Chemistry 125: Lecture 60March 23, 2011

NMR SpectroscopyChemical Shift and

Diamagnetic Anisotropy, Spin-Spin Coupling This

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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

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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