Important
Equivalent nuclei cannot couple to each other

The size of the vicinal couplings is described using the Karplus equations which relates the magnitude of coupling to the angle between the two protons.

Cis & Trans
3J trans 12-16 Hz
3J cis 7-11 Hz

When the protons are closer together the coupling constant is smaller

Positional
180 degrees from each other (both axial) 8-12 Hz
1 axial and 1 equatorial 5-9 Hz
Both equatorial 2-6 Hz

The more favourable the positions of the Hydrogen substituents the smaller the couping constant.

Electronegative and electropositive substituents
ethane-E+ 8.4 Hz
ethane-Cl 7.3 Hz
ethane-3Cl 6.0Hz

As you increase the electronegativity there is a decrease in the coupling constant

Bond length effects on coupling constants
Generally the increase in bond length decreases the coupling constant.
Ranging from 18 to 6 Hz

Angle strain
As the angle strain is decreases there is an increase in the coupling constant.
3 membered ring( with 1 double bond) 0.5-2Hz
6 membered ring (with 1 double bond) 8-10 Hz

Measuring coupling constants
Peak * field strength
a-b = coupling constant
e.g doublet
3.54ppm 3.54 x 100MHz = 354 Hz
3.60ppm 3.60 x 100MHz = 360 Hz
3J(a-b) = 360-354 = 6Hz

As you increase the hertz scale you get the same seperation between 2 peaks which are coupling. The spread of the peaks will just change.

	Created by abbeyjarvis over 10 years ago

Undergraduate CHM2WS1 Note on H NMR Spectroscopy, created by abbeyjarvis on 21/11/2013.

H NMR Spectroscopy