2dnmr. coupling constants geminal coupling are usually negative: ch 2 vicinal coupling usually...

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