ORIGINAL PAPER A combined extraction and DFT study on the complexation of H 3 O + with a hexaarylbenzene-based receptor Petr Toman • Emanuel Makrlı ´k • Petr Vanˇura • Va´clav Kasˇicˇka • Rajendra Rathore Received: 24 February 2010 / Accepted: 16 April 2010 / Published online: 12 May 2010 Ó Springer-Verlag 2010 Abstract From extraction experiments in the two-phase water/nitrobenzene system and c-activity measurements, the stability constant of a hexaarylbenzene-based receptorH 3 O ? complex species dissolved in nitrobenzene saturated with water was determined. By using quantum-mechanical density functional level of theory (DFT) calculations, the most prob- able structures of this complex were derived. Keywords Receptors  Macrocycles  Complexation  DFT  Structure Introduction Hexaarylbenzene (HAB) derivatives attract great attention because of their unique propeller-shaped structure and potential applications in molecular electronics and nano- technology. It has previously been described by employing nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography that the HAB-based receptor 1 (Scheme 1) binds a single potassium cation because it interacts both with the polar ethereal fence and with the central benzene ring via cation–p interaction [1]. Cation–p interaction is a well-established phenomenon in gas phase and in solid state [2] and is known to play an important role in the stabilization of tertiary structures of various proteins [3]. The dicarbollylcobaltate anion [4] and some of its halogen derivatives are very useful reagents for extraction of various metal cations (especially Cs ? , Sr 2? , Ba 2? , Eu 3? , and Am 3? ) from aqueous solutions into a polar organic phase, both under laboratory conditions for purely theoretical or analytical purposes [5–28] and on the technological scale for separation of some high-activity isotopes in reprocessing of spent nuclear fuel and acidic radioactive waste [29–31]. Recently, protonation of valinomycin, some calix[4]are- nes, and dibenzo-18-crown-6 has been investigated [32–40]. In the current work, the stability constant of the HAB-based receptorH 3 O ? (1H 3 O ? ) complex species is determined in the organic phase of the water/nitrobenzene extraction system. Moreover, by applying quantum-mechanical density func- tional level of theory (DFT) calculations, the most probable structures of this cationic complex species are derived. Results and discussion Extraction experiments With regard to previous results [32, 41–44], the two-phase water–HCl/nitrobenzene–CsDCC–1 (hexaarylbenzene-based P. Toman Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic E. Makrlı ´k (&) Faculty of Applied Sciences, University of West Bohemia, Pilsen, Czech Republic e-mail: makrlik@centrum.cz P. Vanˇura Department of Analytical Chemistry, Faculty of Chemical Engineering, Institute of Chemical Technology, Prague, Czech Republic V. Kasˇicˇka Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic R. Rathore Department of Chemistry, Marquette University, Milwaukee, WI, USA 123 Monatsh Chem (2010) 141:737–741 DOI 10.1007/s00706-010-0313-5