Coupling Flashcards
Scalar coupling
= spin-spin coupling
= J-coupling
Through-bond interaction
How does J-coupling arise?
The magnetic moments of other NMR-active nuclei surrounding the observed nucleus produce small fields, in addition to the spectrometer field and the chemical shift fields
i.e. the observed nucleus does not see just one net magnetic field - there is the possibility of several extra fields depending on the number and nature of the surrounding NMR-active nuclei
Why is J-coupling independent of the spectrometer field strength?
The magnitude of J-coupling depends only on the interaction between the nuclear magnetic dipoles
(J is constant at different external magnetic field strengths)
Homonuclear coupling
Between the same isotope i.e. proton-proton coupling
Heteronuclear coupling
Between different isotopes/elements i.e. proton-deuterium coupling
What coupling is observed in an NMR spectrometer?
Coupling exists between ALL NMR active nuclei present in a molecule
However, coupling between identical nuclei in identical positions in a molecule is not observed
What are the 2 distinct types of magnetic interaction (i.e. coupling) between nuclei with non-zero spin?
- Direct interaction (dipole-dipole coupling, D)
2. Indirect/scalar coupling (spin-spin splitting, J)
Direct interaction
= dipole-dipole coupling
= through-space coupling
Basically the magnetic effect on nucleus 1 caused by the magnetic field generated by nucleus 2
Approx. 1000x larger than scalar coupling - but its magnitude is affected by the natural abundance of the NMR active nuclei and the distance between the nuclei
Dipolar coupling is completely averaged out by the random motion of molecules in mobile isotropic liquids, meaning no direct effects are seen - i.e. does not lead to multiplets in liquid-state NMR spectra
Properties of J coupling
Always reported in Hz
Field-independent
Mutual (i.e. Jax = Jxa)
Why does the magnitude of J coupling fall off rapidly as the number of intervening bonds increases?
Because the J coupling effect is usually transmitted through bonding electrons
Through-space mechanism of coupling between two spin 1/2 nuclei, A and X
In the absence of X, A resonates at a frequency vA in a particular field Bz
When X is present, the orientation of the spin of X with respect to A will create 4 possible spin states: aa, ab, ba, bb - each of these is a different energy
In the presence of X, A resonates at two different frequencies, separated by 2Bx = J (vA + J/2 and vA - J/2)
And X will resonate at vX + J/2 and vX - J/2
What does the energetic ordering of the ab and ba states depend on?
The vA and vX values
NMR selection rule
Deltaml = +/- 1
i.e. only aa to ab and ba to bb transitions are allowed (for X)
(for A would be aa to ba and ab to bb)
Why does the ‘through space’ mechanism average spin-spin interactions to zero?
In a solution or liquid, the orientations of the spins are totally random which means the field experienced by A due to X averages out to zero without restriction of movement
Coupling is orientation dependent
The dipole orientations of A and X are only fixed relative to each other in a solid
Fermi contact interaction
The through-bond magnetic interaction between an electron and an atomic nucleus
Responsible for the appearance of coupling in NMR
How is coupling transmitted?
By the polarisation of electrons in bonding MOs
Fermi contact mechanism
For A-X
- The nuclear spin on A polarises the electron spin to be antiparallel (or parallel if gamma is negative)
- This, in turn, polarises the other electron in the bond to be antiparallel as is demanded by Pauli’s exclusion principle
- This electron then polarises the nuclear spin on X - which could be aligned with A (parallel spins, unstable, higher energy, J is negative) or be opposed to A (antiparallel spins, lower energy, J is positive)
When is J positive?
When the nuclear spins are opposed
Why does the magnitude of the coupling constant decrease as the number of bonds between 2 nuclei increases?
Because the ability for efficient polarisation decreases
What does J depend on?
Number of bonds
Bond angles
Effect of bond angles on J
Known as the Karplus relationship Torsional/dihedral angle: 0 degrees = 8 Hz 60 degrees = 2 Hz 90 degrees = 0 Hz 180 degrees = 10 Hz
Effect of gyromagnetic ratio on J
For a given pair of nuclei, the magnitude of the scalar coupling is proportional to the product of the gyromagnetic ratios of the nuclei, if all other molecular factors stay the same
J proportional to yAyB
Why can coupling constants only be compared for isotope of the same element? (e.g. H/D)
Because only isotopes have the same electron cloud
Comparing coupling constant for isotopes of the same element
Where two NMR-active isotopes of a given element exist, e.g. 10B and 11B, the ratio of coupling constants to a third nucleus will be the ratio of the gyromagnetic ratios