Interpretation of more complex spectra

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• Interpretation of more complex spectra

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Diasteriotopic nuclei chemically different
nuclei with different chemical shifts Two nuclei
or groups attached to the same atom that are
present in environments that are not related to
each other by an axis of symmetry. Operational
definition If sequentially replacing the nuclei
in question with an isotopic nucleus leads to
diasteriomers, the nuclei in question are
diasteriotopic.
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Predict the 1H spectrum of CH2F2 spin of F is
½ Next Predict the 1H spectrum of

• H 2 ppm
• F 99 ppm
• JHH 5 Hz
• JHF 40, 10 Hz
• JFF 50 Hz

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Chemical equivalence Two nuclei related by an
element of symmetry including a plane of symmetry
in an achiral environment are chemically
identical. Magnetic equivalence Two chemically
identical nuclei may not necessarily be
magnetically equivalent. Two chemically
equivalent nuclei will have the same chemical
shift but may become magnetically non-equivalent
if they are coupled to the same nucleus with
different coupling constants.
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J12 J34 5.1 Hz J13 J24 1.3 J14 1.95 J15
J46 -0.42 J16 J45 -0.18 J23 1.95 J25
J36 1.4 J26 J35 1.4 J5,6 0.1
H1 H4 6.22 ppm H2 H3 6.53 H5 H6 5.85
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Chemical shift reagents Europium
dipivaloylmethane Eu(dpm) La(fod)
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Chiral Shift Reagents
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Are the hydrogens on the CH2 group
equivalent? CH3CH2OCH(CH3)2 Always equivalent?
Enantiotopic nuclei Enantiotopic nuclei two
nuclei that are related to each other by a plane
of symmetry but not an axis of symmetry. These
nuclei have the same chemical shift in an achiral
environment but have different chemical shifts in
a chiral environment
Operational definition If replacing one hydrogen
by deuterium leads to a new asymmetric carbon
atom, the two hydrogens are enantiotopic
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Non-First Order 1H NMR Spectra Cases where ?
(chemical shift) lt 10 J (coupling constant) AX
case ? AB case ? A2 case
?
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Analysis of AB Case
C
three unknowns ?A, ?B, JAB
JAB
JAB
?4 ?3 ?2 ?1
?? (?1- ?4)(?2 - ?3)1/2 ?A - ?B ½(?A
?B) C C (?1 ?2 ?3 ?4)/4
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JAB 247.5-231.5 16 JAB 211.5-195.7 15.8
JAB (avg) 15.9 Hz
2C 2(?1 ?2 ?3 ?4)/4 2C
(247.5231.5211.5195.7)/2 443.1 ?A ?B
?A - ?B (?1- ?4)(?2 - ?3)1/2
(247.5-195.7)(231.5-195.7)1/2 32.2
?A ?B 443.1 ?A - ?B 32.2 ?A 237.6 ?B
205.5 ?? 32.2
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A2X A2B Case
A3
A3
X
A2
A2B
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Top and bottom
9 transitions can be observed, 1 weak Numbering
the lines from the B nucleus
A2B
8 7 6 5 4
3 2 1
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Top and bottom
9 transitions can be observed, 1 weak Numbering
the lines from the B nucleus
?B ?3 9 Hz ?A 1/2( ?5 ?8)
½(31.939.1) 35.5 JAB 1/3?( ?1 - ?4 ?6 -
?8)? 1/3(0-15.632.5-40.7) 7.9 How do you
know that you have analyzed a spectrum correctly?
?8 ?7 ?6 ?5 ?4 ?3 ?2 ?1
40.7 39.1 32.5 31.9 15.6 9.0 6.7
0
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Raccoon ?B 9 Hz ?A 35.5 35.5 Hz JAB
7.9 JAA ? ?A- ?B 35.5-9 26.9
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Calculated A2B spectra for different values of JAB
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ABX spectrum What do you need to know to analyze
an ABX spectrum? You need to know 3 chemical
shifts ?A, ?B, ?X 3 coupling constants JAB,
JAX, JBX What does an ABX spectrum look like?
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AB
X
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ignore step 6
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JAB appears four times in the AB portion of the
spectrum 8-6 98.9-83.7 15.2 5-2 80.4-64.7
15.7 3-1 72-56.4 15.6 JAB 15.45 7-4
94.6-79.3 15.3
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JAB appears four times in the AB portion of the
spectrum 8-6 98.9-83.7 15.2 5-2 80.4-64.7
15.7 3-1 72-56.4 15.6 JAB 15.45 7-4
94.6-79.3 15.3
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JAB 15.5
The chemical shift of X is the average of all X
lines ?X (9101112)/4 183.9
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JAB 15.5 ?X 183.9
The average of all AB lines is equal to ½(?A
?B) (?A ?B) 2(12345678)/8 157.5
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JAB 15.5 ?X 183.9 (?A ?B) 157.5
6
5
4
2
8
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½ ?(JAXJBX)? the separation of the centers of
the two AB quartets ?(JAXJBX)?
2(8652)/4-(7431)/4 12.7
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JAB 15.5 ?X 183.9
5
6
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(?A ?B) 157.5 ?(JAXJBX)? 12.7
2
8
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2D separation of the 1st and 3rd lines of
first AB quartet 2D- separation of the 1st and
3rd lines of second AB quartet Chose 2D as the
larger value D ½(4-1) 11.45 ½(7-3) 11.3
Dav 11.4 D- ½(6-2) 9.2 ½(5-8) 9.5 D-av
9.35
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JAB 15.45 ?X 183.9
4
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(?A ?B) 157.5 ?(JAXJBX)? 12.7 D 11.4 D-
9.35
2
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2M (4D2 JAB2) ½ 2N (4D-2 JAB2) ½ M
½(411.42 15.52) ½ 8.36 N ½(49.352
15.52) ½ 5.23 1. ?A - ?B MN 13.6
½(JAX JBX) M-N 3.13 2. ?A - ?B M-N
3.13 ½(JAX JBX) MN 13.6
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(7,4,3,1) ab quartet because D 11.3 ab
centered at 75.6 (8,6,5,2) ab- quartet because
D- 9.5 ab- centered at 81.9 Assign ?(JAXJBX)
a sign if ab is centered at a higher
frequency than ab- or a sign if the reverse is
true
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• (?A - ?B) 13.6 ½(JAX-JBX) 3.13 (JAX
  JBX) 6.26
• (?A ?B) 157.5 (JAXJBX) -12.7
• ?A 85.65 JAX -3.2
• ?B 71.85 JBX -9.5
• 2. (?A - ?B) 3.13 ½(JAX-JBX) 13.6
  (JAX JBX) 27.2
• (?A ?B) 157.5 (JAXJBX) -12.7
  
• ?A 80.3 JAX 7.25
• ?B 77.2 JBX -19.95

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All values in Hz 72.2
JAB J12 15.45 ?X ?3 183.9
Solution 1 ?A ?1 85.65 JAX J13 -3.2 ?B
?2 71.85 JBX J23 -9.5
Solution 2 ?A ?1 80.3 JAX J13 7.25 ?B
?2 77.2 JBX J23 -19.95
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Some special cases of ABX like systems Deceptively
simple spectra
?A 452 Hz JAB 8 Hz ?B 452.5 Hz JAX
0 HZ ?X 473 Hz JBX 3 HZ
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When two chemical shifts are fortuitously very
similar, an ABX case can give a deceptively
simple spectrum. Only by simulating the
spectrum can you convince yourself the the
structure is more complex than the nmr suggests
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Whenever the chemical shifts of the A and B
nuclei are very similar, a deceptively simple
spectrum can be obtained 100 MHz 100, 101, 700,
8, 6 10 Hz
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Deceptively complex spectra Virtual coupling
Suppose ?A 100 Hz ?B 104 Hz ?X 200
Hz and JAX 0 Hz JBX 8 JAB 6 Hz Predict
the first order spectrum for X
This is an example of an ABX system
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2,6-dimethylquinone and 2,4-dimethylquinone

?1 ?4 6 ?2 ?3 ?5 ?6 2 J12 J13
J45J466 J14 1 J15 J16 J24J34 2 J23
J56 -10 J25J26 J35J36 0