Complexed Solvents

I was once asked by an inorganic chemist: why do I have two THF signals in the spectrum of my compound dissolved in THF-d8? Many inorganic compounds crystallize with complexed solvent molecules as a fundamental component of their structure. This is particularly true of tetrahydrofuran (THF). The complexed solvent molecules are released when the solid compound is re-dissolved in solution and can easily be detected by high resolution NMR. The figure below shows the 500 MHz ¹H NMR spectrum of an inorganic compound containing complexed THF which was re-dissolved in THF-d8. One can see the spectrum of the residual protons of the THF-d8 solvent and the spectrum of the complexed THF that was released when the solid was dissolved. The signals are separated due the isotope effect.