1
Crown Ethers and Binding of Metals Lara Dakkak Under the supervision of Dr. Khan 12-crown-4 15-crown- 5 18- crown-6 INTRODUCTION • Guest-host binding interactions of four crown ethers with alkali metal ions were studied by electrospray ionization mass spectrometry (ESI-MS). • Crown ethers are compounds that are repeating units of ethylene oxide (C2H4O). The unique binding quality of crown ethers allows for them to bind strongly with ions. The interior of the ring is the portion where Oxygen (in most cases) binds with the metal, whilst the exterior remains hydrophobic. This creates a polar substance that is soluble in a non-polar solvent. •The origin of the name, crown ether stems from the fact that when an ion enters the middle of the crown it bears a resemblance to a crown placed on a head. • The process of Electrospray Ionization begins with the pumping of a sample that has been dissolved in a polar solvent through a narrow, stainless steel capillary. A high voltage is applied to the tip of the capillary resulting in the emergence of the sample as highly charged ions engulfed in droplets of solvent. The spray of charged droplets is directed with the aide of Nitrogen towards the mass spectrometer. Next, a warm flow of drying gas (also Nitrogen) results in the evaporation of the droplet, leaving behind highly charged ions. The charged ions, now free from a solvent, pass through a sampling cone towards an intermediate vacuum area, and finally enter the analyzer of the mass spectrometer, which is under a high vacuum. • Typically, several ions (guest substances) and one crown ether (host complex) are included in a mixture and are passed through the ESI-MS in order to analyze the mixture to see which guest complex best serves a specific host substance. ETHER CAVITY (A) BEST FIT (A) 12-crown- 4 .6 - .75 Na + (1.02) 15-crown- 5 .86 - .92 Na + (1.02) 18-crown- 6 1.34 - 1.43 K + (1.38) 15-crown-5 + Na 15-crown-5 + K15-crown-5 + Cs An ESI-MS Lara Dakkak and Heather McCormick working in the lab. Binding efficiencies reflect a best match for cavity size and ion size. Cs+, 1.67 Angstroms, is too large for these crown ethers!
