18-Crown-6 and Its Hydrates:  Bridging but Versatile Hydrogen Bonding. A Theoretical Study of Static and Dynamic Properties

Abstract

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...

Infrared spectroscopy of gas-phase hydrated K+:18-crown-6 complexes: Evidence for high energy conformer trapping using the argon tagging method

“…The low internal energies limit the degree to which these species can surmount barriers to rearrangement to lower energy structures, and thus higher energy conformers may be trapped. This seems to be the case in the B conformer where the neutral bidentate (18c6)H 2 O, mentioned earlier [15] as one of the preferred configurations for the monohydrated neutral system, may first be formed in the argon cluster with a sufficient barrier for K + to replace the bidentate H 2 O inside the 18c6 cavity. We performed DFT calculations on the neutral (18c6)H 2 O system to gain insight on the B conformer, since the aforementioned hydrated alkali metal ion systems [18] showed that the high energy conformers, trapped in our previous experiments, mirror robust neutral structures that failed to undergo rearrangement after ion impact.…”

“…Further calculation details are available in supporting information section. Previous theoretical work [15] indicates that there are three low lying conformations for 18c6(H 2 O), two of which feature single hydrogen bonds and one with a bidentate hydrogen-bonded water. In K + (18c6)(H 2 O), previous ab initio calculations by Feller [10] indicate that the most favored configuration features K + nestled inside the 18c6 cavity with H 2 O coordinated to K + and forming a hydrogen bond with one 18c6 oxygen.…”

Quantum mechanical calculations of the interactions between diazacrowns and the sodium cation: an insight into Na+ complexation in diazacrown-based synthetic ion channels

“…To the best of our knowledge no QM studies of the complexation of Na + and diaza-18-crown-6 have been reported but many simulation studies have been made involving the complexation of crown ethers and their derivatives with ions. Many insights into the complexation and conformation of the crown ethers have been previously gained by quantum mechanical [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] and force eld simulations. [39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] These studies, due to the similarity of 18-crown-6 to the diaza-18-crown-6, provide us with a point of reference for comparison.…”

A sandwich strand supramolecular complex: aquachloro(nitrilotriacetato-κ2 N,O)copper(II) 18-crown-6-ether trihydrate

“…The wings link to the water molecule O9 via hydrogen bonds (Table 2), and the water molecules also hydrogen bond to the crown ether O atoms, stabilizing the extensive sandwich array of Cu(H 2 NTA)-(H 2 O)Cl units between successive macrocyclic 18-crown-6ether units. Every 18-crown-6-ether ligand associated with two water molecules on opposite sides adopts the conventional conformation with pseudo-D 3d symmetry (Schurhammer et al, 2003). This gives rise to a hydrogen-bonded one-dimensional chain with a succession of Cu(H 2 NTA)(H 2 O)Cl, H 2 O and 18crown-6 molecules.…”