empirical correlations

Empirical Correlations

for methylene of the type Y-CH2-X by adding i to the chemical shift of methane;

= 0.23 + X + Y

substituent constants Zi are added to the chemical shift of ethene;

= 5.28 + Zgem + Zcis + Ztrans

substituent constants Si are added to the chemical shift of benzene;

= 7.27 + ∑Si

First-Order Spin-Spin Coupling

As closer the coupling nuclei as better they can interact and as higher is the coupling constant J. The number of bonds between two coupling nuclei is given as a prefix (e.g. a 3-bond coupling constant is given as 3J).

-1/2

+1/2

+1/2

-1/2OR

RO

H H

OR

RO

H H

Each proton has a unique absorption (chemical shift ) but this absorption is altered by magnetic nuclei nearby. In 1H-NMR other protons but also any other magnetic nuclei such as deuterium, 13C, 19F, and 31P.First-order coupling is found for absorptions that satisfy J > 10 and any pair of chemically equivalent nuclei must also be magnetically equivalent.

Pascal’s triangle

OR

H

H H

C-H sees CH2 protons CH2 sees C-H proton

(+1, 0, -1) (+1/2, -1/2)

Typical A2X3 ethyl pattern

quartet

triplet

H-coupling in ethylbenzene

H-coupling and Integration (isopropylbenzene)

1H-NMR of Ethanol in CDCl3

Rapid exchange of OH: do not see coupling

CH3CH2OH

1H-NMR of Ethanol in DMSO

CH3CH2OH

No exchange

Doublet of Quartets

CH3CH2OH

Methylene group couples to OH and CH3; 3J(CH2-OH) and 3J(CH3-CH2)

Benzyl Acetate

Integration 5:2:3

At high resolution see multiplet