Spin systems

Spin System: Pople Notation

• Each Chemical Shift is designated by a letter• Dn -> Difference in Shift in Hz• J -> Coupling in Hz• If the ratio Dn/J is Small (<6), Letters used to designate the

shift are closeAB, ABC …This represent case of second order spectra. This case is also called strongly coupled.

• If the ratio Dn/J is large (>6), Letters used to designate the shift are farAM, AX …This case give rise to first order type spectra Is is also refer to as weakly coupled case

Pople Notation

CH2 CHOR

ORY

A2X (if the shift difference of CH2 and CH is large compare to coupling).

A2B (if the shift difference of CH2 and CH is small compare to coupling).

3 Spins AMX -> if the 3 spins have large chemical shift difference

ABX -> if 2 spins are close and 1 is far away

ABC -> if 3 spins are close

When nuclei have identical shift but different magnetic coupling, prime symbol is used. For example:

AA’BB’ or AA’XX’

Pople Notation

CH2 CHOR

ORY

A2X (if the shift difference of CH2 and CH is large compare to coupling).

A2B (if the shift difference of CH2 and CH is small compare to coupling).

3 Spins AMX -> if the 3 spins have large chemical shift difference

ABX -> if 2 spins are close and 1 is far away

ABC -> if 3 spins are close

When nuclei have identical shift but different magnetic coupling, prime symbol is used. For example:

AA’BB’ or AA’XX’

A2 spin System

Chemical shift equivalence : Isochronous nuclei

These nuclei are interchangeable by symmetry or rapid exchange

H

Cl

H

Cl

CH2Cl2

A2

CH2CF2

A2X2

H

H Cl

ClCl

Cl

A4

AA’BB’

N

Cl

HA'

HB'

HB

HA

2 sets of homotopic protons : magnetically non-equivalent

H

H

H

A

M

X

JAX = Jcis = 10 Hz JAM = Jtrans = 17 HzJMX = Jgem = 2 Hz

Jtrans

M X

Jcis