We report a molecular dynamics study of the solvation of 18-crown-6 (''18C6'') and of its K 1 , Cs 1 and Sr 21 complexes in a room-temperature ionic liquid (IL) based on 1-butyl-3-methyl-imidazolium 1 , PF 6 À . ''Dry''models of the IL are compared, demonstrating the importance of solvent humidity on the solvation properties. Upon ''dissolution'' of a piece of crystal, 18C6 is found to undergo a conformational change from C i to D 3d , mainly due to enhanced interactions with the BMI 1 solvent cations and H 2 O molecules, when present. The complexes were first studied with dissociated counterions. In the dry IL, the complexed K 1 and Sr 21 cations are locked at the center of the crown by 1 þ 1 (K 1 ), 1 þ 2 or 1 þ 3 (Sr 21 ) PF 6 À anions in facial positions, respectively. The Cs 1 cation is perched over the crown, solvated by 3 PF 6 À anions. In the humid IL, the complexed K 1 also binds to 1 þ 1 PF 6 À facial anions only (no water), whereas Sr 21 is asymmetrically coordinated to at least 3 H 2 O molecules. When co-complexed with Cl À or NO 3 À counterions, Sr 21 is shielded from the dry IL, but coordinates up to 3 additional H 2 O molecules in the humid IL, while K 1 is not hydrated. The solvation of the ''naked'' K 1 , Cs 1 and Sr 21 ions also markedly depends on the solvent humidity. K 1 is coordinated to 4 PF 6 À anions in the dry IL and by 2 PF 6 À plus 3-5 H 2 O in the humid IL. The most spectacular difference concerns Sr 21 , whose first shell is purely anionic (5 PF 6 À ) in the dry IL, but all neutral (8 H 2 O) in the humid IL. According to an energy component analysis, the 18C6 crown, the cations and their complexes are better solvated by the humid than by the dry IL. Finally, we report simulations of 18C6 and on its Sr C 18C6(NO 3 ) 2 complex at the aqueous interface with the ionic liquid, showing enhanced solvent mixing, compared to the interface with classical organic liquids. The microscopic views obtained by these simulations show the active role of the ionic and aqueous components of the liquid on the solvation of the free crown, the free cations and their complexes.

Quantum chemical (QM), classical molecular dynamics (MD), and Car-Parrinello (CP-MD) studies are reported for 18-crown-6 (18C6) and its first 18C6(H 2 O) n hydrates, focusing on the D 3d and C i forms of the crown. They reveal the importance of dynamics and the surrounding medium on its conformational and hydrogen-bonding properties. In the gas phase, the two forms of the free crown are found to be quasi-isoenergetic at several computational levels, but during CP-MD simulations, the D 3d form is more mobile than C i and undergoes conformational changes in such a way as "to fill its own cavity". Among several forms of the monohydrate, the one with a D 3d -type crown and a bridging water molecule is most stable. Along its 10-ps CP-MD trajectory, the H 2 O molecule undergoes a "merry-go-round" dynamics, exchanging between the three "top" oxygens of the deformed crown, and is thus more often instantaneously monodentate than bidentate. The static and dynamic results of different forms of the mono-and dihydrates confirm the importance of dynamic bridging coordination to 18C6. These results solve the apparent contradiction between IR spectroscopic results in humid CCl 4 or supercritical-CO 2 solutions that hint at an equilibrium between monodentate and bidentate hydrogen bonds, whereas in other humid phases (solid-state structures, liquid hydrates, simulated aqueous solutions), the hydrated crown is always D 3d -like and the first coordinated H 2 O molecules are bridging. IntroductionLike (poly)cyclic complex-forming molecules, 1,2 18-crown-6 (18C6) was early recognized to act as a selective host for charged atoms and small molecules and to display fundamental features of molecular recognition: preorganization and macrocyclic effects, flexibility, induced fit upon ligand binding, and a solvent effect on its recognition properties. 3-5 From a basic point of view, it is important to understand the structure of 18C6 as a function of its environment. In the solid state, 18C6 alone is of an elongated shape of C i symmetry (no cavity), and the D 3d form (Figure 1) with a cavity is commonly observed with cationic guests (e.g., K + , R-NH 3 + , H 3 O + , NH 4 + ) as well as in interaction with dipolar molecules (e.g., acetonitrile, water). 6-8 Other symmetries are observed for 18C6 itself (e.g., C 1 within the Na + complex, another C i form in the benzene-sulfonamide adduct) or its derivatives (e.g., dicyclohexyl, dibenzo). 6 These data and computer simulations in the gas phase 9-12 and in solution [13][14][15][16][17][18] show that the structure of 18C6 is highly versatile and depends on its environment.The present study was motivated by spectroscopic IR results on 18C6 hydrates formed in humid organic phases such as CCl 4 19 or supercritical CO 2 (SC -CO 2 ), 20 according to which there is an equilibrium between the monodentate and bidentate coordination of water in the 1:1 adduct with 18C6 ( Figure 2). The corresponding conformation of 18C6 was not established. However, early theoretical simulations on 18C6 in aqueous soluti...

We present a molecular dynamics study of 18C6, 15C5 crown ethers and their acyclic polyether analogues at the water/supercritical-CO 2 interface. The aqueous and CO 2 components form distinct phases, separated by an interface, where in all systems, the ethers are found to concentrate. Simulations of the inclusive K + &18C6 Pic À complexes led to decomplexation of K + and accumulation of the crown ethers on the CO 2 side of the interface, while Pic À anions stack in the aqueous phase. When the K + cations are constrained to form inclusive complexes with 18C6, all complexes concentrate at the interface, be the counterions free to move, or constrained to coordinate to K + . In the presence of nitric acid, modeled by equimolar mixtures of HNO 3 , NO 3À and H 3 O + forms, the ethers remain surface active, as does the neutral form of the acid. These simulations demonstrate the importance of interfacial phenomena in assisted ion extraction to supercritical-CO 2 .