2DNMR Coupling Constants Geminal Coupling are usually negative:

2DNMR Coupling Constants Geminal Coupling are usually negative:

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 O 2 CH3

NOE enhancement observed C NMR 13 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

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 HH Identification of 1,1-Me Identification of 5-Me

Identification of H4 Identification of 10-Me Identification of H3 Identification of H2 p=3

q=2 Selective Decoupling irradiate H4 doublet

triplet little interaction normal spectrum 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

i j h d f

e ab g c 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

j h i i h

f g e d f g

d c f e a,b a,b

AX3 XAM2 where JAX= JAM JXM = 0 Can we assign the peaks?

carbon count 2 =CC2 =CH 1 3 CH3

2 CH2 8,9,10 1 4,5 2,6 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 4. 9 b a bb

a 2 6 irradiated irradiated

10 9 8 c b a irradiated

2 6 c Irradiated b

irradiated a irradiated irradiated

1 5 4 C NMR 13

4,5 9,10,8 3,7 2,6 10 4 1

1 CH2OH 3 2 5

8 6 7 9 8 9,10

4 5 OH 1 H NMR 1

6 2

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