2dnmr. coupling constants geminal coupling are usually negative: ch 2 vicinal coupling usually...
Post on 19-Dec-2015
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2DNMR
Coupling Constants
Geminal Coupling are usually negative: CH2
Vicinal coupling usually positive:
CH-CH
Why?Coupling is through bonding electrons;
consider the following model
2D NMR basically provides information about connectivity and proximity
COSY spectroscopy (correlation spectroscopy)(various pulse sequences are available
providing similar information)
HH COSY
H H COSY-90
Long range HH COSY
H C COSY
HSQC method
gradient select
HMQC
method
gradient select
HMBC
method
2. Off-resonance decoupling
CH3
O
2
NOE enhancement observed
13C NMR
Coupling is reduced, but multiplicity is retained; 1H irradiation 1 ppm upfield from TMS; to observe multiplicity at C-3, 1H irradiation could be set to 10 ppm downfield from TMS
signals can be integrated
Demonstration of the effect of NOE on decoupling
C-9 signal as a function of H decoupling
C-9
3. Attached protons
4. Identification of nearest neighbors
Identification of 1,1’-Me
Identification of 5-Me
Identification of 10-Me
Identification of H4
Identification of H3
Identification of H2
HH
p = 3
q = 2
irradiate
normal spectrum
H4 doublet
little interaction
triplet
Selective Decoupling
Results are often reported as difference spectra
10 Note: peak s 2 and 3 are mislabeled
Measurement of CH coupling constants Heteronuclear J resolved Spectrum
Identify the carbons by chemical shift
j
i
h d
g
f e
a bc
Let test our assignments using ACD
An INADEQUATE spectrum (incredible natural abundance double quantum transfer experiment spectroscopy) measures 13C –13C transfer of magnetization between two 13 C nuclei using natural abundance C (detection of 1 in 10000 molecules).
j i
e
d
g
f
h
c
i
id
g
f
f
e a,b
hj
a,b
XAM2
where JAX= JAM
JXM = 0
AX3
Can we assign the peaks?
carbon count
1 3 CH3
2 CH2
=CH2 =CC2
1
2,6
8,9,10
4,5
NOE Difference Spectra
Some NOE operates through space as well as through bonding electrons. The through space interaction decreases as the inverse of the sixth power of the through space distance of the nuclei. The through space interaction occurs between nuclei that interact by a dipolar interaction
The NOE difference spectrum is obtained by subtracting a normal spectrum from one in which a specific proton is irradiated. An measurable interaction can be expected up to about 4Å.
irradiated
irradiated
a
a
bb
b
62
9
irradiated
Irradiated
irradiated
a
a
b
b
c
c
62
10 9 8
irradiated
irradiated
1
5 4
1H NMR
13C NMR
CH2OH
1
234
56
78 9
10
1
1
3,7 2,6 4,5 9,10,8
62
45
OH
89,10