Exchange and Medium Effects

Intermolecular exchange involving a separation of the coupled atoms will appear as zero coupling if the exchange is fast on the NMR time scale, but when the intermolecular exchange involves no separation of the coupled atoms an average coupling 

There is a gas-to-liquid shift in the BF coupling constant in BF3. The density dependence of this coupling in the gas phase and the temperature dependence in the pure liquid (liquid density varies with temperature) are both measures of the effects of van der Waals interactions on the BF coupling constant. The effects of solvent on coupling constants have been reviewed by Barfield and Johnston.  

---