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The design of cesium-selective ionophores must include the nature of cesium-water interactions. The authors have carried out extensive ab initio and density functional theory calculations of hydrated cesium cations to obtain reasonably accurate energetics, thermodynamic quantities, and IR spectra. An extensive search was made to find the most stable structures. Since water...water interactions are important in the aqua-Cs+ clusters, the authors investigated the vibrational frequency shifts as a function of the number of water molecules and the frequency characteristics with and without the presence of outer-shell water molecules. The predicted vibrational frequencies were then compared with the infrared photodissociation spectra of argon-tagged hydrated cesium cluster ions. This comparison allowed the identification of specific hydrogen-bonding structures present in the experimental spectra.

IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine-(H(2)O)(n) clusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of L-phenylalanine (L-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for L-Phe-(H(2)O)(1), and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In L-Phe-(H(2)O)(2), only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of L-Phe-(H(2)O)(3) the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the pi-system. Finally, in isolated L-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initio calculation.

Using molecular-orbital analysis, we have elucidated the quantum-chemical origin of the intriguing phenomena in sequential hydration energies of the gold cation, which is known to be the most conspicuous among all transition metals. The hydration energy of Au+ with the second water molecule is found to be much larger than that with the first water molecule. Owing to the large relativistic effect of gold (i.e., significant lowering of the 6s orbital energy and significant raising of the 5d orbital energy), the highest occupied molecular orbital of the hydrated gold cation has a large portion of the 6s orbital. As the electron density of the 6s orbital populates in a large outer spherical shell far off the gold nucleus, the p orbitals (or sp hybridized lone-pair orbitals) of the water molecules are able to overlap with the outer part of the 6s orbital in the dihydrated gold cation, resulting in the unusual skewed overlap of p-6s-p orbitals (not the atom-to-atom bond overlap). No previous molecular-orbital analysis has reported this peculiar skewed orbitals overlap. Since this skewed orbitals overlap is saturated with two water molecules, this property is responsible for the low coordination number of the gold ion. [ABSTRACT FROM AUTHOR]

Investigates the possibility of the benzene-water van der Waals bonded systems to exhibit a pi-H kind of bonding interaction. Ab initio calculations on different conformations of the fluorobenzene-water van der Waals bond; Involvement of the bond formed between benzene and water in the formation of a six-membered ring system with the F-C-C-H of the aromatic ring.

We investigate the interaction of the water trimer with the aromatic π systems (benzene, toluene, fluorobenzene, and p-difluorobenzene) at the second-order Møller-Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets. The minimum-energy structures, binding energies, and the harmonic vibrational frequencies were calculated for all these systems. The minimum-energy structures reveal that the water trimer has only a π type of interaction with benzene and toluene, but has both a π and σ type (H-bond with the fluorine atom) of interaction with fluorobenzene and p-difluorobenzene. The binding energies reveal that, contrary to what has been observed in previous theoretical studies, the strength of the interaction of these π systems with the water trimer is greater than with the water dimer. A good agreement is obtained between the calculated and experimentally measured vibrational frequencies of these complexes. An analysis of the calculated OH stretching modes reveals some unusual facets of the π and σ type of interaction observed in the fluorobenzene and p-difluorobenzene complexes. Thus one observes that the OH red-shift associated with the π type of interaction is larger than with the σ type of interaction. This is particularly interesting because in both the water monomer and water dimer complexes, the OH red-shift associated with the interaction of the water cluster with fluorobenzene and p-difluorobenzene is due to the formation of the H-bond with the fluorine atom. Thus the increase in the size of the water cluster seems to favor the π type of interaction over the σ type of interaction. This inference is also supported by the magnitude of the two-body terms associated with the π and σ type of interaction in both the fluorobenzene and p-difluorobenzene complexes. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The correlation of various properties of water clusters (H[sub 2]O)[sub 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: ν[sub 3][sup da] (single donor-single acceptor) ≅ν[sub 3][sup daa] (single donor-double acceptor) >ν[sub 3][sup dda] (double donor-single acceptor) >ν[sub 1][sup dda]>ν[sub 1][sup da]> (or ≅) ν[sub 1][sup daa]. 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 (ν[sub 3][sup da] and ν[sub 3][sup daa]). The intensities of ν[sub 1][sup daa] and ν[sub 1][sup da] 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 ν[... [ABSTRACT FROM AUTHOR]

The interaction of the water dimer with both the olefinic and aromatic π systems (ethene, benzene, toluene, fluorobenzene, and p-difluorobenzene) has been investigated using both the supermolecular [second-order Møller-Plesset (MP2)] and perturbational (symmetry adapted perturbation theory) approaches. The geometry optimizations, harmonic vibrational frequencies, and the components of the binding energy were evaluated using fairly large basis sets (6-31+G[sup *] and aug-cc-pVDZ). The minimum energy structures obtained at the MP2/6-31+G[sup *] and MP2/aug-cc-pVDZ levels of theory indicate that the water dimer exhibits a π-type of interaction with ethene, benzene, and toluene and a σ-type of interaction with both fluorobenzene and p-difluorobenzene. This is demonstrated from the vibrational frequencies which are in good agreement with the experimentally determined numbers. Our calculations indicate that the nature and strength of the interaction of the donor water molecule (water dimer) with the π system has a significant bearing on the total binding energy of the complex. Apart from the interaction of the water dimer with the π system, we also show how this interaction influences the hydrogen bond characteristics of the water dimer by evaluating the hydrogen bond strengths. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]