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...

Electron-bound water clusters [e(-)(H(2)O)(n)] show very strong peaks in mass spectra for n=2, 6, 7, and (11), which are called magic numbers. The origin of the magic numbers has been an enigma for the last two decades. Although the magic numbers have often been conjectured to arise from the intrinsic properties of electron-bound water clusters, we attributed them not to their intrinsic properties but to the particularly weak stability of the corresponding neutral water clusters (H(2)O)(n=2,6,7, and (11)). As the cluster size increases; this nonsmooth characteristic feature in stability of neutral water clusters is contrasted to the smooth increase in stability of e(-)-water clusters. As the magic number clusters have significant positive adiabatic electron affinities, their abundant distributions in atmosphere could play a significant role in atmospheric thermodynamics.

We have studied the electronic structures, energetics, electron vertical detachment energies (VDEs), and O–H vibrational spectra of various conformers of water clusters with an excess electron [e+(H2O)n, n=2–5] or anionic water clusters [(H2O)n−] using comprehensive ab initio calculations. As noted in our preliminary work [J. Kim et al., Phys. Rev. A 59, 930 (1999)], the structure of the water dimer anion is characterized to be linear-like (slightly towards the cis conformer) but very floppy with large wide-ranging zero point vibration motion at 0 K. The lowest energy structures of the water trimer to pentamer anion are all cyclic with very small VDEs (< 0.05 eV). However, these cyclic structures which are metastable are prone to become the neutral species by releasing an excess electron because the transition barriers seem to be very small. Thus, observation of such cyclic structures would not be feasible. On the other hand, a linear water trimer structure which is 0.8 kcal/mol higher in energy than the cyclic form gives the VDE (0.14 eV) close to the experimentally observed value. A large VDE observed in the pentamer also corresponds to a slightly high energy conformer. This suggests that formation of anionic water clusters in experiments seems to be dynamically and kinetically driven.

Using the computer-aided molecular design approach, we recently reported the synthesis of calix[4]hydroquinone (CHQ) nanotube arrays self-assembled with infinitely long one-dimensional (1-D) short hydrogen bonds (H-bonds) and aromatic-aromatic interactions. Here, we assess various calculation methods employed for both the design of the CHQ nanotubes and the study of their assembly process. Our calculations include ab initio and density functional theories and first principles calculations using ultrasoft pseudopotential plane wave methods. The assembly phenomena predicted prior to the synthesis of the nanotubes and details of the refined structure and electronic properties obtained after the experimental characterization of the nanotube crystal are reported. For better characterization of intriguing 1-D short H-bonds and exemplary displaced pi-pi stacks, the X-ray structures have been further refined with samples grown in different solvent conditions. Since X-ray structures do not contain the positions of H atoms, it is necessary to analyze the system using quantum theoretical calculations. The competition between H-bonding and displaced pi-pi stacking in the assembling process has been clarified. The IR spectroscopic features and NMR chemical shifts of 1-D short H-bonds have been investigated both experimentally and theoretically. The dissection of the two most important interaction components leading to self-assembly processes would help design new functional materials and nanomaterials.