Chemistry 125: Lecture 60March 24, 2010

NMR Spectroscopy Isotropic J and Dynamics

This

For copyright notice see final page of this file

ZERO!

average over

sphere

Electrons Orbiting

Other NucleiBapplied

Isotropic JH-H is mediated by

bonding electrons(the anisotropic through-space part

is averaged to zero by tumbling)

15.44.jpg

Not spatial proximity!

3.07 Å1.85 Å2.38 Å

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.

15.44.jpg

good p-p

good s-s

bad p-p

bad s-s2 bad s-p good s-p; good p-s

s-p > s-s and pp (lecture 13 frame 2)

Better Overlap!

+ +

+ +

H

No “handle” if same chem shift(see Frame 11 below)

2-13 Hz, depends on conformation (overlap)

13 Hz2 Hz Hgauche ~7 Hz

11 Hz

(approximate way to measure torsional angle!)

J C-H13

Amplify 30x

126 Hz

Coupling electron must be on the C13 nucleus.

99% of sample is C12

(instrumentalartifact)

Hybridization and J coupling

sp3

sp2

sp

Units for and Jppm Hz

Energy Scale

Same Frequency (Hz) Scale(relative to TMS)Same ppm () Scale

0 Hz300 Hz

Two unsymmetrical Doublets

a b

Lose the “handle”

as becomes smaller than J

By increasing (Hz) big magnet reduces asymmetry

and gives “ideal” pattern. Note: is measured in Hz (not ppm) between the

weighted average positions of the lines in the

two doublets

Slope up toward splitting partner

quartet

*

* *

*doubletsof

*

* *

*

Cf. Fig. 15.53, p. 738

Dynamics(the NMR Time Scale / Decoupling)

Sec. 15.9 pp. 746-749Sec. 15.6e p. 739

ROH chemical

shift?

CH3CH2OHspin-spin splitting?

average of manyH-bond structures

(dependent on concentration & temperature)

average for exchange among many molecules

(H+ / solvent)

Three OH peaks from three different kinds of molecules

(with different neighboring CH2 proton spins)

Fig. 15.53p. 733

5.3

2.3 1.8

Form AN O O

O

H

H+

F

Cl -

HO

HO

H

H

Form A

N O O

O

H

H+

F

Cl -

Why doesn’t IR show OH averaging?

110 x 1010 Hz 102 x 1010 Hz Difference ~ 1011 Hz

(This particular sample is a solid, but no averaging is observed in solution spectra either.)

How long does it take to measure frequency precisely?

1 second

20 Hz

22 Hz

21 HzBut a 1 sec pulse samples full range of phases.

0.5 sec (1/) is long enough to sample the full range, favorable

and unfavorable.

Match with a short pulse of20 Hz light is nearly as good.No net interaction with light.

1 sec light pulse distinguishes 20 Hz from 21 Hz.

light field

What if protons are “exchanging” faster than 1/?

1 second

20 Hz

22 Hz

21 Hz

22 Hz 22 Hz20 Hz 20 Hz 22 Hz 20 Hz

Very good match with the 21Hz average frequency single, sharp peak.

When do Peaks Average?

When atoms don’t stay put long enough to tell the

difference in frequency.e.g. If two peaks differ by 100 Hz, you must

count for ~0.01 sec to tell them apart.

These IR peaks differ by 1011 Hz. Exchange of position is not that fast.

One average chemical shift(no splitting observed)

equatorial

axial

15.58

Compared to What?

The NMR Time Scale

p. 746

29 Hz

Coalescence at ~30 flips per second

axialequatoriald11 to avoid complications from spin-spin splitting.

Chemistry 125 Seventh Examination Answers April 16, 2004

1. (3 min) Name the functional group in the molecule on the right and suggest

reagents and conditions for its preparation (no mechanism necessary).

The functional group is hemiacetal (or, more loosely, hemiketal) This hemiacetal is formed by reacting acetaldehyde with isopropanol using either acid or base catalysis (or by reacting the full ketal with water using acid catalysis).

2. (9 min) Sketch the proton nmr spectrum you would expect for the compound in Question 1. Say a few words about your

selection of chemical shift and splitting pattern for each of the 6 labelled (sets of) protons. Group Chemical shift Splitting a almost anyplace ( 1-6)depending Probably no splitting (broad singlet) on concentration and temperature because of rapid OH proton exchange (averaged H-bonding) among molecules b slightly deshielded by two oxygen 1:1 doublet from single H on neighboring atoms on neighboring carbon. carbon (J about 7 Hz) 1-2 (a real analogue is at 1.3)

H3

C H

H O

C H3

C H3

HO( a )

( b )( c )

( d )

( f )

( e )

aProbably no splitting (broad singlet)

because of rapid OH proton exchangeamong different molecules

1:1 doublet from single H on neighboring carbon (J about 7 Hz)

Almost anyplace ( 1-6) dependingon concentration and temperature

(averaged H-bonding)

Slightly deshielded by two oxygenatoms on neighboring carbon

1-2 (a real analogue is at 1.3)

b

c Deshielded by two oxygenatoms on the same carbon

4-5 (a real analogue is at 4.7)

1:3:3:1 quartet from three H atoms on neighboring carbon (J about 7 Hz)

Chemistry 125 Seventh Examination Answers April 16, 2004

1. (3 min) Name the functional group in the molecule on the right and suggest

reagents and conditions for its preparation (no mechanism necessary).

The functional group is hemiacetal (or, more loosely, hemiketal) This hemiacetal is formed by reacting acetaldehyde with isopropanol using either acid or base catalysis (or by reacting the full ketal with water using acid catalysis).

2. (9 min) Sketch the proton nmr spectrum you would expect for the compound in Question 1. Say a few words about your

selection of chemical shift and splitting pattern for each of the 6 labelled (sets of) protons. Group Chemical shift Splitting a almost anyplace ( 1-6)depending Probably no splitting (broad singlet) on concentration and temperature because of rapid OH proton exchange (averaged H-bonding) among molecules b slightly deshielded by two oxygen 1:1 doublet from single H on neighboring atoms on neighboring carbon. carbon (J about 7 Hz) 1-2 (a real analogue is at 1.3)

H3

C H

H O

C H3

C H3

HO( a )

( b )( c )

( d )

( f )

( e )

d 1:1 doublet from single H on neighboring C (J about 7 Hz)

Seven line multiplet from 6 H atoms on neighboring carbons (J about 7 Hz)

Slightly deshielded by oxygen atom on neighboring carbon

1 (a real analogue is at 1.1)

Deshielded by oxygen atom onthe same carbon (but less than c) 4 (a real analogue is at 3.8)

e

fSlightly deshielded by oxygenatom on neighboring carbon.

1 (a real analogue is at 1.1)

1:1 doublet from single H on neighboring carbon (J about 7 Hz)

different from d (diastereotopic)

End of Lecture 60March 24, 2010

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