empirical correlations
DESCRIPTION
Empirical Correlations. for methylene of the type Y-CH 2 -X by adding D i to the chemical shift of methane ; d = 0.23 + D X + D Y. substituent constants Z i are added to the chemical shift of ethene ; d = 5.28 + Z gem + Z cis + Z trans. - PowerPoint PPT PresentationTRANSCRIPT
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