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Cation-[Pi] 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. The interaction energies of [n]collarenes with cations have been evaluated by using ab initio calculations. The selectivity of these [n]collarenes in aqueous solution was revealed by using statistical perturbation theory in conjunction with Monte Carlo and molecular dynamics simulations. It has been observed that in [n]collarenes the ratio of the interaction energies of a cation with it and the cation with the basic building unit (benzene) can be correlated to its ion selectivity. We find that collarenes are excellent and efficient ionophores that bind cations through cation-[Pi] interactions. [6]Collarene is found to be a selective host for [Li.sup.+] and [Mg.sup.2+], [8]collarene for [K.sup.+] and [Sr.sup.2+], and [10]collarene for [Cs.sup.+] and [Ba.sup.2+]. This finding indicates that [10]collarene and [8]collarene could be used for effective separation of highly radioactive isotopes, 137Cs and 90Sr, which are major constituents of nuclear wastes. More interestingly, collarenes of larger cavity size can be useful in capturing organic cations. [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 ionophores/model-receptors capable of heralding a new direction in molecular recognition and host-guest chemistry.

A series of strained amides such as N-formylazetidine, N-formylaziridine and their protonated tautomers were analyzed to characterize the mechanisms of N- and O-protonation in strained molecular systems. Ab initio calculations of strained N-formylazetidine and N-formylazidine indicated the presence of planar conformations that mediated the hydrolysis of strained amide systems. The stereoselectivity of the strained molecular systems containing a nitrogen lone pair was also dependent on stereoelectronic and electrostatic effects.

The two dimensional (2D) to three dimensional (3D) transition for the protonated water cluster has been controversial, in particular, for H+(H2O)7. For H+(H2O)7 the 3D structure is predicted to be lower in energy than the 2D structure at most levels of theory without zero-point energy (ZPE) correction. On the other hand, with ZPE correction it is predicted to be either 2D or 3D depending on the calculational levels. Although the ZPE correction favors the 3D structure at the level of coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] using the aug-cc-pVDZ basis set, the result based on the anharmonic zero-point vibrational energy correction favors the 2D structure. Therefore, the authors investigated the energies based on the complete basis set limit scheme (which we devised in an unbiased way) at the resolution of the identity approximation Mo\ller-Plesset second order perturbation theory and CCSD(T) levels, and found that the 2D structure has the lowest energy for H+(H2O)7 [though nearly isoenergetic to the 3D structure for D+(D2O)7]. This structure has the Zundel-type configuration, but it shows the quantum probabilistic distribution including some of the Eigen-type configuration. The vibrational spectra of MP2/aug-cc-pVDZ calculations and Car-Parrinello molecular dynamics simulations, taking into account the thermal and dynamic effects, show that the 2D Zundel-type form is in good agreement with experiments. [ABSTRACT FROM AUTHOR]

Despite intensive studies of the neutral tropyl radical, none of its structure, energetics, and vibrational modes are still clear. This system has puzzled scientists for over a decade since one vibrational mode frequency sharply varies from imaginary number 3000i cm-1 to the real number 6000 cm-1, depending on the calculation methods employed. We find that the origin of this peculiar mode is due to the pseudorotation (ωirot) involved in the interconversion of two nearly isoenergetic Jahn-Teller configurations (elongated structure 2B1 and compressed structure 2A2 with C2v symmetry). Here, we first report that this interconversion is not via D7h or C2v symmetry configuration but via Cs symmetry (i.e., by changing the C2v axis). This interconversion barrier is found negligibly small. Thus, the two conformers are considered to be not two different structures but a dynamically identical structure with partial quantum statistical distributions on the potential energy surface. Owing to the nearly barrierless pseudorotation, the overall structure in a short time scale (less than femtosecond) would be Cs-like between 2A2 and 2B1 configurations with small fluctuation of bond distances. However, the dynamical transitions between the 2B1 and 2A2 configurations via 14 different pseudorotation pathways would make the tropyl radical have the effective D7h structure in either a nonshort time scale (greater than femtosecond) or at nonlow temperatures, which explains the high temperature electron spin resonance experiments. [ABSTRACT FROM AUTHOR]

Electron-bound water clusters [e-(H2O)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 (H2O)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. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated whether cyclacenes and collarenes can behave as ionophores and receptors. Methodology used to conduct the study; Information on the trend observed in the complexes of [n]collarenes with alkaline-earth metals; Findings of the study.

X[sup -]·(H[sub 2]O)[sub 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[sup -]·(H[sub 2]O)[sub n] and X[sup -]·(H[sub 2]O)[sub 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[sup -]·(H[sub 2]O)[sub n], a significant charge transfer is noted in the S[sub 0] ground state, which results in much less significant charge transfer in the S[sub 1] excited state compared with other hydrated halide clusters which show near full charge transfers in the S[sub 1] excited states. Finally, the nature of the stabilization interactions operative in these clusters has been explained in terms of many-body interaction energies. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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.

The possible utilization of [n]beltenes as a new family of ionophores, which exhibit a cation-olefinic pi type of interaction in contrast to the cation-aromatic pi type of interaction exhibited by [n]collarenes, has been investigated using both ab initio calculations and molecular dynamic simulations. Like [n]collarenes, n ethene groups are linked by -CH(2)- linkages in the [n]beltenes. Our calculations indicate that these [n]beltenes exhibit strong binding affinities and high selectivity for alkali metal cations ([5]beltene to Li(+), [6]beltene to Na(+), [7]beltene to Na(+) and K(+), [8]beltene to K(+) and Rb(+), and [9]beltene to Cs(+) and Rb(+)). Compared to [n]collarenes, [n]beltenes are expected to have a finer ion selectivity because their cavity sizes can be varied with integral number n, while that of the former can be varied with an even number n. Suitable substituents could be employed to enhance both the binding and specificity of various sizes of [n]beltenes to different cations, as well as to increase the solubility.