X − ⋅(H 2 O) n=1–4  [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor internal structures with negligible hydrogen bonding between water molecules, while the latter favors surface structures with significant hydrogen bonding between water molecules. These characteristics are well featured in their O–H spectra of the clusters. However, the spectra are forced to be very sensitive to the temperature, which explains some differences between different spectra. In case of F−⋅(H2O)n, a significant charge transfer is noted in the S0 ground state, which results in much less significant charge transfer in the S1 excited state compared with other hydrated halide clusters which show near full charge transfers in the S1 excited states. Finally, the nature of the stabilization interactions operative in these clusters has been explained in terms of many-body interaction energies.

The correlation of various properties of water clusters (H2O)n=1–10 to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O–H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O–H vibrational frequencies serve as sensitive indicators of the status of proton donation (“d”) and acceptance (“a”) (i.e., the structural signature of H-bond type) for each water monomer in the cluster. In general, the magnitudes of the O–H frequencies (ν) for each cluster can be arranged in the following order: ν3da (single donor–single acceptor) ≅ν3daa (single donor–double acceptor) >ν3dda (double donor–single acceptor) >ν1dda>ν1da> (or ≅) ν1daa. The increase in the cluster size has a pronounced effect on the decrease of the lower frequencies. However, there are small changes in the higher frequencies (ν3da and ν3daa). The intensities of ν1daa and ν1da are very high, since the increased atomic charges can be correlated to the enhanced H-bond relay effect. On the other hand, the intensities of the ν1dda modes are diminished by more than half. Most of the above data have been compared to the available experimental data. Keeping in view the recent experimental reports of the HOH bending modes, we have also analyzed these modes, which show the following trend: ν2dda>ν2daa≅ν2da. The present study therefore would be useful in the assignments of the experimental O–H stretching and HOH bending modes.

It's a kinda magic! Contrary to conventional wisdom that OH bonds associated with dangling hydrogen atoms and those in the H3O+ ion in molecular clusters display characteristic peaks in IR spectra, a dynamic effect makes such peaks disappear, even in the gas phase at low temperatures. This finding helps solve the long‐standing problems of magic and antimagic protonated water clusters with 21 (top structures) and 22 (bottom structures) water molecules.

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Cation-interactions are important forces in molecular recognition by biological receptors, enzyme catalysis, and crystal engineering. We have harnessed these interactions in designing molecular systems with circular arrangement of benzene units that are capable of acting as ionophores and models for biological receptors.[n]Collarenes are promising candidates with high selectivity for a specific cation, depending on n, because of their structural rigidity and well-defined cavity size. [12]Collarene exhibits a pronounced affinity for tetramethylammonium cation and acetylcholine, which implies that it could serve as a model for acetylcholinestrase. Thus, collarenes can prove to be novel and effective ionophores͞model-receptors capable of heralding a new direction in molecular recognition and host-guest chemistry.The design of novel ionophores and receptors has attracted considerable interest in the recent past (1-5). The cationinteraction discovered by Dougherty and coworkers (6, 7) has received much attention as a new type of binding force important in biological molecular recognition (8-15), enzyme catalysis (16, 17), and crystal engineering (3, 18). This cationinteraction is known to be responsible for the binding of acetylcholine (ACh) to its deactivating enzyme (8, 9), acetylcholinestrase (19,20), which has served as a target receptor in designing therapeutic agents against various ailments like myasthenia gravis, glaucoma, and Alzheimer's disease (21-23).A lot of effort and concern has been evinced on the disposal of nuclear wastes. A major bottleneck, however, stems from the effective separation of major hazardous isotopes (such as 137 Cs and 90 Sr, which have fairly long half-lives of Ϸ30 years) from these wastes (24-27). There have been extensive attempts to design and develop systems that can be used in nuclear waste separations as effective ionophores. In this context the use of various types of ionophores has been reported (25-27), and there have been extensive attempts to explore new types of ionophores that have a high selectivity for these hazardous isotopes.It therefore is appealing to investigate the possibility of designing novel ionophores and model receptors based on the principle of the cation-interaction. There have been reports of the systems in which -orbitals are oriented vertically to the plane of the rings, namely belt-shape carbocyclic-conjugated systems (such as annulenes, beltenes, cyclacenes, and collarenes) (28-37). The recent discoveries of fully conjugated systems with a curved surface like fullerenes and carbon nanotubes (38, 39) further adds fuel to such a kind of study. However, belt-shape carbocyclic-conjugated systems have hardly been studied apart from the synthetic study of cyclacene precursors and collarenes (32-37).Thus, it would be of importance to investigate whether cyclacenes (comprised of only benzene moieties) and collarenes (having CH 2 linkages between benzene units) can behave as ionophores and receptors. These molecular structures can be understood f...