Hydration of the bisulfate ion: atmospheric implications

Using molecular dynamics configurational sampling combined with ab initio energy calculations, we determined the low energy isomers of the bisulfate hydrates. We calculated the CCSD(T) complete basis set (CBS) binding electronic and Gibbs free energies for 53 low energy isomers of HSO(4)(-)(H(2)O)(n=1-6) and derived the thermodynamics of adding waters sequentially to the bisulfate ion and its hydrates. Comparing the HSO(4)(-)/H(2)O system to the neutral H(2)SO(4)/H(2)O cluster, water binds more strongly to the anion than it does to the neutral molecules. The difference in the binding thermodynamics of HSO(4)(-)/H(2)O and H(2)SO(4)/H(2)O systems decreases with increasing number of waters. The thermodynamics for the formation of HSO(4)(-)(H(2)O)(n=1-5) is favorable at 298.15 K, and that of HSO(4)(-)(H(2)O)(n=1-6) is favorable for T < 273.15 K. The HSO(4)(-) ion is almost always hydrated at temperatures and relative humidity values encountered in the troposphere. Because the bisulfate ion binds more strongly to sulfuric acid than it does to water, it is expected to play a role in ion-induced nucleation by forming a strong complex with sulfuric acid and water, thus facilitating the formation of a critical nucleus.     

Bond energies and structures of ammonia-sulfuric acid positive cluster ions

New particle formation in the atmosphere is initiated by nucleation of gas-phase species. The small molecular clusters that act as seeds for new particles are stabilized by the incorporation of an ion. Ion-induced nucleation of molecular cluster ions containing sulfuric acid generates new particles in the background troposphere. The addition of a proton-accepting species to sulfuric acid cluster ions can further stabilize them and may promote nucleation under a wider range of conditions. To understand and accurately predict atmospheric nucleation, the stabilities of each molecular cluster within a chemical family must be known. We present the first comprehensive measurements of the ammonia-sulfuric acid positive ion cluster system NH(4)(+)(NH(3))(n)(H(2)SO(4))(s). Enthalpies and entropies of individual growth steps within this system were measured using either an ion flow reactor-mass spectrometer system under equilibrium conditions or by thermal decomposition of clusters in an ion trap mass spectrometer. Low level ab initio structural calculations provided inputs to a master equation model to determine bond energies from thermal decomposition measurements. Optimized ab initio structures for clusters up through n = 3, s = 3 are reported. Upon addition of ammonia and sulfuric acid pairs, internal proton transfer generates multiple NH(4)(+) and HSO(4)(-) ions within the clusters. These multiple-ion structures are up to 50 kcal mol(-1) more stable than corresponding isomers that retain neutral NH(3) and H(2)SO(4) species. The lowest energy n = s clusters are composed entirely of ions. The addition of acid-base pairs to the core NH(4)(+) ion generates nanocrystals that begin to resemble the ammonium bisulfate bulk crystal starting with the smallest n = s cluster, NH(4)(+)(NH(3))(1)(H(2)SO(4))(1). In the absence of water, this cluster ion system nucleates spontaneously for conditions that encompass most of the free troposphere.     

Theoretical studies on the coupling interactions in H2SO4···HOO˙···(H2O)n (n = 0-2) clusters: toward understanding the role of water molecules in the uptake of HOO˙ radical by sulfuric acid aerosols

A detailed knowledge of coupling interactions among sulfuric acid (H(2)SO(4)), the hydroperoxyl radical (HOO˙), and water molecules (H(2)O) is crucial for the better understanding of the uptake of HOO˙ radicals by sulfuric acid aerosols at different atmospheric humidities. In the present study, the equilibrium structures, binding energies, equilibrium distributions, and the nature of the coupling interactions in H(2)SO(4)···HOO˙···(H(2)O)(n) (n = 0-2) clusters have been systematically investigated at the B3LYP/6-311++G(3df,3pd) level of theory in combination with the atoms in molecules (AIM) theory, natural bond orbital (NBO) method, energy decomposition analyses, and ab initio molecular dynamics. Two binary, five ternary, and twelve tetramer clusters possessing multiple intermolecular H-bonds have been located on their potential energy surfaces. Two different modes for water molecules have been observed to influence the coupling interactions between H(2)SO(4) and HOO˙ through the formations of intermolecular H-bonds with or without breaking the original intermolecular H-bonds in the binary H(2)SO(4)···HOO˙ cluster. It was found that the introduction of one or two water molecules can efficiently enhance the interactions between H(2)SO(4) and HOO˙, implying the positive role of water molecules in the uptake of the HOO˙ radical by sulfuric acid aerosols. Additionally, the coupling interaction modes of the most stable clusters under study have been verified by the ab initio molecular dynamics.     

Measurement of the thermodynamics of the hydrated dimer and trimer of sulfuric acid

We report measurements of the equilibrium constants, K(p), for the water mediated clustering of two and three sulfuric acid molecules for atmospheric temperatures and relative humidities. Limits for K(p) for the formation of the sulfuric acid tetramer and higher clusters as well as the kinetics of small sulfuric acid cluster growth are also presented. These results put strong constraints on the rate of nucleation in the atmosphere via the H(2)SO(4)/H(2)O neutral mechanism. We show that the neutral nucleation of H(2)SO(4) and H(2)O is slower than ion-induced nucleation of H(2)SO(4) and H(2)O for most conditions found in the middle and upper troposphere. These laboratory-based upper limits to the nucleation rates are also much lower than the predictions of the liquid drop model/classical nucleation theory.     

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.     

Quasi-unary homogeneous nucleation of H2SO4-H2O

We show that the binary homogeneous nucleation (BHN) of H2SO4-H2O can be treated as quasi-unary nucleation of H2SO4 in equilibrium with H2O vapor. A scheme to calculate the evaporation coefficient of H2SO4 molecules from H2SO4-H2O clusters is presented and a kinetic model to simulate the quasi-unary nucleation of H2SO4-H2O is developed. In the kinetic model, the growth and evaporation of sulfuric acid clusters of various sizes are explicitly simulated. The kinetic quasi-unary nucleation model does not have two well-recognized problems associated with the classical BHN theory (violation of the mass action law and mismatch of the cluster distribution for monomers) and is appropriate for the situations where the assumption of equilibrium cluster distribution is invalid. The nucleation rates predicted with our quasi-unary kinetic model are consistent with recent experimental nucleation experiments in all the cases studied, while the most recent version of the classical BHN model systematically overpredicts the nucleation rates. The hydration of sulfuric acid clusters, which is not considered in the classical model but is accounted for implicitly in our kinetic quasi-unary model, is likely to be one of physical mechanisms that lead to lower nucleation rates. Further investigation is needed to understand exactly what cause the difference between the kinetic quasi-unary model and the classical BHN model.     

Direct observation of triple ions in aqueous solutions of nickel(II) sulfate: a molecular link between the gas phase and bulk behavior

Electrospray ionization of an aqueous solution of nickel(II) sulfate provides direct experimental evidence for the formation of triple ions of the type [Ni(2)(SO(4))(H(2)O)(n)](2+) and [Ni(SO(4))(2)](2-), whose existence in aqueous solution has previously been proposed based on relaxation spectroscopy [Chen et al. J. Sol. Chem. 2005, 34, 1045]. Formally, these triple ions are formed by aggregation of the solvated ions Ni(2+) and SO(4)(2-), respectively, with the neutral ion pair NiSO(4). In addition, also higher adducts are observed, e.g. the "pentuple ions" [Ni(3)(SO(4))(2)(H(2)O)(n)](2+) (n = 7-9) and [Ni(2)(SO(4))(3)](2-), of which the dicationic is extensively hydrated, whereas the anionic is not. The structures of the dinuclear nickel clusters are derived from ab initio calculations and their infrared spectra are compared with experimental data obtained for the gaseous ions [Ni(2)SO(4)(H(2)O)(5)](2+) and [Ni(2)(SO(4))(3)](2-), respectively. The calculations show that the structures are crucially controlled by the degree of solvation of nickel ion. Explicit consideration of solvating water molecules within the first coordination sphere suggest that the dicationic triple ion [Ni(2)SO(4)](aq)(2+) is bent and thus bears a permanent dipole moment, whereas the [Ni(SO(4))(2)](aq)(2-) dianion tends to be quasi-linear. The experimental and theoretical data for the gaseous ions thus support the elegant, but indirect, deductions previously made based on solution-phase studies.     

Ab initio vibrational calculations for H2SO4 and H2SO4 x H2O: spectroscopy and the nature of the anharmonic couplings

Vibrational frequencies for fundamental, overtone, and combination excitations of sulfuric acid (H2SO4) and of sulfuric acid monohydrate cluster (H2SO4 x H2O) are computed directly from ab initio MP2/TZP potential surface points using the correlation-corrected vibrational self-consistent field (CC-VSCF) method, which includes anharmonic effects. The results are compared with experiment. The computed transitions show in nearly all cases good agreement with experimental data and consistent improvement over the harmonic approximation. The CC-VSCF improvements over the harmonic approximation are largest for the overtone and combination excitations and for the OH stretching fundamental. The agreement between the calculations and experiment also supports the validity of the MP2/TZP potential surfaces. Anharmonic coupling between different vibrational modes is found to significantly affect the vibrational frequencies. Analysis of the mean magnitude of the anharmonic coupling interactions between different pairs of normal modes is carried out. The results suggest possible mechanisms for the internal flow of vibrational energy in H2SO4 and H2SO4 x H2O.     

Quantum vibrational analysis of hydrated ions using an ab initio potential

We present full-dimensional potential energy surfaces (PESs) for hydrated chloride based on the sum of ab initio (H(2)O)Cl(-), (H(2)O)(2), and (H(2)O)(3) potentials. The PESs are shown to predict minima and corresponding harmonic frequencies accurately on the basis of comparisons with previous and new ab initio calculations for (H(2)O)(2)Cl(-), (H(2)O)(3)Cl(-), and (H(2)O)(4)Cl(-). An estimate of the effect of the 3-body water interaction is made using a simple 3-body water potential that was recently fit to tens of thousands of ab initio 3-body energies. Anharmonic, coupled vibrational calculations are presented for these clusters, using the "local monomer model" for the high frequency intramolecular modes. This model is tested against previous "exact" calculations for (H(2)O)Cl(-). Radial distribution functions at 0 K obtained from quantum zero-point wave functions are also presented for the (H(2)O)(2)Cl(-) and (H(2)O)(3)Cl(-) clusters.     

Collision induced complex formation following electron capture of SO2-H2O complex interacting with argon atoms

Electron capture dynamics of SO(2)-H(2)O(Ar)(n) complexes (n = 0-2) have been investigated by means of direct ab initio molecular dynamics (MD) method in order to elucidate the effects of solvent argon on the reaction dynamics of SO(2)-H(2)O. The neutral complex of SO(2)-H(2)O has a C(s) symmetry, and the sulfur of SO(2) interacts with the oxygen of H(2)O with an eclipsed form. In the SO(2)-H(2)O(Ar)(n) complexes, the dipole of H(2)O interacts with the argon atoms in the most stable structure. Following the electron capture of the complex SO(2)-H(2)O, the complex anion SO(2)(-)(H(2)O) is dissociated directly into SO(2)(-) + H(2)O. On the other hand, the electron capture of SO(2)(H(2)O)(Ar)(n) argon complex (n = 1-2) leads to the anion-water complex SO(2)(-)(H(2)O) because the collision of H(2)O with the Ar atom causes a rebound of H(2)O from Ar atom to the SO(2)(-) anion. The argon solvent enhanced the SO(2)(-)(H(2)O) complex formation. The reaction mechanism of SO(2)(H(2)O) in the participation of argon atoms was discussed on the basis of the present results.     

Size-dependent charge-separation reaction for hydrated sulfate dianion cluster, SO(2-)(H2O)n, with n=3-7

The decrease in the reaction rate for the charge separation in SO(4) (2-)(H(2)O)(n) with increasing cluster size is examined by first-principles calculations of the energetics, activation barriers, and thermal stability for n=3-7. The key factor governing the charge separation is the difference in the strength of solvation interaction: while interaction with water is strong for the reactant SO(4) (2-) and the product OH(-), it is relatively weak for HSO(4) (-). It gives rise to a barrier for charge separation as SO(4) (2-) is transformed into HSO(4) (-) and OH(-), although the overall reaction energy is exothermic. The barrier is high when more than two H(2)O are left to solvate HSO(4) (-), as in the case of symmetric solvation structure and in the case of large clusters. The entropy is another important factor since the potential surface is floppy and the thermal motion facilitates the symmetric distribution of H(2)O around SO(4) (2-), which leads to the gradual reduction in reaction rate and the eventual switch-off of charge separation as cluster size increases. The experimentally observed products for n=3-5 are explained by the thermally most favorable isomer at each size, obtained by ab initio molecular-dynamics simulations rather than by the isomer with the lowest energy.     

Ionic dissociations of chlorosulfonic acid in microsolvated clusters: A density functional theory and ab initio MO study

Ionic dissociation of chlorosulfonic acid (HSO3Cl) in the molecular clusters HSO3Cl-(H2O)n (n = 1-4) and HSO3Cl-NH3-(H2O)n (n = 0-3) was investigated by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and thermodynamic properties, such as relative enthalpy and relative Gibbs free energy, and were calculated using the hybrid density func- tional (B3LYP) method and the second order M?ller-Plesset approximation (MP2) method with the 6-311 G** basis set. Chlorosulfonic acid was found to require a minimum of three water molecules for ionization to occur and at least one water molecule to protonate ammonia. The corresponding clusters with fewer water molecules were found to be strongly hydrogen-bonded. The related properties and acid strength of chlorosulfonic acid were discussed and compared to the acid strengths of perchloric acid and sulfuric acid in the context of clusters with ammonia and water. The relative stabilities of these clusters were also investigated.     

Photoinduced electron transfer and solvation dynamics in aqueous clusters: comparison of the photoexcited iodide-water pentamer and the water pentamer anion

Upon photoexcitation of iodide-water clusters, I(-)(H(2)O)(n), an electron is transferred from iodide to a diffuse cluster-supported, dipole-bound orbital. Recent femtosecond photoelectron spectroscopy experiments have shown that, for photoexcited I(-)(H(2)O)(n) (n≥ 5), complex excited-state dynamics ultimately result in the stabilization of the transferred electron. In this work, ab initio molecular dynamics simulations of excited-state I(-)(H(2)O)(5) and (H(2)O)(5)(-) are performed, and the simulated time evolution of their structural and electronic properties are compared to determine unambiguously the respective roles of the water molecules and the iodine atom in the electron stabilization dynamics. Results indicate that, driven by the iodine-hydrogen repulsive interactions, excited I(-)(H(2)O)(5) rearranges significantly from the initial ground-state minimum energy configuration to bind the excited electron more tightly. By contrast, (H(2)O)(5)(-) rearranges less dramatically from the corresponding configuration due to the lack of the same iodine-hydrogen interactions. Despite the critical role of iodine for driving reorganization in excited I(-)(H(2)O)(5), excited-electron vertical detachment energies appear to be determined mostly by the water cluster configuration, suggesting that femtosecond photoelectron spectroscopy primarily probes solvent reorganization in photoexcited I(-)(H(2)O)(5).     

Ab initio theoretical calculations of the electronic excitation energies of small water clusters

A direct ab initio molecular dynamics method has been applied to a water monomer and water clusters (H(2)O)(n) (n = 1-3) to elucidate the effects of zero-point energy (ZPE) vibration on the absorption spectra of water clusters. Static ab initio calculations without ZPE showed that the first electronic transitions of (H(2)O)(n), (1)B(1)←(1)A(1), are blue-shifted as a function of cluster size (n): 7.38 eV (n = 1), 7.58 eV (n = 2) and 8.01 eV (n = 3). The inclusion of the ZPE vibration strongly affects the excitation energies of a water dimer, and a long red-tail appears in the range of 6.42-6.90 eV due to the structural flexibility of a water dimer. The ultraviolet photodissociation of water clusters and water ice surfaces is relevant to these results.     

Microscopic solvation of NaBO2 in water: anion photoelectron spectroscopy and ab initio calculations

We investigated the microscopic solvation of NaBO(2) in water by conducting photoelectron spectroscopy and ab initio studies on NaBO(2)(-)(H(2)O)(n) (n = 0-4) clusters. The vertical detachment energy (VDE) of NaBO(2)(-) is estimated to be 1.00 ± 0.08 eV. The photoelectron spectra of NaBO(2)(-)(H(2)O)(1) and NaBO(2)(-)(H(2)O)(2) are similar to that of bare NaBO(2)(-), except that their VDEs shift to higher electron binding energies (EBE). For the spectra of NaBO(2)(-)(H(2)O)(3) and NaBO(2)(-)(H(2)O)(4), a low EBE feature appears dramatically in addition to the features observed in the spectra of NaBO(2)(-)(H(2)O)(0-2). Our study shows that the water molecules mainly interact with the BO(2)(-) unit in NaBO(2)(-)(H(2)O)(1) and NaBO(2)(-)(H(2)O)(2) clusters to form Na-BO(2)(-)(H(2)O)(n) type structures, while in NaBO(2)(-)(H(2)O)(3) and NaBO(2)(-)(H(2)O)(4) clusters, the water molecules can interact strongly with the Na atom, therefore, the Na-BO(2)(-)(H(2)O)(n) and Na(H(2)O)(n)···BO(2)(-) types of structures coexist. That can be seen as an initial step of the transition from a contact ion pair (CIP) structure to a solvent-separated ion pair (SSIP) structure for the dissolution of NaBO(2).     

Hydrated copper and gold monovalent cations: Ab initio study

To understand the hydration phenomena of noble transition metals, we investigated the structures, hydration energies, electronic properties, and spectra of the Cu(+)(H(3)O)(1-6) and Au(+)(H(2)O)(1-6) clusters using ab initio calculations. The coordination numbers of these clusters are found to be only two, which is highly contrasted to those of Ag(+)(H(2)O)(n) (which have the coordination numbers of 3-4) as well as the hydrated alkali metal ions (which have the coordination numbers of approximately 6). For the possible identification of their interesting hydration structures, we predict their IR spectra for the OH stretch modes.     

Micro-hydration of the MgNO3+ cation in the gas phase.

Coordination complexes of the magnesium nitrate cation with water [MgNO(3)(H(2)O)(n)](+) up to n=7 are investigated by experiment and theory. The fragmentation patterns of [MgNO(3)(H(2)O)(n)](+) clusters generated via electrospray ionization indicate a considerable change in stability between n=3 and 4. Further, ion-molecule reactions of mass-selected [MgNO(3)(H(2)O)(n)](+) cations with D(2)O reveal the occurrence of consecutive replacement of water ligands by heavy water, and in this respect the complexes with n=4 and 5 are somewhat more reactive than their smaller homologs with n=1-3 as well as the larger clusters with n=6 and 7. For the latter two ions, the theory suggests the existence of isomers, such as complexes with monodentate nitrato ligands as well as solvent-separated ion pairs with a common solvation shell. The reactions observed and the ion thermochemistry are discussed in the context of ab initio calculations, which also reveal the structures of the various hydrated cation complexes.     

Structural rearrangements and magic numbers in reactions between pyridine-containing water clusters and ammonia

Molecular cluster ions H(+)(H(2)O)(n), H(+)(pyridine)(H(2)O)(n), H(+)(pyridine)(2)(H(2)O)(n), and H(+)(NH(3))(pyridine)(H(2)O)(n) (n = 16-27) and their reactions with ammonia have been studied experimentally using a quadrupole-time-of-flight mass spectrometer. Abundance spectra, evaporation spectra, and reaction branching ratios display magic numbers for H(+)(NH(3))(pyridine)(H(2)O)(n) and H(+)(NH(3))(pyridine)(2)(H(2)O)(n) at n = 18, 20, and 27. The reactions between H(+)(pyridine)(m)(H(2)O)(n) and ammonia all seem to involve intracluster proton transfer to ammonia, thus giving clusters of high stability as evident from the loss of several water molecules from the reacting cluster. The pattern of the observed magic numbers suggest that H(+)(NH(3))(pyridine)(H(2)O)(n) have structures consisting of a NH(4)(+)(H(2)O)(n) core with the pyridine molecule hydrogen-bonded to the surface of the core. This is consistent with the results of high-level ab initio calculations of small protonated pyridine/ammonia/water clusters.     

Photochemistry of HI on argon and water nanoparticles: hydronium radical generation in HI·(H2O)n

Photochemistry of HI molecules on large Ar(n) and (H(2)O)(n), n ～ 100-500, clusters was investigated after excitation with 243 nm and 193 nm laser radiation. The measured H-fragment kinetic energy distributions pointed to a completely different photodissociation mechanism of HI on water than on argon clusters. Distinct features corresponding to the fragment caging (slow fragments) and direct exit (fast fragments) were observed in the spectra from HI photodissociation on Ar(n) clusters. On the other hand, the fast fragments were entirely missing in the spectrum from HI·(H(2)O)(n) and the slow-fragment part of the spectrum had a different shape from HI·Ar(n). The HI·(H(2)O)(n) spectrum was interpreted in terms of the acidic dissociation of HI on (H(2)O)(n) in the ground state, and hydronium radical H(3)O formation following the UV excitation of the ionically dissociated species into states of a charge-transfer-to-solvent character. The H(3)O generation was proved by experiments with deuterated species DI and D(2)O. The experiment was complemented by ab initio calculations of structures and absorption spectra for small HI·(H(2)O)(n) clusters, n = 0-5, supporting the proposed model.     

CuCl3]- and [CuCl4]2- hydrates in concentrated aqueous solution: a density functional theory and ab initio study

In this work, structures and thermodynamic properties of [CuCl(3)](-) and [CuCl(4)](2-) hydrates in aqueous solution were investigated using density functional theory and ab initio methods. Contact ion pair (CIP) and solvent-shared ion pair (SSIP) structures were both taken into account. Our calculations suggest that [CuCl(3)(H(2)O)(n)](-) clusters might favor a four-coordinated CIP structure with a water molecule coordinating with the copper atom in the equatorial position for n = 3 and 4 in aqueous solution, whereas the four-coordinated SSIP structure with one chloride atom dissociated becomes more stable as n increases to 5. For the [CuCl(4)](2-) cluster, the four-coordinated tetrahedron structure is more stable than the square-planar one, whereas for [CuCl(4)(H(2)O)(n)](2-) (n ≥ 1) clusters, it seems that four-coordinated SSIP structures are slightly more favorable than CIP structures. Our calculations suggest that Cu(2+) perhaps prefers a coordination number of 4 in CuCl(2) aqueous solution with high Cl(-) concentrations. In addition, natural bond orbital (NBO) calculations suggest that there is obvious charge transfer (CT) between copper and chloride atoms in [CuCl(x)](2-x) (x = 1-4) clusters. However, compared with that in the [CuCl(2)](0) cluster, the CT between the copper and chloride atoms in [CuCl(3)](-) and [CuCl(4)](2-) clusters becomes negligible as the number of attached redundant Cl(-) ions increases. This implies that the coordination ability of Cl(-) is greatly weakened for [CuCl(3)](-) and [CuCl(4)](2-) clusters. Electronic absorption spectra of these different hydrates were obtained using long-range-corrected time-dependent density functional theory. The calculated electronic transition bands of the four-coordinated CIP conformer of [CuCl(3)(H(2)O)(n)](-) for n = 3 and 4 are coincident with the absorption of [CuCl(3)](-)(aq) species (～284 and 384 nm) resolved from UV spectra obtained in CuCl(2) (ca. 10(-4) mol·kg(-1)) + LiCl (>10 mol·kg(-1)) solutions, whereas the calculated bands of [CuCl(3)(H(2)O)(n)](-) in their most stable configurations are not when n = 0 - 2 or n > 4, which means that the species [CuCl(3)](-)(aq) exists in those CuCl(2) aqueous solutions in which the water activity is neither too low nor too high. The calculated bands of [CuCl(4)(H(2)O)(n)](2-) clusters correspond to the absorption spectra (～270 and 370 nm) derived from UV measurements only when n = 0, which suggests that [CuCl(4)](2-)(aq) species probably exist in environments in which the water activity is quite low.