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Magnetic dipole–dipole interaction, also called dipolar coupling, refers to the direct interaction between two magnetic dipoles. Roughly speaking, the magnetic field of a dipole goes as the inverse cube of the distance, and the force of its magnetic field on another dipole goes as the first derivative of the magnetic field. It follows that ...
It is related to the prototypical Ising model, where at each site of a lattice, a spin {} represents a microscopic magnetic dipole to which the magnetic moment is either up or down. Except the coupling between magnetic dipole moments, there is also a multipolar version of Heisenberg model called the multipolar exchange interaction .
Examples of dipole-dipole and quadrupole-quadrupole exchange interactions in J=1 case. Blue arrow means the transition comes with a phase shift. [21] There are four major mechanisms to induce exchange interactions between two magnetic moments in a system: [20] 1). Direct exchange 2). RKKY 3). Superexchange 4). Spin-Lattice.
Example 1 H NMR spectrum (1-dimensional) of ethanol plotted as signal intensity vs. chemical shift.There are three different types of H atoms in ethanol regarding NMR. The hydrogen (H) on the −OH group is not coupling with the other H atoms and appears as a singlet, but the CH 3 − and the −CH 2 − hydrogens are coupling with each other, resulting in a triplet and quartet respectively.
There are four basic coupling mechanisms: conductive, capacitive, magnetic or inductive, and radiative. Any coupling path can be broken down into one or more of these coupling mechanisms working together. Conductive coupling occurs when the coupling path between the source and victim is formed by direct electrical contact with a conducting body.
A key example of this phenomenon is the spin–orbit interaction leading to shifts in an electron's atomic energy levels, due to electromagnetic interaction between the electron's magnetic dipole, its orbital motion, and the electrostatic field of the positively charged nucleus.
For a fully oriented molecule, the dipolar coupling for an 1 H-15 N amide group would be over 20 kHz, and a pair of protons separated by 5 Å would have up to ~1 kHz coupling. However the degree of alignment achieved by applying magnetic field is so low that the largest 1 H- 15 N or 1 H- 13 C dipolar couplings are <5 Hz. [ 19 ]
Since 1 H dipole-dipole couplings provide the primary means of cross-relaxation for organic molecules in solution, spins undergoing cross-relaxation are those close to one another in space. Therefore, the cross peaks of a NOESY spectrum indicate which protons are close to each other in space.