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Chemistry 125: Lecture 61March 26, 2010
NMR Spectroscopy Decoupling
& CorrelationPreliminary This
For copyright notice see final page of this file
Proton Decoupling
13C H
100 MHz25 MHz
J ~ 125 Hz
(in frame rotating at 100 MHz)
C13 NMR spectrum
irradiate H (100 MHz) and pulse (25MHz)
to observe C13
H average
oror
H NMR spectrum
C13 up C13 downC12
H up H down
40 db (inverse log measure of rf power)
CH2
CHCH3
C
CH2CH
CDCl3
Observe 13C while decoupling 1H at various powers.
1 db (inverse log measure of rf power)
CH2
CHCH3
C
CH2CH
CDCl3
Observe 13C while decoupling 1H at various powers.
NOE(Nuclear Overhauser
Enhancement)
RF excitation of a nucleus strengthens
the signal from nearby nuclei.
Bad for integrationGood for
determining structure
(see below)
Proton-Decoupled 13C NMR Assignments forthe Artificial Sweetner Neotame Monohydrate
Prof. Eric Munson, Kansas Univ.
ON
O
O N O
O
1 3
14
15
18-20
16 17HH
13
7-12
6
542
- +H
One peak per carbon, pretty
well spread out
Why no 13C-13C splitting?
Only 1% of 13Cs have a 13C neighbor
in the same molecule.C=O Carom
C-Xe-neg
Calkane
Double Labeling
Introduction:Lanosterol Biogenesis
Cf. Frames 6-13 of Lecture 52and Sec. 12.13 pp. 554-562
Squalene
+
+
+
+
+
HO
HH
H
CH3
HH
CH3
H+
CH3H3C
H3C CH3
CH3
CH3
CH3H3C
Lanosterol(source of cholesterol& steroid hormones)
Squalene
+
+
+
+
+
HO
HH
H
CH3
HH
CH3
H+
CH3H3C
H3C CH3
CH3
CH3
CH3H3C
Lanosterol(source of cholesterol& steroid hormones)
3°
3°
3°
3°3°
Cute StoryIs it True?
(Wait for NMR)
HO
HH
CH3H3C
H3C CH3
CH3
CH3
CH3H3C
Squalene
Lanosterol
OPP
13C Label
Single Label Enrichment
Enriched Peaks
(100x stronger than natural-abundance
peaks)
HO
HH
CH3H3C
H3C CH3
CH3
CH3
CH3H3C
Squalene
Lanosterol
OPP
13C Label
Single Label Enrichment
Enriched Peaks
Proves that they entered as a unit.
The dilute double label experiment enhances the same 12 13C peaks as the single label experiments, but only 8 of them show spin-spin splitting (because their C-C bond stays intact).
HO
HH
CH3H3C
H3C CH3
CH3
CH3
CH3H3C
13C Double Label
Squalene
OPP
DILUTE !
Double-Doublet (proton decoupled)13C-13C splitting (neighboring 13Cs)
Power of Correlation
both labeled, but notin the same molecule
Few single presursor molecules have any C13 label, but thosethat are labeled have two C13s.
These are both labeled, in the same molecule
Strongly confirms the rearrangement scheme.
1H to 1HCorrelation by NOE
(through-spacemagnetic interaction)
protonsin proteinpolymer
proximity(< 6Å) of
With Molecular Mechanics Constraints gives 3-D Structure (without crystal!)
NH at 7.25
is within 6Å of NHs at 8.9, 8.3, 8.25, 7.7
Narrow range; mostly HN-C=O protons
Identify NH withamino acid by
coupling throughCH to R
H
HO
NHR O
NHR
Less-congested, off-diagonal peaks appear when “tickling” one signal on the diagonal enhances another.
diago
nal s
hows n
orm
al sp
ectru
m
heav
ily co
nges
ed b
y ove
rlapp
ing si
gnals
H3C CH3
H3C
H3C
CH3
CH3
CH3
+
A
C C
B
D
B
1H vs. 1H Correlation
in TIME
H3C CH3
H3C
H3C
CH3
CH3
CH3
+
CH3
CH3
H3C
H3C
CH3
CH3
CH3
+
A
A
C C
B
D
B
C
C
B
D
B
0.3 sec 40°C
(ppm)
Note: ppm scale is slanted and "wackbards". The protons in methyls C and D are near + charge (see resonance
structures), thus deshielded from lack of electron density, and appear furthest to right - at highest .
(Range of peaks is 150 Hz in 60 MHz spectrometer = 2.5 ppm.)
+
+
+
+
+
+
++
+ +
+
C A
D B
B C
Methide Shift: 1-2 (as shown)
“2-Dimensional”NMR
H3C CH3
H3C
CH3
CH3
H3C CH3
H3C
CH3
CH3
CH3
+
A B
C D
A D
or 1-Anywhere?
Thin LayerChromatography
of partially purified extractof brown algae
looking for ecdysteroids
Developed with CHCl3 : MeOH : C6H6 (25 : 5 : 3)
Developed withEtOAc : 96%EtOH : H2O (80 : 15 : 5)
http://www.chromsource.com/books/Milestones-TLC.pdf
Thin LayerChromatography
of partially purified extractof brown algae
looking for ecdysteroids
better resolution
in 2 - D
http://www.chromsource.com/books/Milestones-TLC.pdf
("3-D" becauseecdysone spots turn turquoise
after vanillin/ H2SO4 spray)
Electrophilic Aromatic Substitution
H
HH
H
H
H
D2SO4
H
HH
H
D
H
Sec. 16.4
H
HH
H
H
H
D
via A/D intermediate
+
D2SO4C6D6
etc.
Observable!
Or other electrophiles in place of D+
e.g. NO2+, Br+, HOSO2, R+, R-C=O R-C=O+HOSO2
+
H3C CH3
H3C
H3C
CH3
CH3
CH3
+
A
C C
B
D
B
1H vs. 1Hcorrelation in time
H3C CH3
H3C
H3C
CH3
CH3
CH3
+
CH3
CH3
H3C
H3C
CH3
CH3
CH3
+
A
A
C C
B
D
B
C
C
B
D
B
0.3 sec 40°C
CA
DB
Remember
+
+
+
SHMo2 (Simple Hückel Molecular Orbital Program)
Benzene Pentadienyl
Cation addition converts ring to chain.
H
HH
H
H
H
D+
H
HH
H
H
H
D
+
Locus ofodd electron in radical,
+ charge(LUMO)
in cation.
- charge (HOMO)
in anion,SOMO
(nonbonding)
+
+
+
Cf.
X
NO2
++
+ +
HO2N
X
+++
+
H
O2N
X
+
+ +
H
Substituent Effects on RateX
O2N
X
O2N
X X
NO2
+ +NO2
+
(from HONO2/H2SO4)
X Relative Rate (overall)
H [1] Cl 0.03 NO2 6 10-
8
CH3 25 OH 1000
(CH3)3N+ 1 10-
8
donation / withdrawal
(or ) e-donation eases formation of cation intermediates
(or ) e-withdrawal retards formation of cation intermediates
Why is -NO2 e-Withdrawing when -OH is e-Donating?
-NO2 6 10-
8
e-withdrawal retards formation of cation intermediates
-OH 1000 e-donation eases
N
O O
O
HpC
C=C pO-1
0
High HOMO; Good Overlap with Phenyl
Low OMO;Good Overlapwith Phenyl
(but poor E-match)
High HOMO;
Low LUMO;Good Overlapwith Phenyl
OH is a Donor
NO2 is a Acceptor
No Overlap with Phenyl
NO2 isallylic
(willing but not able)
Substituent Effects on Orientation (p. 763)
X
NO2
+
(from HONO2/H2SO4)
X
NO2
++
+ +
HO2N
X
+++
+
H
H
X
+
+ +
NO2
X- Relative Rate (per replaceable H)
H- [1] [1] [1] Cl- 0.0008 0.03 0.13
O2N- 9 10-
8
0.6 10-8 0.03 10-8
H3C- 1 39 46 (CH3)3C- 4 6 72
DirectingActivating
Deactivating
Act
Deact
Deact
(CH3)3N- 3 10-
8
0.6 10-8 + _
ortho meta para
EtOC=O 0.006 0.0006
- 0.003
o/p
m
o/p
“e-donating”
“e-withdrawing”
(steric hindrance)?
Preparation ProblemsNO2
Cl
NO2
Cl2
FeCl3
NO2
Cl
Cl2
FeCl3
(Cl+ FeCl4-)
NH2
Cl
HNO3
H2SO4
OH
Cl
“reduction”
(Zn/HCl?)
H2O
100°C
HONO
(NaNO2/HCl)
N2
Cl
+
diazonium salt
Cl-
or :HalCNNO2
H
Cl
End of Lecture 61March 26, 2010
Copyright © J. M. McBride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).
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