Association patterns in (HF)(m)(H2O)(n) (m + n = 2-8) clusters

In an attempt to understand the phase behavior of aqueous hydrogen fluoride, the clustering in the mixture is investigated at the molecular level. The study is performed at the mPW1B95/6-31+G(d,p) level of theory. Several previous studies attempted to describe the dissociation of HF in water, but in this investigation, the focus is only on the association patterns that are present in this binary mixture. A total of 214 optimized geometries of (HF)n(H2O)m clusters, with m + n as high as 8, were investigated. For each cluster combination, several different conformations are investigated, and the preferred conformations are presented. Using multiple linear regressions, the average strengths of the four possible H-bonding interactions are obtained. The strongest H-bond interaction is reported to be the H2O...H-F interaction. The most probable distributions of mixed clusters as a function of composition are also deduced. It is found that the larger (HF)n(H2O)m clusters are favored both energetically and entropically compared to the ones that are of size m + n < or = 3. Also, the clusters with equimolar contributions of HF and H2O are found to have the strongest interactions.     

In search of CS2(H2O)(n=1-4) clusters

Gaussian-3 and MP2/aug-cc-pVnZ methods have been used to calculate geometries and thermochemistry of CS(2)(H2O)n, where n=1-4. An extensive molecular dynamics search followed by optimization using these two methods located two dimers, six trimers, six tetramers, and two pentamers. The MP2/aug-cc-pVDZ structure matched best with the experimental result for the CS(2)(H2O) dimer, showing that diffuse functions are necessary to model the interactions found in this complex. For larger CS(2)(H2O)n clusters, the MP2/aug-cc-pVDZ minima are significantly different from the MP2(full)6-31G* structures, revealing that the G3 model chemistry is not suitable for investigation of sulfur containing van der Waals complexes. Based on the MP2/aug-cc-pVTZ free energies, the concentration of saturated water in the atmosphere and the average amount of CS(2) in the atmosphere, the concentrations of these clusters are predicted to be on the order of 10(5) CS(2)(H2O) clusters.cm(-3) and 10(2) CS(2)(H2O)(2) clusters.cm(-3) at 298.15 K. The MP2/aug-cc-pVDZ scaled harmonic and anharmonic frequencies of the most abundant dimer cluster at 298 K are presented, along with the MP2/aug-cc-pVDZ scaled harmonic frequencies for the CS(2)(H(2)O)(n) structures predicted to be present in a low-temperature molecular beam experiment.     

Structure and bonding in cyclic isomers of B2AlH(n)(m) (n = 3-6, m = -2 to +1): a comparative study with B3H(n)(m), BAl2H(n)(m) and Al3H(n)(m)

The structure, bonding and energetics of B(2)AlH(n)(m) (n = 3-6, m = -2 to +1) are compared with corresponding homocyclic boron, aluminum analogues and BAl(2)H(n)(m) using density functional theory (DFT). Divalent to hexacoordinated boron and aluminum atoms are found in these species. The geometrical and bonding pattern in B(2)AlH(4)(-) is similar to that for B(2)SiH(4). Species with lone pairs on the divalent boron and aluminum atoms are found to be minima on the potential energy surface of B(2)AlH(3)(2-). A dramatic structural diversity is observed in going from B(3)H(n)(m) to B(2)AlH(n)(m), BAl(2)H(n)(m) and Al(3)H(n)(m) and this is attributable to the preference of lower coordination on aluminum, higher coordination on boron and the higher multicenter bonding capability of boron. The most stable structures of B(3)H(6)(+), B(2)AlH(5) and BAl(2)H(4)(-) and the trihydrogen bridged structure of Al(3)H(3)(2-) show an isostructural relationship, indicating the isolobal analogy between trivalent boron and divalent aluminum anion.     

IR spectroscopy on isolated Co(n)(alcohol)m cluster anions (n=1-4, m=1-3): structures and spin states

Isolated cobalt-alcohol cluster anions containing n=1-4 cobalt and m=1-3 alcohol molecules (alcohol=methanol, ethanol, propanol) are produced in a supersonic beam by using a laser ablation source. By applying IR photodissociation spectroscopy vibrational spectra in the OH stretching region are obtained. Several structures in different spin states are discussed for the (n,m) clusters. In comparison with density functional theory calculations applied to both the Co/alcohol clusters and the naked Co cluster anions, an unambiguous structural assignment is achieved. It turns out that structures are preferred with a maximum number of hydrogen bonds between the OH groups and the Co···Co units. These hydrogen bonds are typical for anionic species leading to an activation of the OH groups which is indicated by large red-shifts of the OH stretching frequencies compared to the naked alcohols. For each (n,m) cluster, the frequency shifts systematically with respect to the different alcohols, but the type of structure is identical for all alcohol ligands. The application of IR spectroscopy turns out to be an ideal tool not only as a probe for structures but also for spin states which significantly influence the predicted OH stretching frequencies.     

Photoelectron spectroscopic study of the hydrated nucleoside anions: Uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1)

The hydrated nucleoside anions, uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1), have been prepared in beams and studied by anion photoelectron spectroscopy in order to investigate the effects of a microhydrated environment on parent nucleoside anions. Vertical detachment energies (VDEs) were measured for all eight anions, and from these, estimates were made for five sequential anion hydration energies. Excellent agreement was found between our measured VDE value for thymidine(-)(H(2)O)(1) and its calculated value in the companion article by S. Kim and H. F. Schaefer III.     

Collision-induced dissociation studies of protonated ether--(H2O)n (n = 1-3) clusters

We have studied the protonated ether-(H2O)n (n = 1-3) complexes containing tetrahydrofuran, dimethyl, diethyl, dibutyl, and butylmethyl ethers using a flowing afterglow triple-quadrupole mass spectrometer. Collision-induced dissociation, CID, of all clusters with n = 1, 2 shows sequential water loss. The n = 3 cluster of dimethyl ether shows sequential water loss, while all other ether clusters display selective product formation. The CID spectra are interpreted based on known energetics, and theoretical studies of the dimethyl and diethyl ether systems.     

Theoretical studies on structures of neptunyl carbonates: NpO2(CO3)m(H2O)n(q-) (m = 1-3, n = 0-3) in aqueous solution

Extensive ab inito computations have been carried out to study the equilibrium structure, infrared spectra, and bonding characteristics of a variety of hydrated NpO2(CO3)m(q-) complexes by considering the solvent as a polarizable dielectric continuum as well as the corresponding anhydrate complexes in the gas phase. The computed structural parameters and vibrational results at the MP2 level in aqueous solution are in good agreement with Clark et al.'s experiments and provide realistic pictures of the neptunyl complexes in an aqueous environment. Our computed hydration energies reveal that the complex with water molecules directly bound to it yields the best results. Our analysis of the nature of the bonding of neptunyl complexes provides insight into the nature of 6d and 5f bonding in actinide complexes.     

Geometric and electronic structures of multiple-decker one-end open sandwich clusters: Eu(n)(C8H8)n- (n = 1-4)

We have measured the photoelectron spectra of the multiple-decker 1:1 sandwich clusters of Eu(n)(COT)n- (n = 1-4; COT = 1,3,5,7-cyclooctatetraene), synthesized in the gas phase, and studied theoretically the bonding scheme, charge distribution, valence orbital energies, and photodetachment energies. We calculated the ground electronic state X- and the first excited electronic state A-, both of which have strong ionic bonding and a characteristic charge distribution. Moreover, we found that the valence orbital energies of Eu (6s) and COT (L delta) depend strongly on cluster size and their positions in the clusters. With the calculated vertical detachment energies for these valence orbitals, we assigned the peaks in the experimental photoelectron spectra and analyzed the origin of their interesting behavior by employing simple point charge models. From these analyses, it became clear that the characteristic behavior of the spectra is due to the strong ionic bonding and the charge distribution. In addition, using the point charge models, we estimated the vertical detachment energies of the one-dimensional polymer [Eu(COT)]infinity-.     

Exploring the rich energy landscape of sulfate-water clusters SO4(2-) (H2O)(n=3-7): an electronic structure approach

We present a reinvestigation of sulfate-water clusters SO4(2-) (H2O)(n=3-7), which involves several new aspects. Using a joint molecular mechanics/first principles approach, we perform exhaustive searches for stable cluster geometries, showing that the sulfate-water landscape is much richer than anticipated previously. We check the compatibility of the new structures with experiment by comparing vertical detachment energies (VDEs) calculated at the B3LYP/6-311++G** level of theory and determine the energetic ordering of the isomers at the RI-MP2/aug-cc-pVTZ level. Our results are bench-marked carefully against reference energies of estimated CCSD(T)/aug-cc-VTZ quality and VDEs of CCSD(T)/aug-cc-pVDZ quality. Furthermore, we calculate anharmonic vibrational corrections for up to the n = 6 clusters, which are shown to be significant for isomer energy ordering. We use energy decomposition analysis (EDA) based on the absolutely localized fragment (ALMO) expansion to gain chemical insight into the binding motifs.     

Comparison of hydrated hydroperoxide anion (HOO-)(H2O)n clusters with alkaline hydrogen peroxide (HOOH)(OH-)(H2O)(n-1) clusters, n = 1-8, 20: an ab initio study

Hydroperoxide anion (HOO(-)), the conjugate base of hydrogen peroxide (HOOH), has been relatively little studied despite the importance of HOOH in commercial processes, atmospheric science, and biology. The anion has been shown to exist as a stable species in alkaline water. This project explored the structure of gas phase (HOO(-))(H(2)O)(n) clusters and identified the lowest energy configurations for n ≤ 8 at the B3LYP/6-311++G** level of theory and for n ≤ 6 at the MP2/aug-cc-pVTZ level of theory. As a start toward understanding equilibration between HOO(-) and HOOH in an alkaline environment, (HOOH)(OH(-))(H(2)O)(n-1) clusters were likewise examined, and the lowest energy configurations were determined for n ≤ 8 (B3LYP/6-311++G**) and n ≤ 6 (MP2/aug-cc-pVTZ). Some studies were also done for n = 20. The two species have very different solvation behaviors. In low energy (HOOH)(OH(-))(H(2)O)(n-1) clusters, HOOH sits on the surface of the cluster, is 4-coordinated (each O is donor once and acceptor once), and donates to the hydroxide ion. In contrast, in low energy (HOO(-))(H(2)O)(n) clusters, (HOO(-)) takes a position in the cluster center surrounded on all sides by water molecules, and its optimum coordination number appears to be 7 (one O is donor-acceptor-acceptor while the other is a 4-fold acceptor). For n ≤ 6 the lowest (HOOH)(OH(-))(H(2)O)(n-1) cluster lies 1.0-2.1 kcal/mol below the lowest (HOO(-))(H(2)O)(n) cluster, but the lowest clusters found for n = 20 favor (HOO(-))(H(2)O)(20). The results suggest that ambient water could act as a substantial kinetic brake that slows equilibration between (HOOH)(OH(-)) and (HOO(-))(H(2)O) because extensive rearrangement of solvation shells is necessary to restabilize either species after proton transfer.     

The identification of a solvated electron pair in the gaseous clusters of Na(-)(H2O)n and Li(-)(H2O)n

By first principles calculations, we explore the possibility that Na(-)(H(2)O)(n) and Li(-)(H(2)O)(n) clusters, which have been measured previously by photoelectron experiments, could serve as gas-phase molecular models for the solvation of two electrons. Such models would capture the electron-electron interaction in a solution environment, which is missed in the well-known anionic water clusters (H(2)O)(n) (-). Our results show that by n = 10, the two loosely bound s electrons in Li(-)(H(2)O)(n) are indeed detached from lithium, and they could exist in either the singlet (spin-paring) or the triplet (spin-coupling) state. In contrast, the two electrons would prefer to stay on the sodium atom in Na(-)(H(2)O)(n) and on the surface of the cluster. The formation of a solvated electron pair and the variation in solvation structures make these two cluster series interesting subjects for further experimental investigation.     

Density functional theory investigation of the geometric and electronic structures of [UO2(H2O)m(OH)n](2 - n) (n + m = 5)

Gradient corrected density functional theory has been used to calculate the geometric and electronic structures of the family of molecules [UO2(H2O)m(OH)n](2 - n) (n + m = 5). Comparisons are made with previous experimental and theoretical structural and spectroscopic data. r(U-O(yl)) is found to lengthen as water molecules are replaced by hydroxides in the equatorial plane, and the nu(sym) and nu(asym) uranyl vibrational wavenumbers decrease correspondingly. GGA functionals (BP86, PW91 and PBE) are generally found to perform better for the cationic complexes than for the anions. The inclusion of solvent effects using continuum models leads to spurious low frequency imaginary vibrational modes and overall poorer agreement with experimental data for nu(sym) and nu(asym). Analysis of the molecular orbital structure is performed in order to trace the origin of the lengthening and weakening of the U-O(yl) bond as waters are replaced by hydroxides. No evidence is found to support previous suggestions of a competition for U 6d atomic orbitals in U-O(yl) and U-O(hydroxide)pi bonding. Rather, the lengthening and weakening of U-O(yl) is attributed to reduced ionic bonding generated in part by the sigma-donating ability of the hydroxide ligands.     

Global search for minimum energy (H2O)n clusters, n = 3-5

The Gaussian-3 (G3) model chemistry method has been used to calculate the relative deltaG(o) values for all possible conformers of neutral clusters of water, (H2O)n, where n = 3-5. A complete 12-fold conformational search around each hydrogen bond produced 144, 1728, and 20,736 initial starting structures of the water trimer, tetramer, and pentamer. These structures were optimized with PM3, followed by HF/6-31G* optimization, and then with the G3 model chemistry. Only two trimers are present on the G3 potential energy hypersurface. We identified 5 tetramers and 10 pentamers on the potential energy and free-energy hypersurfaces at 298 K. None of these 17 structures were linear; all linear starting models folded into cyclic or three-dimensional structures. The cyclic pentamer is the most stable isomer at 298 K. On the basis of this and previous studies, we expect the cyclic tetramers and pentamers to be the most significant cyclic water clusters in the atmosphere.     

Photoionization and ab initio study of Ba(H2O)n (n = 1-4) clusters

An experimental and theoretical study of the photoionization energies (IE's) of Ba(H(2)O)(n) clusters containing up to n = 4 water molecules has been performed. The clusters were generated by a pick-up source combining laser vaporization with pulsed supersonic expansion, and then photoionized by radiation of 272.5-340 nm. The experimentally determined IE(e)'s for n = 1 to 4 are 4.56 ± 0.05, 4.26 ± 0.05, 3.90 ± 0.05 and 3.71 ± 0.05 eV. This cluster size dependence of IE is reproduced within ±0.06 eV employing the mPW1PW91 density-functional and CCSD(T, Full) quantum-chemical methods combined with the 6-311++G(d,p) basis set for the H and O atoms and three different relativistic effective core potentials for Ba atoms. The calculations indicate that the lowest energy hydration structures represent the most relevant contributions to both the vertical and adiabatic ionization energies. Experimental and theoretical evidence correlates with the progressive surface-delocalization of the electron from the hydration cavity around the Ba atom and suggests that the intra-cluster electron transfer is possible even for small aggregates.     

Theoretical characterization of structures and energies of benzene-(H2S)n and (H2S)n (n=1-4) clusters

An ab initio study was performed in clusters up to four H(2)S molecules and benzene using calculations at MP26-31+G(*) and MP2/aug-cc-pVDZ levels. Differences between both sets of calculations show the importance of using large basis sets to describe the intermolecular interactions in this system. The obtained binding energies reflect that benzene has not the same behavior in H(2)S as in water, pointing to a higher solubility of this molecule in H(2)S than in water. The Bz-cluster binding energy was fitted to an asymptotic representation with a maximum value of the energy of -8.00 kcal/mol that converges in a cluster with 12 H(2)S molecules. The obtained intermolecular distance in the Bz-H(2)S dimer is similar to the experimental value; however, the difference is much larger for the angles defining the orientation. The influence of benzene produces a distortion of the (H(2)S)(n) clusters, so the intermolecular distances change with regard to the (H(2)S)(n) isolated clusters. Frequency shifts are larger in clusters with benzene than without it. In the smallest clusters the shift associated to the stretching of the S-H bonded to benzene is the largest one, but for the cluster with three H(2)S molecules this stretching is combined with the other S-H stretching of the molecule so the resulting shift is not the largest one.     

Vibrational spectroscopy and structures of Ni+(C2H2)(n) (n =1-4) complexes

Nickel cation-acetylene complexes of the form Ni(+)(C(2)H(2))(n), Ni(+)(C(2)H(2))Ne, and Ni(+)(C(2)H(2))(n)Ar(m) (n = 1-4) are produced in a molecular beam by pulsed laser vaporization. These ions are size-selected and studied in a time-of-flight mass spectrometer by infrared laser photodissociation spectroscopy in the C-H stretch region. The fragmentation patterns indicate that the coordination number is 4 for this system. The n = 1-4 complexes with and without rare gas atoms are also investigated with density functional theory. The combined IR spectra and theory show that pi-complexes are formed for the n = 1-4 species, causing the C-H stretches in the acetylene ligands to shift to lower frequencies. Theory reveals that there are low-lying excited states nearly degenerate with the ground state for all the Ni(+)(C(2)H(2))(n) complexes. Although isomeric structures are identified for rare gas atom binding at different sites, the attachment of rare gas atoms results in only minor perturbations on the structures and spectra for all complexes. Experiment and theory agree that multiple acetylene binding takes place to form low-symmetry structures, presumably due to Jahn-Teller distortion and/or ligand steric effects. The fully coordinated Ni(+)(C(2)H(2))(4) complex has a near-tetrahedral structure.     

The structural and electronic properties of Ag-adsorbed (SiO2)n (n=1-7) clusters

Equilibrium geometries, charge distributions, stabilities, and electronic properties of the Ag-adsorbed (SiO(2))(n) (n=1-7) clusters have been investigated using density functional theory with generalized gradient approximation for exchange-correlation functional. The results show that the Ag atom preferably binds to silicon atom with dangling bond in nearly a fixed direction, and the incoming Ag atoms tend to cluster on the existing Ag cluster leading to the formation of Ag islands. The adsorbed Ag atom only causes charge redistributions of the atoms near itself. The effect of the adsorbed Ag atom on the bonding natures and structural features of the silica clusters is minor, attributing to the tendency of stability order of Ag(SiO(2))(n) (n=1-7) clusters in consistent with silica clusters. In addition, the energy gaps between the highest occupied and lowest unoccupied molecular orbitals remarkably decrease compared with the pure (SiO(2))(n) (n=1-7) clusters, eventually approaching the near infrared radiation region. This suggests that these small clusters may be an alternative material which has a similar functionality in treating cancer to the large gold-coated silica nanoshells and the small Au(3)(SiO(2))(3) cluster.     

Low-lying electronic states of M(3)O(9)(-) and M(3)O(9)(2-) (M = Mo, W)

Multiple low-lying electronic states of M(3)O(9)(-) and M(3)O(9)(2-) (M = Mo, W) arise from the occupation of the near-degenerate low-lying virtual orbitals in the neutral clusters. We used density functional theory (DFT) and coupled cluster theory (CCSD(T)) with correlation consistent basis sets to study the structures and energetics of the electronic states of these anions. The adiabatic and vertical electron detachment energies (ADEs and VDEs) of the anionic clusters were calculated with 27 exchange-correlation functionals including one local spin density approximation functional, 13 generalized gradient approximation (GGA) functionals, and 13 hybrid GGA functionals, as well as the CCSD(T) method. For M(3)O(9)(-), CCSD(T) and nearly all of the DFT exchange-correlation functionals studied predict the (2)A(1) state arising from the Jahn-Teller distortion due to singly occupying the degenerate e' orbital to be lower in energy than the (2)A(1)' state arising from singly occupying the nondegenerate a(1)' orbital. For W(3)O(9)(-), the (2)A(1) state was predicted to have essentially the same energy as the (2)A(1)' state at the CCSD(T) level with core-valence correlation corrections included and to be higher in energy or essentially isoenergetic with most DFT methods. The calculated VDEs from the CCSD(T) method are in reasonable agreement with the experimental values for both electronic states if estimates for the corrections due to basis set incompleteness are included. For M(3)O(9)(2-), the singlet state arising from doubly occupying the nondegenerate a(1)' orbital was predicted to be the most stable state for both M = Mo and W. However, whereas M(3)O(9)(2-) was predicted to be less stable than M(3)O(9)(-), W(3)O(9)(2-) was predicted to be more stable than W(3)O(9)(-).     

Structure and stability of (TiO2)n, (SiO2)n, and mixed Ti(m)Si(n-m)O(2n) [n = 2-5, m = 1 to (n - 1)] clusters

Structures, energetics, and vibrational spectra are investigated for small pure (TiO(2))(n), (SiO(2))(n), and mixed Ti(m)Si(n-m)O(2n) [n = 2-5, m = 1 to (n - 1)] oxide clusters by density functional theory (DFT). The BP86/ATZP level of theory is employed to obtain constitutional isomers of the oxide clusters. In accordance with previous studies, our calculations show three-dimensional compact structures are preferred for pure (TiO(2))(n) with oxo-stabilized higher hexavalent states, and linear chain structures are favored for pure (SiO(2))(n) with tetravalent states. However, the herein theoretically first reported mixed Ti(m)Si(n-m)O(2n) oxide clusters prefer either three-dimensional compact or linear chain structures depending upon the stoichiometry of the compound. Vibrational analysis of the important modes of some highly stable structures is provided. Coupled-cluster single and double excitation (with triples) [CCSD(T)] computed energy gaps for the TiO(2) dimers compare well with results from previous study. Excitation energies are computed by use of time-dependent (TD) DFT and equation-of-motion coupled-cluster calculations with singles and doubles (EOM-CCSD) for the most stable isomers.     

Electron scattering on OH-(H2O)n clusters (n = 0-4)

The cross sections for electron scattering on OH-(H2O)n for n = 0-4 were measured from threshold to approximately 50 eV. All detachment cross sections were found to follow the classical prediction given earlier [Phys. Rev. Lett. 74, 892 (1995)] with a threshold energy for electron-impact detachment that increased upon sequential hydration, yielding values in the range from 4.5 eV +/- 0.2 eV for OH- to 12.10 eV +/- 0.5 eV for OH-(H2O)4. For n > or = 1, we found that approximately 80% of the total reaction events lead to electron detachment plus total dissociation of the clusters into the constituent molecules of OH and H2O. Finally, we observed resonances in the cross sections for OH-(H2O)3 and for OH-(H2O)4. The resonances were located at approximately 15 eV and were ascribed to the formation of dianions in excited states.