A Combined Experimental and Theoretical Study on the Conformational Behavior of a Calix[6]arene Be ´ atrice Boulet, †,‡ Laurent Joubert, † Ge ´ rard Cote, † Ce ´ line Bouvier-Capely, ‡ Catherine Cossonnet, ‡ and Carlo Adamo* ,† Laboratoire d’Electrochimie et Chimie Analytique, UMR CNRS-ENSCP 7575, Ecole Nationale Supe ´ rieure de Chimie de Paris, 11 rue P. et M. Curie, F-75231 Paris Cedex 05, France, and IRSN/DRPH/SDI/LRC, BP 17, F-92262 Fontenay-aux-Roses CEDEX, France ReceiVed: NoVember 11, 2005; In Final Form: February 21, 2006 An experimental and theoretical study on the conformational behavior of the 1,3,5-OMe-2,4,6-OCH 2 CONHOH- p-tert-butylcalix[6]arene has been carried out. In particular, semiempirical (AM1) and density functional theory (DFT) calculations have been performed in order to identify the possible conformers. The obtained results show that the cone structure is the most stable conformer at any level of theory, even if significant differences have been obtained for the other species. The inclusion of solvent effect, through a continuum model, also points out the relevant role played by the solvent in the stabilization of the cone structure in solution. These latter results have been confirmed by NMR experiments, which clearly show the presence of only the cone conformer in a polar solvent, such as DMSO. Finally, 1 H and 13 C NMR spectra on model systems, i.e., two successive phenol rings (Ar 1 -CH 2 -Ar 2 ), have been computed at the DFT level and compared with the experimental spectra of the complete molecule. The results show an overall good agreement with the experimental data, thus leading to an unambiguous assignment of the experimental spectra. 1. Introduction Calixarenes have been extensively studied during the past decade, due to their versatility in terms of complexing capability, conformational flexibility, and reactivity. Such peculiar chemical characteristics have been exploited in a large variety of chemical applications, including catalysis, host-guest chemistry, separa- tion chemistry, selective ion transport, and sensors. 1 Within these applications, these macrocyclic molecules are particularly interesting for their properties related to the actinide extraction. 2,3 In fact, they can selectively extract neutral or charged molecules, and such properties can be easily modified and tuned, since they depend on the size of the molecular cavity, the geometry of coordination, and the functionalization of lateral groups. 4 A large number of experimental studies on the complexation mechanism have been carried out in the past years. 3-8 Thus, calix[6]arenes bearing carboxylic, phosphonic, or hydroxamic acid groups have been widely studied as specific ligands for the uranyl ion (UO 2 2+ ). 9-17 More recently, it has been shown that calix[6]arenes bearing hydroxamic functions, such as the 1,3,5-OMe-2,4,6OCH 2 CONHOH-p-tert-butylcalix[6]arene (Fig- ure 1) are particularly suitable to complex selectively the uranyl ion. 11-13,18 In an experimental context, the understanding of the confor- mation behavior of calixarenes is a necessary prerequisite to rationalize their properties and to drive future syntheses. Experimental studies showed that calix[n]arenes present a preorganized flexible structure in solution. 8 These macrocyclic molecules are composed of n phenolic units linked by methylene bridges at ortho positions, with various groups on the upper (the para position) and lower rims (phenolic OR groups). 19 Their flexibility is due to two possible rotational modes of the phenol units, the so-called “oxygen-through-the-annulus” (OTA) and the “para-substituent-through-the-annulus“ (PSTA) rotations. 20 Experimental conformational features of calixarenes have been deeply analyzed by NMR spectroscopy, using 1 H, 13 C, time- scale dependence, NOESY, and ROESY techniques. 20-24 In particular, Van Duynhoven and co-workers studied the influence of lower rim alkyl substituents on the conformations of 1,3,5- OR 1 -2,4,6-OR 2 -p-tert-butylcalix[6]arenes. 23 They suggested that methyl or ethyl R 1 groups favor a highly symmetric (C 3V ) cone * Corresponding author: carlo-adamo@enscp.fr. † ENSCP. ‡ IRSN. Figure 1. Structure of most stable conformer of 1,3,5-OMe-2,4,6- OCH2CONHOH-p-tert-butylcalix[6]arene (cone conformation). Hy- drogen atoms are omitted for clarity. 5782 J. Phys. Chem. A 2006, 110, 5782-5791 10.1021/jp0565305 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006