Unusual complex between 18-Crown-6 and tetramethylammonium cation—detection by electrospray ionization mass spectrometry

Abstract

Complexes between crown ethers and quaternary ammonium cations have been studied by electrospray ionisation mass spectrometry (ESI-MS). The ESI-MS method has been shown to allow observation of not only stable inclusion complexes between large crown ethers and tetramethylammonium cation (e.g. [DB30C10 + (CH3)4N]+ ion) but also of unstable inclusion complexes between smaller crown ethers and quaternary ammonium cations which are difficult to observe by other methods, namely [18C6 + (CH3)4N]+ ion. Stability of the complexes between crown ethers containing aromatic ring and tetramethylammonium cation is enhanced by cation-Π interactions. The molecule of 18C6 does not contain aromatic rings, thus [18C6 + (CH3)4N]+ ion exists due to the formation of C–H···O hydrogen bonds. Such a complex is quite unusual, since C–H···O hydrogen bonds are very weak and usually coexist with other strong interactions.