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High level ab initio calculations are employed to investigate the excess electron attachment to the hydrated hydrohalogen acids. The excess electron leads to the dissociation of hydrogen halide acids, which results in the release of a hydrogen radical. Neutral HCl, HBr, and HI are dissociated by tetrahydration. Upon binding an excess electron, these hydrated hydrohalogen acids show that (i) the H–X bond strength weakens with redshifted H–X stretching frequencies, (ii) HX can have a bound-electron state, a dissociated structure, or a zwitter-ionic structure, and (iii) HCl/HBr is dissociated by tri/mono-hydration, while HI is dissociated even without hydration. This dissociation is in contrast to the case of electron attachment to hydrated hydrogen fluoric acids for which HF is not dissociated by more than ten water molecules. [ABSTRACT FROM AUTHOR]

On the basis of density functional theory (DFT) and high level ab initio theory, we report the structures, binding energies, thermodynamic quantities, IR spectra, and electronic properties of the hydride anion hydrated by up to six water molecules. Ground state DFT molecular dynamics simulations (based on the Born-Oppenheimer potential surface) show that as the temperature increases, the surface-bound hydride anion changes to the internally bound structure. Car-Parrinello molecular dynamics simulations are also carried out for the spectral analysis of the monohydrated hydride. Excited-state ab initio molecular dynamics simulations show that the photoinduced charge-transfer-to-solvent phenomena are accompanied by the formation of the excess electron-water clusters and the detachment of the H radical from the clusters. The dynamics of the detachment process of a hydrogen radical upon the excitation is discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have studied the dissociation phenomena of sodium halides by water molecules. The structures, binding energies, electronic properties, and IR spectroscopic features have been investigated by using the density-functional theory, second-order Mo\ller-Plesset perturbation theory, and coupled clusters theory with single, double, and perturbative triplet excitations. In the case that the sodium halides are hydrated by three water molecules, the most stable structures show the partial (or half) dissociation feature. The dissociated structures are first found for NaX(H2O)n=5 for X=Br/I, though these structures are slightly higher in energy than the global minimum-energy structure. In the case of hexahydrated sodium halides the global minimum-energy structures (which are different from the structures reported in any previous work) are found to be dissociated (X=F/I) or partially/half dissociated (X=Cl/Br), while other nearly isoenergetic structures are undissociated, and the dissociated cubical structures are higher in energy than the corresponding global minimum-energy structure. [ABSTRACT FROM AUTHOR]

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]

We have carried out ab initio and density functional theory calculations of hydrated rubidium cations. The calculations involve a detailed evaluation of the structures, thermodynamic properties, and IR spectra of several plausible conformers of Rb+(H2O)n=1–8 clusters. An extensive search was made to find out the most stable conformers. Since the water-water interactions are important in hydrated Rb+ complexes, we investigated the vibrational frequency shifts of the OH stretching modes depending on the number of water molecules and the presence/absence of outer-shell water molecules. The predicted harmonic and anharmonic vibrational frequencies of the aqua-Rb+ clusters reflect the H-bonding signature, and would be used in experimental identification of the hydrated structures of Rb+ cation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

To understand the mechanism of aqueous base dissociation chemistry, the ionic dissociation of cesium–hydroxide in water clusters is examined using density functional theory and ab initio calculations. In this study, we report hydrated structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of CsOH·(H2O)n=0–4. With the addition of water molecules, the Cs–OH bond lengthened significantly from 2.46 Å for n=1 to 3.08 Å for n=4, which causes redshift in Cs–O stretching frequency. It is found that three water molecules are needed for the dissociation of Cs–OH, in contrast to the case of strong acid dissociation which requires at least four water molecules. However, the dissociation for n=3 could be considered as incomplete because a very weak CS...OH stretch mode is still present, while that for n=4 is complete since the Cs...OH mode no longer exists. This study can be related with hydration chemistry of cations and anions, and extended into the intra- and intercharge-transfer phenomena. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using ab initio and density functional calculations, we studied photoexcitation of a charge-balanced electron donor–acceptor (DA) complex comprised of tetracyanoethylene (TCE) and tetramethylethylene (TME). We considered both the TCE-TME stacked conformer and a possible conformer with a solvent molecule (dichloromethane) inserted between TCE and TME. The photoexcitation of the DA complex can directly form a charge transfer (CT) state. Our theoretical investigations show that the CT state can also be produced from the decay of higher excited states. Using the continuum model, we investigated the solvent effects on CT absorption, local excitation, and CT emission in the polar solvent. The equilibrium solvation energies of the ground and excited states of the DA complex were calculated using the self-consistent reaction field method, and then the correction of nonequilibrium solvation energies for the vertical transitions was made. The transition energies (i.e., CT absorption for the DA complexes and CT emission for the contact ion pair complexes) in the polar solvent show redshifts relative to those in the gas phase. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The study of electron solvation dynamics is vital for understanding the phenomena related to the electron transfer process in solvents. On the basis of a recent femtosecond dynamics study of charge-transfer-to-solvent states in photoexcited iodide–water clusters [Lehr et al., Science 284, 635 (1999)], we have investigated the solvent rearrangement process for the excited electron in the iodide–water hexamer using ab initio calculations. Upon excitation of iodide–water hexamer, an electron transfers from the iodide anion to the water cluster. This results in release of the iodine atom and thereby formation of anionic water hexamer which undergoes rearrangement process toward the most stable conformation. The transformation pathway from the low-lying energy structures of the iodide–water hexamer to those of the electron–water hexamer is thus elucidated from the potential energy surface including the global and local minima and transition states of the electron–water hexamer. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Owing to the utility of redox phenomena of silver in many chemical systems, it is important to understand the coordination chemistry of Ag[sup +] ion and hence the hydration structure. The lowest-energy conformations of Ag[sup +](H[sub 2]O)[sub 1–6] are sensitive to the calculation method employed. The coordination number (N[sub c]) of Ag[sup +](H[sub 2]O)[sub n] is predicted to be 2 for n=2–6 at the density functional theory level, while the N[sub c] for n=3–5 is 3, and that for n=6 is 4 at the second-order Møller–Plesset perturbation level. Further accurate analysis based on coupled-cluster singles and doubles theory with perturbative corrections for triple excitations agrees with the MP2 results except that N[sub c] of 4 is also as competitive as N[sub c] of 3 for n=5. To identify the correct N[sub c], it would be useful to facilitate the IR experimental characterization. We thus provide the OH spectra for various possible structures. It is interesting to note that the hydration chemistry of Ag[sup +] ion is somewhat different from that of alkali metal ions. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies photoswitch molecules of the 1,2-bis-(3-thienyl)-ethene derivatives class. Structures; Photophysics; Molecular properties at the ground and excited states; Applications in the design of nonlinear optical switches.

The water hexamer is known to have a number of isoenergetic structures. The first experimental identification of the O-H stretching vibrational spectra of the water hexamer was done in the presence of benzene. It was followed by the identification of the pure water hexamer structure by vibration-rotational tunneling (VRT) spectroscopy. Although both experiments seem to have located only the Cage structure, the structure of the benzene-water hexamer complex is not clearly known, and the effect of benzene in the water hexamer is unclear. In particular, it is not obvious how the energy difference between nearly isoenergetic water hexamer conformers changes in the presence of benzene. Thus, we have compared the benzene complexes with four low-lying isoenergetic water hexamers, Ring, Book, Cage, and Prism structures, using ab initio calculations. We also investigated the effects of the presence of benzene on the structures, harmonic vibrational frequencies, and infrared (IR) intensities for the four low-lying energy conformers. There is little change in the structure of the water hexamer upon its interaction with the benzene molecule. Hence the deformation energies are very small. The dominant contribution to the benzene-water cluster interaction mainly comes from the π-H interactions between benzene and a single water molecule. As a result of this π-H interaction, O-H[sub π] bond length increases and the corresponding stretching vibrational frequencies are redshifted. The IR spectral features of both (H[sub 2]O)[sub 6] and benzene-(H[sub 2]O)[sub 6] are quite similar. From both the energetics and the comparison of calculated and experimental spectra of the benzene-(H[sub 2]O)[sub 6], the water structure in these complexes is found to have the Cage form. In particular, among the four different Cage structures, only one conformer matches the experimental O-H vibrational frequencies. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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]

A number of experimental and theoretical studies have been carried out on the anionic water hexamer in the last decade. However, none of these studies have reported the adiabatic electron detachment energy. The present study employing extensive high-level ab initio calculations report the adiabatic electron detachment energy, which explains the unusual stability of the anionic water hexamer. This stability can be correlated to the unusually intense peak observed in the photoelectron-detachment spectra. It is also shown that our previously predicted pyramid structure reproduces the important characteristics of the experimental O-H vibrational spectra. © 2000 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]