2 3 4 5 6 7 8

13. C. O H CH3C OCH2CH3 J in Hz vs. Chemical Shift in
NMR Problem 1
(of 40)
Influence of CH2 on CH3 must be the same as that of CH3 on CH2
CH3C
OCH2CH3 O C. H
and independent of Bo
Triplet
(1:2:1)
Eight (23) sets of molecules that differ in spins of adjacent H nuclei
J in Hz
Quartet
(1:3:3:1)
7.3
7.3 1: 1
7.3
7.3
7.3 1 2 2: 1 3 3: 4: 1 6 4
binomial
coefficients
d (ppm) 1 2 3 4 5 6 7 8
vs. Chemical Shift in
(Orbiting driven by Bo)

14. HO-CH2-CH3 ? ? 5.1 Hz 7.2 Hz 124 Hz 13CH3 ? Subtle Asymmetry d
Dd ppm
5.1 Hz
7.2 Hz
× 400 MHz
J = 7.2 Hz
DMSO-d5
Subtle
Asymmetry
HO-CH2-CH3
1.070
1.052 d
Doublet of Quartets
7.2
5.1
124 Hz
D is a weaker magnet than H.
13CH3
1:4:6:4:1
Quintet?
D can be oriented 3 ways in Bo.
7.2
CD3SCD2H O
1.8 Hz
1.1% of C
1:2:3:2:1
Quintet ? ?
H2O
d (ppm) ?

15. What determines
the Strength of
Spin-Spin Splitting?

16. is mediated by bonding electrons
Isotropic JH-H
is mediated by bonding electrons
(the anisotropic through-space part is averaged to zero by tumbling)

17. Not spatial proximity! Might overlap be greater for anti C-H bonds ??
In tumbling molecules, nuclear spins communicate not through space, but through paired electrons on the nuclei.
When the “up” electron of this MO is on Nucleus A
HOMO-3
J depends on the s-orbital content of molecular orbitals.
only its “down” electron is
available to be on Nucleus B
J = 6-8 Hz
J = 1-3 Hz
J = 0-1 Hz
Might overlap be greater for anti C-H bonds ??
3.07 Å
1.85 Å
2.38 Å
J = 0-3 Hz
J = Hz
J = 6-12 Hz
Not spatial proximity!
Through-space interaction of dipoles averages to zero on tumbling.

18.  Better Overlap! Which gives better overlap?
Examine the overlap of the components. + +
Backside overlap
is counterintuitive. 
Better Overlap!
good p-p
bad p-p
2 bad s-p +
good s-p; good p-s +
s-p > s-s or p-p
(See Lecture 12)
good s-s
bad s-s

19. C Overlap s-ps s-s p-ps Overlap Integral 1.0 0.8 0.6 0.4 0.2 0.0 1.2
1.3
1.4
1.5 Å C
s-ps
Interesting that s-p is greater than EITHER s-s or p-p
s-s
p-ps
C Overlap

20. (approximate way to measure a rigid torsional angle!)
10-20
No “handle” for rf if same chem shift
(see Frame 26 below)
invisible H H
11 Hz
13 Hz
2 Hz
gauche ~7 Hz
2-13 Hz, depends on conformation (overlap)
(approximate way to measure a rigid torsional angle!)

21. End of Lecture 60 March 23, 2011
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