Cation-p interaction of Tl þ with [6]helicene: Experimental and DFT study Emanuel Makrlík a, * , David Sýkora b , Stanislav B € ohm b , Petr Va  nura b , Vladimír Církva c , Jan Storch c , Miroslav Pol  a  sek d a Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kamýck a 129, 165 21 Prague 6, Czech Republic b Institute of Chemical Technology, Prague, Technick a 5, 166 28 Prague 6, Czech Republic c Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojov a 2/135, 165 02 Prague 6, Czech Republic d J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej skova 2155/3, 182 23 Prague 8, Czech Republic article info Article history: Received 19 May 2014 Received in revised form 26 May 2015 Accepted 25 June 2015 Available online 30 June 2015 Keywords: Univalent thallium cation [6]Helicene Complexation DFT calculations Structures abstract By using electrospray ionization mass spectrometry (ESI-MS), it was proven experimentally that the univalent thallium cation forms with [6]helicene (C 26 H 16 ) the cationic complex species [Tl(C 26 H 16 )] þ in the gas phase. Further, applying quantum mechanical DFT calculations, the most probable structure of the [Tl(C 26 H 16 )] þ complex was derived. In the resulting complex, the “central” cation Tl þ is bound by six bonds to six carbon atoms from the two terminal benzene rings of the parent [6]helicene ligand via cation-p interaction. Finally, the interaction energy, E(int), of the considered cation-p complex [Tl(C 26 H 16 )] þ was found to be 144.8 kJ/mol, confirming the formation of this cationic complex species as well. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Helicenes are polycyclic aromatic compounds consisting of ortho-fused benzene rings with nonplanar topology with C 2 -sym- metric axis perpendicular to the axis of helicity as a result of the steric repulsive interaction between terminal aromatic rings [1e3]. This makes them chiral even though they have no center of chirality. The highly delocalized large p-electron system of fully aromatic helicenes along with the previously mentioned inherent chirality predetermines their unique optical [4] and electronic [5] properties, as well as their use in many fields of research, including supramolecular chemistry [6], molecular recognition [7,8], and asymmetric organoeor transition metal catalysis [9,10]. Since helicenes are commercially available, their derivatization studies have emerged [11]. Cation-p interaction refers to the noncovalent attraction be- tween a cation (e.g., Li þ , Na þ , or K þ ) and a p-system (e.g., benzene) [12]. Its strength is often comparable with the interaction between a cation and traditional ligands, including water, alcohols, and amines. As a result of the cation-p interaction, there are extraor- dinarily important driving forces in molecular recognition pro- cesses in many biological and artificial systems [13e16]. The cation- p interaction is a well established phenomenon in the gas phase, as well as in the solid state [17e23], and is known to play an important role in the stabilization of tertiary structures of various proteins [24]. In the current work, electrospray ionization mass spectrometry (ESI-MS) was used as an experimental technique for characteriza- tion of the cation-p interaction between the univalent thallium cation (Tl þ ) and the electroneutral [6]helicene (C 26 H 16 ; see Scheme 1) ligand in the gas phase. At this point it should be noted that the numerous arene complexes of Tl(I) have been described in the solid state [25], however, up to now, a cation-p complex with [6]helicene has not been proven. Moreover, applying quantum mechanical DFT calculations, the most probable structure of the experimentally proven cationic complex [Tl(C 26 H 16 )] þ was predicted. 2. Experimental [6]Helicene (puriss., 99%) was purchased from Lach-ner, Czech Republic, while thallium(I) trifluoromethanesulfonate (puriss., * Corresponding author. E-mail address: makrlik@centrum.cz (E. Makrlík). Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc http://dx.doi.org/10.1016/j.molstruc.2015.06.080 0022-2860/© 2015 Elsevier B.V. All rights reserved. Journal of Molecular Structure 1100 (2015) 150e153