The Karplus Relationship for Spins Separated by Two Bonds

The magnitude of the J-coupling results from the overlap of the occupied molecular orbitals connecting the coupling participants to the atoms between them. When the two orbitals are orthogonal, the overlap of the orbitals is minimized thus the coupling is likewise minimized. 

The presence of additional electron density between the spins separated by three bonds serves to enhance the amount of coupling that takes place. H s that are trans across a C=C bond exhibit a usually 

The magnitude of the produced by geminal H s also varies as a function of bond angle. Just as increasing orbital overlap increases the J-coupling for vicinal H s, so too does the increase of orbital 

Bond angle. If one atom is bonded to two other atoms, the bond angle is the geometrical angle between the vectors connecting the common atom to the two other atoms. 

Valence shell electron pair repulsion, VSEPR. A mental tool used to predict deviations from standard hybridized bond angles based on how much of the surface area of an atom a given electron pair in its outer shell occupies. 

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