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1st-order spin-spin coupling
We observe 1st-order NMR spectra when the frequency difference between the chemical shifts of any given pair of nuclei is much larger than the value of the coupling constant between them
/J > 10
and any set of chemically equivalent nuclei is also magnetically equivalent.
1st-order NMR spectra exhibit a number of simple characteristics:
• Multiplicities that result from coupling reflect the 2nI + 1 rule (IH = ½).• The intensities of spin-spin multiplets correspond to Pascal’s triangle for I = 1/2• Nuclei with the same chemical shift do not split each other, even when the
coupling constant between them is not zero.• Spacings between adjacent components of a spin-spin multiplet are equal to J.• Spin-spin multiplets are centred on the resonance frequency
2nd-order spin-spin coupling
We observe 2nd-order NMR spectra when the frequency difference between the chemical shifts of any given pair of nuclei is small compared to the value of the coupling constant between them
/J < 10
and/or any set of chemically equivalent nuclei is not magnetically equivalent.
Nuclei are chemically equivalent if they can be interchanged by a symmetry operation of the molecule. Nuclei that are interchangeable by a rotation (Cn) are said to be homotopic. Nuclei related only by a mirror plane are termed enantiotopic. Chemically equivalent nuclei are isochronous (same chemical shift) but the converse is not necessarily true.
Nuclei are magnetically equivalent if they are isochronous and if all the coupling constants for couplings to any other nucleus are equal for each nucleus (isogamous coupling).
Enantiotopic and Diastereotopic Protons
H
H3C
Cl
H
H
Cl
CH3
H
Enantiotopic protons by inversion (i)
H H
CO2HH3C
HHO
Diastereotopic protons of methylene groups
chiral molecule
H2H1
CO2HHO
H
H2 H1
HO2C
plane makes H1’s and H2’sequivalent
Diastereotopic protons can not be placed in same chemical environment
achiral molecule
no plane through CH2’s thus the protons are diastereotopic
Staggered Rotamers – non-chiral
H2H1
Cl
Br
H4H3
H2
H1Cl
H4H3
Br
H2
H1
Cl
H3
Br
H4
anti rotamer: H1 and H2 as well as H3 and H4 are enantiotopic interchanged through a plane of symmetry other rotamers: no symmetry, H1 and H2 as well as H3 and H4 are diastereotopicget chemical shifts for the anti and gauche rotamersrapid rotation gives one chemical shift for H1 and H2 and another for H3 and H4
anti gauche
H2H1
Cl
Br
H4Cl
H2
H1Cl
H4Cl
Br
H2
H1
Cl
Cl
Br
H4
H1 and H2 are not equivalent as cannot be interchanged by a symmetry operation
• no plane, axis or inversion center• not interchanged by rapid rotation • averaged chemical shift is not identical
Staggered Rotamers – chiral centre next to methylene group
Magnetic Equivalence
If chemical shift equivalent nuclei couple equally to other nuclei then they are magnetically equivalent
magnetic equivalent if symmetrically disposed with respect to each nuclei in the spin system.
NO2
Cl
H1 H1'
H2H2'
H1 and H1’ chemical shift equivalentJ12 J12’ = 7-10 Hz, J1’2 J1’2’ = 1 Hz
since H1 and H1’ couple differently to H2 magnetically not equivalent AA’XX’ spectrum is complex
Spectrum of 1-Chloro-4-nitrobenzene
AA’XX’ spectrum
AA’BB’ Spectrum
AMX Spectrum of Styrene
AB
AB
AB
AB
AB
AB
AB
AB AB
AB spin systems
Analysis of AB spin systems
AB = (4C2 – J2)
The ratio of intensities between larger inner and smaller outer peaks is
(1+J/2C)/(1-J/2C)
O
O
H
H
-21.5
Geminal couplings
Geminal (2J) couplings are usually negative and usually reach values of up to 30 Hz. Geminal protons attached to double and triple bonds can have positive coupling constants.
Lone pairs of electrons can donate electron density and make 2J more positive.
O
H
H
+42
Coupling of magnetically equivalent protons do not appear in the NMR spectrum but the coupling constants can be determined by deuteriation or from 13C satellites.
H
H
H
HH
H
8-13 Hz
1-6 Hz
0-5 Hz
Vicinal CouplingsVicinal (3J) couplings are often positive and usually reach values of up to 20 Hz.
Vicinal Couplings (3J) depend on dihedral angle
Long-Range Couplings
All couplings between protons that are more than 3 bonds apart are called long-range couplings (4J, 5J, etc.). Their coupling constant can reach values between 0.5-3 Hz if both sets of protons are connected to the same -electron system.
O
H
Br
t.-Bu
H1.7 Hz
N
H
H
0.9 Hz
H
CH3
CHO
H
1.45 Hz