Effect of additional hydrogen peroxide to H2O2...(H2O)n, n=1 and 2 complexes: quantum chemical study

Hydrogen peroxide, H2O2, acts as a particularly strong reactant in aqueous environment. It has been demonstrated earlier that agglomerates with a single peroxide interacting with one and two water molecules manifest in several stable conformers within a narrow energy range. In the present study we seek structural changes brought out by adding an extra H2O2 to these systems at molecular level employing ab initio quantum chemical methods, viz., restricted Hartree-Fock and the second order Moller-Plesset perturbation theory. These clusters exhibit consistent trends in energy hierarchy at both the levels. Further, a many body interaction energy analysis quantifies the strength and cooperativity of hydrogen bonding in the (H2O2)2...(H2O)n, (n=1 and 2) clusters, bringing out structuring/destructuring effects attributed to attachment of water and hydrogen peroxide molecules.     

Microsolvation effect, hydrogen-bonding pattern, and electron affinity of the uracil-water complexes U-(H2O)n (n = 1, 2, 3)

To achieve a systematic understanding of the influence of microsolvation on the electron accepting behaviors of nucleobases, the reliable theoretical method (B3LYP/DZP++) has been applied to a comprehensive conformational investigation on the uracil-water complexes U-(H(2)O)(n) (n = 1, 2, 3) in both neutral and anionic forms. For the neutral complexes, the conformers of hydration on the O2 of uracil are energetically favored. However, hydration on the O4 atom of uracil is more stable for the radical anions. The electron structure analysis for the H-bonding patterns reveal that the CH...OH(2) type H-bond exists only for di- and trihydrated uracil complexes in which a water dimer or trimer is involved. The electron density structure analysis and the atoms-in-molecules (AIM) analysis for U-(H(2)O)(n) suggest a threshold value of the bond critical point (BCP) density to justify the CH...OH(2) type H-bond; that is, CH...OH(2) could be considered to be a H-bond only when its BCP density value is equal to or larger than 0.010 au. The positive adiabatic electron affinity (AEA) and vertical detachment energy (VDE) values for the uracil-water complexes suggest that these hydrated uracil anions are stable. Moreover, the average AEA and VDE of U-(H(2)O)(n) increase as the number of the hydration waters increases.     

Isotope exchange in reactions between D2O and size-selected ionic water clusters containing pyridine, H+ (pyridine)m(H2O)n

Pyridine containing water clusters, H(+)(pyridine)(m)(H(2)O)(n), have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. In the experiments, H(+)(pyridine)(m)(H(2)O)(n) with m = 1-4 and n = 0-80 are observed. For the cluster distributions observed, there are no magic numbers, neither in the abundance spectra, nor in the evaporation spectra from size selected clusters. Experiments with size-selected clusters H(+)(pyridine)(m)(H(2)O)(n), with m = 0-3, reacting with D(2)O at a center-of-mass energy of 0.1 eV were also performed. The cross-sections for H/D isotope exchange depend mainly on the number of water molecules in the cluster and not on the number of pyridine molecules. Clusters having only one pyridine molecule undergo D(2)O/H(2)O ligand exchange, while H(+)(pyridine)(m)(H(2)O)(n), with m = 2, 3, exhibit significant H/D scrambling. These results are rationalized by quantum chemical calculations (B3LYP and MP2) for H(+)(pyridine)(1)(H(2)O)(n) and H(+)(pyridine)(2)(H(2)O)(n), with n = 1-6. In clusters containing one pyridine, the water molecules form an interconnected network of hydrogen bonds associated with the pyridinium ion via a single hydrogen bond. For clusters containing two pyridines, the two pyridine molecules are completely separated by the water molecules, with each pyridine being positioned diametrically opposite within the cluster. In agreement with experimental observations, these calculations suggest a "see-saw mechanism" for pendular proton transfer between the two pyridines in H(+)(pyridine)(2)(H(2)O)(n) clusters.     

First principles study on the solvation and structure of C2O4(2-)(H2O)n, n = 6-12

The structures and energies of hydrated oxalate clusters, C2O4(2-)(H2O)n, n = 6-12, are obtained by density functional theory (DFT) calculations and compared to SO4(2-)(H2O)n. Although the evolution of the cluster structure with size is similar to that of SO4(2-)(H2O)n, there are a number of important and distinctive futures in C2O4(2-)(H2O)n, including the separation of the two charges due to the C-C bond in C2O4(2-), the lower symmetry around C2O4(2-), and the torsion along the C-C bond, that affect both the structure and the solvation energy. The solvation dynamics for the isomers of C2O4(2-)(H2O)12 are also examined by DFT based ab initio molecular dynamics.     

Potential energy surface of O2-(H2O) and factors controlling water-to-O2- binding motifs

The potential energy surface (PES) of O(2)(-)(H(2)O) is investigated by varying the interoxygen distance of O(2)(-) via ab initio calculations with a large basis set. Although two stationary points, C(s) and C(2v) conformers, are found along the interoxygen-distance coordinate, only the C(s) conformer is identified as a minimum-energy species. We find a critical distance, r(c), separating these two conformers in the PES. The C(s) conformer prevails at interoxygen distances of O(2)(-) that are less than r(c), while the C(2v) conformer dominates at the distances larger than r(c). The structural features of these two conformers are also discussed. Although the water deformation energy is shown to be the stabilization source responsible for the prevalence of the C(s) cluster conformer at the interoxygen distances of O(2)(-) less than r(c), the ionic hydrogen bonding is the major driving force for transformation of the water binding motif from C(s) to C(2v) when the interoxygen distance of O(2)(-) increases.     

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 importance of NO+ (H2O)4 in the conversion of NO+ (H2O)n to H3O+ (H2O)n: I. Kinetics measurements and statistical rate modeling

The kinetics for conversion of NO(+)(H(2)O)(n) to H(3)O(+)(H(2)O)(n) has been investigated as a function of temperature from 150 to 400 K. In contrast to previous studies, which show that the conversion goes completely through a reaction of NO(+)(H(2)O)(3), the present results show that NO(+)(H(2)O)(4) plays an increasing role in the conversion as the temperature is lowered. Rate constants are derived for the clustering of H(2)O to NO(+)(H(2)O)(1-3) and the reactions of NO(+)(H(2)O)(3,4) with H(2)O to form H(3)O(+)(H(2)O)(2,3), respectively. In addition, thermal dissociation of NO(+)(H(2)O)(4) to lose HNO(2) was also found to be important. The rate constants for the clustering increase substantially with the lowering of the temperature. Flux calculations show that NO(+)(H(2)O)(4) accounts for over 99% of the conversion at 150 K and even 20% at 300 K, although it is too small to be detectable. The experimental data are complimented by modeling of the falloff curves for the clustering reactions. The modeling shows that, for many of the conditions, the data correspond to the falloff regime of third body association.     

Ab initio molecular dynamics studies of ionic dissolution and precipitation of sodium chloride and silver chloride in water clusters, NaCl(H2O)n and AgCl(H2O)n, n = 6, 10, and 14

An ab initio molecular dynamics method was used to compare the ionic dissolution of soluble sodium chloride (NaCl) in water clusters with the highly insoluble silver chloride (AgCl). The investigations focused on the solvation structures, dynamics, and energetics of the contact ion pair (CIP) and of the solvent-separated ion pair (SSIP) in NaCl(H(2)O)(n) and AgCl(H(2)O)(n) with cluster sizes of n = 6, 10 and 14. We found that the minimum cluster size required to stabilize the SSIP configuration in NaCl(H(2)O)(n) is temperature-dependent. For n = 6, both configurations are present as two distinct local minima on the free-energy profile at 100 K, whereas SSIP is unstable at 300 K. Both configurations, separated by a low barrier (<10 kJ mol(-1)), are identifiable on the free energy profiles of NaCl(H(2)O)(n) for n = 10 and 14 at 300 K, with the Na(+)/Cl(-) pairs being internally solvated in the water cluster and the SSIP configuration being slightly higher in energy (<5 kJ mol(-1)). In agreement with the low bulk solubility of AgCl, no SSIP minimum is observed on the free-energy profiles of finite AgCl(H(2)O)(n) clusters. The AgCl interaction is more covalent in nature, and is less affected by the water solvent. Unlike NaCl, AgCl is mainly solvated on the surface in finite water clusters, and ionic dissolution requires a significant reorganization of the solvent structure.     

The mechanism of proton exchange: guided ion beam studies of the reactions, H(H2O)n+ (n=1-4)+D2O and D(D2O)n+ (n=1-4)+H2O

Reactions of protonated water clusters, H(H(2)O)(n) (+) (n=1-4) with D(2)O and their "mirror" reactions, D(D(2)O)(n) (+) (n=1-4) with H(2)O, are studied using guided-ion beam mass spectrometry. Absolute reaction cross sections are determined as a function of collision energy from thermal energy to over 10 eV. At low collision energies, we observe reactions in which H(2)O and D(2)O molecules are interchanged and reactions where H-D exchange has occurred. As the collision energy is increased, the H-D exchange products decrease and the water exchange products become dominant. At high collision energies, processes in which one or more water molecules are lost from the reactant ions become important, with simple collision-induced dissociation processes, i.e., those without H-D exchange, being dominant. Threshold energies of endothermic channels are measured and used to determine binding energies of the proton bound complexes, which are consistent with those determined by thermal equilibrium measurements and previous collision-induced dissociation studies. A kinetic scheme that relies only on the ratio of isomerization and dissociation rate constants successfully accounts for the kinetic energy dependence observed in the branching ratios for H-D and water exchange products in all systems. Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations confirm the feasibility and establish the details of this kinetic model.     

Infrared spectroscopy of Cu+(H2O)(n) and Ag+(H2O)(n): coordination and solvation of noble-metal ions

M(+)(H(2)O)(n) and M(+)(H(2)O)(n)Ar ions (M=Cu and Ag) are studied for exploring coordination and solvation structures of noble-metal ions. These species are produced in a laser-vaporization cluster source and probed with infrared (IR) photodissociation spectroscopy in the OH-stretch region using a triple quadrupole mass spectrometer. Density functional theory calculations are also carried out for analyzing the experimental IR spectra. Partially resolved rotational structure observed in the spectrum of Ag(+)(H(2)O)(1) x Ar indicates that the complex is quasilinear in an Ar-Ag(+)-O configuration with the H atoms symmetrically displaced off axis. The spectra of the Ar-tagged M(+)(H(2)O)(2) are consistent with twofold coordination with a linear O-M(+)-O arrangement for these ions, which is stabilized by the s-d hybridization in M(+). Hydrogen bonding between H(2)O molecules is absent in Ag(+)(H(2)O)(3) x Ar but detected in Cu(+)(H(2)O)(3) x Ar through characteristic changes in the position and intensity of the OH-stretch transitions. The third H(2)O attaches directly to Ag(+) in a tricoordinated form, while it occupies a hydrogen-bonding site in the second shell of the dicoordinated Cu(+). The preference of the tricoordination is attributable to the inefficient 5s-4d hybridization in Ag(+), in contrast to the extensive 4s-3d hybridization in Cu(+) which retains the dicoordination. This is most likely because the s-d energy gap of Ag(+) is much larger than that of Cu(+). The fourth H(2)O occupies the second shells of the tricoordinated Ag(+) and the dicoordinated Cu(+), as extensive hydrogen bonding is observed in M(+)(H(2)O)(4) x Ar. Interestingly, the Ag(+)(H(2)O)(4) x Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag(+)(H(2)O)(3) x Ar but revived at n=4. Size dependent variations in the spectra of Cu(+)(H(2)O)(n) for n=5-7 provide evidence for the completion of the second shell at n=6, where the dicoordinated Cu(+)(H(2)O)(2) subunit is surrounded by four H(2)O molecules. The gas-phase coordination number of Cu(+) is 2 and the resulting linearly coordinated structure acts as the core of further solvation processes.     

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.     

Ab initio studies on Al(+)(H(2)O)(n), HAlOH(+)(H(2)O)(n-1), and the size-dependent H(2) elimination reaction

We report computational studies on Al(+)(H(2)O)(n), and HAlOH(+)(H(2)O)(n-1), n = 6-14, by the density functional theory based ab initio molecular dynamics method, employing a planewave basis set with pseudopotentials, and also by conventional methods with Gaussian basis sets. The mechanism for the intracluster H(2) elimination reaction is explored. First, a new size-dependent insertion reaction for the transformation of Al(+)(H(2)O)(n), into HAlOH(+)(H(2)O)(n-1) is discovered for n > or = 8. This is because of the presence of a fairly stable six-water-ring structure in Al(+)(H(2)O)(n) with 12 members, including the Al(+). This structure promotes acidic dissociation and, for n > or = 8, leads to the insertion reaction. Gaussian based BPW91 and MP2 calculations with 6-31G* and 6-31G** basis sets confirmed the existence of such structures and located the transition structures for the insertion reaction. The calculated transition barrier is 10.0 kcal/mol for n = 9 and 7.1 kcal/mol for n = 8 at the MP2/6-31G** level, with zero-point energy corrections. Second, the experimentally observed size-dependent H(2) elimination reaction is related to the conformation of HAlOH(+)(H(2)O)(n-1), instead of Al(+)(H(2)O)(n). As n increases from 6 to 14, the structure of the HAlOH(+)(H(2)O)(n-1) cluster changes into a caged structure, with the Al-H bond buried inside, and protons produced in acidic dissociation could then travel through the H(2)O network to the vicinity of the Al-H bond and react with the hydride H to produce H(2). The structural transformation is completed at n = 13, coincident approximately with the onset of the H(2) elimination reaction. From constrained ab initio MD simulations, we estimated the free energy barrier for the H(2) elimination reaction to be 0.7 eV (16 kcal/mol) at n = 13, 1.5 eV (35 kcal/mol) at n = 12, and 4.5 eV (100 kcal/mol) at n = 8. The existence of transition structures for the H(2) elimination has also been verified by ab initio calculations at the MP2/6-31G** level. Finally, the switch-off of the H(2) elimination for n > 24 is explored and attributed to the diffusion of protons through enlarged hydrogen bonded H(2)O networks, which reduces the probability of finding a proton near the Al-H bond.     

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.     

Organic-inorganic hybrids based on novel bimolecular [Si2W22Cu2O78(H2O)]12- polyoxometalates and the polynuclear complex cations [Cu(ac)(phen)(H2O)]n n+ (n=2, 3)

The reaction of a monosubstituted Keggin polyoxometalate (POM) generated in situ with copper-phenanthroline complexes in excess ammonium or rubidium acetate led to the formation of the hybrid metal organic-inorganic compounds A7[Cu2(ac)2(phen)2(H2O)2][Cu3(ac)3(phen)3(H2O)3][Si2W22Cu2O78(H2O)].approximately 18 H2O (A=NH4+ (1), Rb+ (2); ac=acetate; phen=1,10-phenanthroline). These compounds are constructed from inorganic and metalorganic interpenetrated sublattices containing the novel bimolecular Keggin POM, [Si2W22Cu2O78(H2O)]12-, and Cu-ac-phen complexes, [Cu(ac)(phen)(H2O)]n n+ (n=2, 3). The packing of compound 1 can be viewed as a stacking of open-framework layers parallel to the xy plane built of hydrogen-bonded POMs, and zigzag columns of pi-stacked Cu-ac-phen complex cations running along the [111] direction. Magnetic and EPR results are discussed with respect to the crystal structure of the compounds. DFT calculations on [Cu(ac)(phen)(H2O)]n n+ cationic complexes have been performed, to check the influence of packing in the complex geometry and determine the magnetic exchange pathways.     

Computational study on the negative electron affinities of NO2 -.(H2O)n clusters (n=0-30)

Here we report negative electron affinities of NO(2)(-).(H2O)n clusters (n=0-30) obtained from density functional theory calculations and a simple correction to Koopmans' theorem. The method relies on the calculation of the detachment energy of the monoanion and its highest occupied molecular orbital and lowest unoccupied molecular orbital energies, and explicit calculations on the dianion itself are avoided. A good agreement with resonances in the cross section for neutral production in electron scattering experiments is found for n=0, 1, and 2. We find several isomeric structures of NO(2)(-).(H2O)2 of similar energy that elucidate the interplay between water-water and ion-water interactions. The topology is predicted to influence the electron affinity by 0.5 and 0.4 eV for NO(2)(-).(H2O) and NO(2)(-).(H2O)2, respectively. The electron affinity of larger clusters is shown to follow a (n+delta)-1/3 dependence, where delta=3 represents the number of water molecules that in volume, could replace NO(2) (-).     

Sequential hydration energies of the sulfate ion, from determinations of the equilibrium constants for the gas-phase reactions: SO4(H2O)(n)2- = SO4(H2O)(n-1)2- + H2O

Sequential hydration energies of SO4(H2O)(n)2- were obtained from determinations of the equilibrium constants of the following reactions: SO4(H2O)(n)2- = SO4(H2O)(n-1)2- + H2O. The SO4(2-) ions were produced by electrospray and the equilibrium constants Kn,n-1 were determined with a reaction chamber attached to a mass spectrometer. Determinations of Kn,n-1 at different temperatures were used to obtain DeltaG0n,n-1, DeltaH0 n,n-1, and DeltaS0n,n-1 for n = 7 to 19. Interference of the charge separation reaction SO4(H2O)(n)2- = HSO4(H2O)(n-k)- + OH(H2O)(k-1)- at higher temperatures prevented determinations for n < 7. The DeltaS0n,n-1 values obtained are unusually low and this indicates very loose, disordered structures for the n > or = 7 hydrates. The DeltaH0n,n-1 values are compared with theoretical values DeltaEn,n-1, obtained by Wang, Nicholas, and Wang. Rate constant determinations of the dissociation reactions n,n - 1, obtained with the BIRD method by Wong and Williams, showed relatively lower rates for n = 6 and 12, which indicate that these hydrates are more stable. No discontinuities of the DeltaG0n,n-1 values indicating an unusually stable n = 12 hydrate were observed in the present work. Rate constants evaluated from the DeltaG0n,n-1 results also fail to indicate a lower rate for n = 12. An analysis of the conditions used in the two types of experiments indicates that the different results reflect the different energy distributions expected at the dissociation threshold. Higher internal energies prevail in the equilibrium measurements and allow the participation of more disordered transition states in the reaction.     

Thermal effects on energetics and dynamics in water cluster anions (H2O)n(-)

The electron binding energies and relaxation dynamics of water cluster anions (H(2)O)(n)(-) (11 ≤ n ≤ 80) formed in co-expansions with neon were investigated using one-photon and time-resolved photoelectron imaging. Unlike previous experiments with argon, water cluster anions exhibit only one isomer class, the tightly bound isomer I with approximately the same binding energy as clusters formed in argon. This result, along with a decrease in the internal conversion lifetime of excited (H(2)O)(n)(-) (25 ≤ n ≤ 40), indicates that clusters are vibrationally warmer when formed in neon. Over the ranges studied, the vertical detachment energies and lifetimes appear to converge to previously reported values.     

Sequential bond energies and barrier heights for the water loss and charge separation dissociation pathways of Cd(2+)(H2O)n, n = 3-11

The bond dissociation energies for losing one water from Cd(2+)(H(2)O)(n) complexes, n = 3-11, are measured using threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer coupled with a thermal electrospray ionization source. Kinetic energy dependent cross sections are obtained for n = 4-11 complexes and analyzed to yield 0 K threshold measurements for loss of one, two, and three water ligands after accounting for multiple collisions, kinetic shifts, and energy distributions. The threshold measurements are converted from 0 to 298 K values to give the hydration enthalpies and free energies for sequentially losing one water from each complex. Theoretical geometry optimizations and single point energy calculations are performed on reactant and product complexes using several levels of theory and basis sets to obtain thermochemistry for comparison to experiment. The charge separation process, Cd(2+)(H(2)O)(n) → CdOH(+)(H(2)O)(m) + H(+)(H(2)O)(n-m-1), is also observed for n = 4 and 5 and the competition between this process and water loss is analyzed. Rate-limiting transition states for the charge separation process at n = 3-6 are calculated and compared to experimental threshold measurements resulting in the conclusion that the critical size for this dissociation pathway of hydrated cadmium is n(crit) = 4.     

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.     

Proton mobility and stability of water clusters containing the bisulfate anion, HSO4(-)(H2O)n

Bisulfate water clusters, HSO(4)(-)(H(2)O)(n), have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. For the cluster distributions studied, there are some possible "magic number" peaks, although the increase in abundance compared to their neighbours is small. Experiments with size-selected clusters with n = 0-25, reacting with D(2)O at a center-of-mass energy of 0.1 eV, were performed, and it was observed that the rate of hydrogen/deuterium exchange is lower for the smallest clusters (n < 8) than for the larger (n > 11), with a transition taking place in the range n = 8-11. We propose that the protonic defect of the bisulfate ion remains rather stationary unless the degree of hydration reaches a given level. In addition, it was observed that H/D scrambling becomes close to statistically randomized for the larger clusters. Insight into this size dependency was obtained by B3LYP/6-311++G(2d,2p) calculations for HSO(4)(-)(H(2)O)(n) with n = 0-10. In agreement with experimental observations, these calculations suggest pronounced effectiveness of a 'see-saw mechanism' for pendular proton transfer with increasing HSO(4)(-)(H(2)O)(n) cluster size.