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.     

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.     

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.     

Electron impact ionization of water-doped superfluid helium nanodroplets: observation of He(H(2)O)(n)(+) clusters

Electron impact mass spectra have been recorded for helium nanodroplets containing water clusters. In addition to identification of both H(+)(H(2)O)(n) and (H(2)O)(n)(+) ions in the gas phase, additional peaks are observed which are assigned to He(H(2)O)(n)(+) clusters for up to n=27. No clusters are detected with more than one helium atom attached. The interpretation of these findings is that quenching of (H(2)O)(n)(+) by the surrounding helium can cool the cluster to the point where not only is fragmentation to H(+)(H(2)O)(m) (where m < or = n-1) avoided, but also, in some cases, a helium atom can remain attached to the cluster ion as it escapes into the gas phase. Ab initio calculations suggest that the first step after ionization is the rapid formation of distinct H(3)O(+) and OH units within the (H(2)O)(n)(+) cluster. To explain the formation and survival of He(H(2)O)(n)(+) clusters through to detection, the H(3)O(+) is assumed to be located at the surface of the cluster with a dangling O-H bond to which a single helium atom can attach via a charge-induced dipole interaction. This study suggests that, like H(+)(H(2)O)(n) ions, the preferential location for the positive charge in large (H(2)O)(n)(+) clusters is on the surface rather than as a solvated ion in the interior of the cluster.     

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.     

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.     

Ultraviolet and infrared photodissociation of Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar clusters

Ion-molecule complexes of the form Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar are produced by laser vaporization in a pulsed nozzle cluster source. These clusters are mass-selected and studied with ultraviolet (355 nm) photodissociation and resonance-enhanced infrared photodissociation spectroscopy in the C-H stretch region of benzene. In the UV, Si(+)(C6H6)n clusters (n = 1-5) fragment to produce the Si(+)(C6H6)n mono-ligand species, suggesting that this ion has enhanced relative stability. IR photodissociation of Si(+)(C6H6)n complexes occurs by the elimination of benzene, while Si(+)(C6H6)(n)Ar complexes lose Ar. Resonances reveal C-H vibrational bands in the 2900-3300 cm(-1) region characteristic of the benzene ligand with shifts caused by the silicon cation bonding. The IR spectra confirm that the major component of the Si(+)(C6H6)n ions studied have the pi-complex structure rather than the isomeric insertion products suggested previously.     

Alternative low-energy mechanisms for isotopic exchange in gas-phase D2O-H+(H2O)n reactions.

Molecular-dynamics (MD) trajectories and high-level ab initio methods have been used to study the low-energy mechanism for D(2)O-H(+)(H(2)O)(n) reactions. At low collisional energies, MD simulations show that the collisional complexes are long-lived and undergo fast monomolecular isomerization, converting between different isomers within 50-500 ps. Such processes, primarily involving water-molecule shifts along a water chain, require the surmounting of very-low-energy barriers and present sizable non- Rice-Ramsperger-Kassel-Marcus (RRKM) effects, which are interpreted as a lack of randomization of the internal kinetic energy. Interestingly, the rate of water shifts was found to increase upon increasing the size of the cluster. Based on these findings, we propose to incorporate the following steps into the mechanism for low-energy isotopic scrambling these D(2)O-H(+)(H(2)O)(n) reactions: a) formation of the collisional complex [H(+)(H(2)O)(n)D(2)O]* in a vibro-rotational excited state; b) incorporation of the heavy-water molecule in the cluster core as HD(2)O(+) by means of isomerization involving molecular shifts; c) displacement of solvation molecules from the first shell of HD(2)O(+) inducing de-deuteration (shift of a D(+) to a neighbor water molecule); d) reorganization of the clusters and/or expulsion of one of the isotopic variants of water (H(2)O, HDO or D(2)O) from the periphery of the complex.     

Undissociated versus dissociated structures for water clusters and ammonia-water clusters: (H2O)n and NH3(H2O)n-1 (n = 5, 8, 9, 21). Theoretical study

To understand the autoionization of pure water and the solvation of ammonia in water, we investigated the undissociated and dissociated (ion-pair) structures of (H2O) n and NH3(H2O)n-1 (n = 5, 8, 9, 21) using density functional theory (DFT) and second order Moller-Plesset perturbation theory (MP2). The stability, thermodynamic properties, and infrared spectra were also studied. The dissociated (ion-pair) form of the clusters tends to favor the solvent-separated ion-pair of H3O+/NH4+ and OH-. As for the NH3(H2O)20 cluster, the undissociated structure has the internal conformation, in contrast to the surface conformation for the (H2O)21 cluster, whereas the dissociated structure of NH3(H2O)20 has the surface conformation. As the cluster size of (H2O)n/NH3(H2O)n-1 increases, the difference in standard free energy between undissociated and dissociated (ion-pair) clusters is asymptotically well corroborated with the experimental free energy change at infinite dilution of H3O+/NH4+ and OH-. The predicted NH and OH stretching frequencies of the undissociated and dissociated (ion-pair) clusters are discussed.     

Dissociative recombination of H+(H2O)3 and D+(D2O)3 water cluster ions with electrons: cross sections and branching ratios

Dissociative recombination (DR) of the water cluster ions H(+)(H(2)O)(3) and D(+)(D(2)O)(3) with electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). For the first time, absolute DR cross sections have been measured for H(+)(H(2)O)(3) in the energy range of 0.001-0.8 eV, and relative cross sections have been measured for D(+)(D(2)O)(3) in the energy range of 0.001-1.0 eV. The DR cross sections for H(+)(H(2)O)(3) are larger than previously observed for H(+)(H(2)O)(n) (n=1,2), which is in agreement with the previously observed trend indicating that the DR rate coefficient increases with size of the water cluster ion. Branching ratios have been determined for the dominating product channels. Dissociative recombination of H(+)(H(2)O)(3) mainly results in the formation of 3H(2)O+H (probability of 0.95+/-0.05) and with a possible minor channel resulting in 2H(2)O+OH+H(2) (0.05+/-0.05). The dominating channels for DR of D(+)(D(2)O)(3) are 3D(2)O+D (0.88+/-0.03) and 2D(2)O+OD+D(2) (0.09+/-0.02). The branching ratios are comparable to earlier DR results for H(+)(H(2)O)(2) and D(+)(D(2)O)(2), which gave 2X(2)O+X (X=H,D) with a probability of over 0.9.     

Structures, energetics, vibrational spectra of NH4+ (H2O)(n=4,6) clusters: Ab initio calculations and first principles molecular dynamics simulations

Important structural isomers of NH(4) (+)(H(2)O)(n=4,6) have been studied by using density functional theory, Moller-Plesset second order perturbation theory, and coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The zero-point energy (ZPE) correction to the complete basis set limit of the CCSD(T) binding energies and free energies is necessary to identify the low energy structures for NH(4) (+)(H(2)O)(n=4,6) because otherwise wrong structures could be assigned for the most probable structures. For NH(4) (+)(H(2)O)(6), the cage-type structure, which is more stable than the previously reported open structure before the ZPE correction, turns out to be less stable after the ZPE correction. In first principles Car-Parrinello molecular dynamics simulations around 100 K, the combined power spectrum of three lowest energy isomers of NH(4) (+)(H(2)O)(4) and two lowest energy isomers of NH(4) (+)(H(2)O)(6) explains each experimental IR spectrum.     

Tuning of the internal energy and isomer distribution in small protonated water clusters H(+)(H2O)(4-8): an application of the inert gas messenger technique

Infrared spectroscopy of gas-phase hydrated clusters provides us much information on structures and dynamics of water networks. However, interpretation of spectra is often difficult because of high internal energy (vibrational temperature) of clusters and coexistence of many isomers. Here we report an approach to vary these factors by using the inert gas (so-called "messenger")-mediated cooling technique. Protonated water clusters with a messenger (M), H(+)(H(2)O)(4-8)·M (M = Ne, Ar, (H(2))(2)), are formed in a molecular beam and probed with infrared photodissociation spectroscopy in the OH stretch region. Observed spectra are compared with each other and with bare H(+)(H(2)O)(n). They show clear messenger dependence in their bandwidths and relative band intensities, reflecting different internal energy and isomer distribution, respectively. It is shown that the internal energy follows the order H(+)(H(2)O)(n) > H(+)(H(2)O)(n)·(H(2))(2) > H(+)(H(2)O)(n)·Ar > H(+)(H(2)O)(n)·Ne, while the isomer-selectivity, which changes the isomer distribution in the bare system, follows the order H(+)(H(2)O)(n)·Ar > H(+)(H(2)O)(n)·(H(2))(2) > H(+)(H(2)O)(n)·Ne ~ (H(+)(H(2)O)(n)). Although the origin of the isomer-selectivity is unclear, comparison among spectra measured with different messengers is very powerful in spectral analyses and makes it possible to easily assign spectral features of each isomer.     

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.     

Infrared photodissociation spectroscopy of Al(+)(CH(3)OH)(n) (n = 1-4)

Infrared photodissociation spectra of Al(+)(CH(3)OH)(n) (n = 1-4) and Al(+)(CH(3)OH)(n)-Ar (n = 1-3) were measured in the OH stretching region, 3000-3800 cm(-1). For n = 1 and 2, sharp absorption bands were observed in the free OH stretching region, all of which were well reproduced by the spectra calculated for the solvated-type geometry with no hydrogen bond. For n = 3 and 4, there were broad vibrational bands in the energy region of hydrogen-bonded OH stretching vibrations, 3000-3500 cm(-1). Energies of possible isomers for the Al(+)(CH(3)OH)(3),4 ions with hydrogen bonds were calculated in order to assign these bands. It was found that the third and fourth methanol molecules form hydrogen bonds with methanol molecules in the first solvation shell, rather than a direct bonding with the Al(+) ion. For the Al(+)(CH(3)OH)(n) clusters with n = 1-4, we obtained no evidence of the insertion reaction, which occurs in Al(+)(H(2)O)(n). One possible explanation of the difference between these two systems is that the potential energy barriers between the solvated and inserted isomers in the Al(+)(CH(3)OH)(n) system is too high to form the inserted-type isomers.     

The structures and electronic states of zinc-water clusters Zn(n)(H2O)(m) (n = 1-32 and m = 1-3)

Ab initio and Density Functional Theory (DFT) calculations have been carried out for zinc-water clusters Zn(n)-(H2O)(m) (n = 1-32 and m = 1-3, where n and m are the numbers of zinc atoms and water molecules, respectively) to elucidate the structure and electronic states of the clusters and the interaction of zinc cluster with water molecules. The binding energies of H2O to zinc clusters were small at n = 2-3 (2.3-4.2 kcal mol(-1)), whereas the energy increased significantly in n = 4 (9.0 kcal mol(-1)). Also, the binding nature of H2O was changed at n = 4. The cluster size dependency of the binding energy of H2O accorded well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in surface regions of the zinc cluster have a positive charge, whereas those in the interior region have a negative charge with the large electron population in the 4p orbital. The interaction of H2O with the zinc clusters were discussed on the basis of the theoretical results.     

Infrared photodissociation spectroscopy of Mg(+)(H2O)Ar(n) complexes: isomers in progressive microsolvation

Ion-molecule complexes of the form Mg(H2O)Ar(n)+ (n = 1-8) are produced by laser vaporization in a pulsed-nozzle cluster source. These complexes are mass-selected and studied with infrared photodissociation spectroscopy in the O-H stretch region. The spectra are interpreted with the aid of ab initio calculations on the n = 1-5 complexes, including examination of various isomeric structures. The combined spectroscopic and theoretical studies reveal the presence of multiple isomeric structures at each cluster size, as the argon atoms assemble around the Mg(+)(H2O) unit. Distinct infrared resonances are measured for argon-on-metal, argon-on-OH and argon-on-two-OH isomers.     

Ab initio studies on the mechanism of the size-dependent hydrogen-loss reaction in Mg+(H2O)n

The mechanism of size-dependent intracluster hydrogen loss in the cluster ions Mg(+)(H(2)O)(n), which is switched on around n=6, and off around n=14, was studied by ab initio calculations at the MP2/6-31G* and MP2/6-31G** levels for n=1-6. The reaction proceeds by Mg(+)-assisted breaking of an H-O bond in one of the H(2)O molecules. The reaction barrier is dependent on both the cluster size and the solvation structure. As n increases from 1 to 6, there is a dramatic drop in the reaction barrier, from greater than 70 kcal mol(-1) for n=1 to less than 10 kcal mol(-1) for n=6. In the transition structures, the Mg atom is close to the oxidation state of +2, and H(2)O molecules in the first solvation shell are much more effective in stabilizing the transition structures and lowering the reaction barriers than H(2)O molecules in the other solvation shells. While the reaction barrier for trimer core structures with only three H(2)O molecules in the first shell is greater than 24 kcal mol(-1), even for Mg(+)(H(2)O)(6), it drops considerably for clusters with four-six H(2)O molecules in the first shell. The more highly coordinated complexes have comparable or slightly higher energy than the trimer core structures, and the presence of such high coordination number complexes is the underlying kinetic factor for the switching on of the hydrogen-loss reaction around n=6. For clusters with trimer core structures, the hydrogen loss reaction is much easier when it is preceded by an isomerization step that increases the coordination number around Mg(+). Delocalization of the electron on the singly occupied molecular orbital (SOMO) away from the Mg(+) ion is observed for the hexamer core structure, while at the same time this isomer is the most reactive for the hydrogen-loss reaction, with an energy barrier of only 2.7 kcal mol(-1) at the MP2/6-31G** level.     

Mid-infrared characterization of the NH4 +(H2O)n clusters in the neighborhood of the n=20 "magic" number

Vibrational predissociation spectra are reported for size-selected NH4+ (H2O)n clusters (n=5-22) in the 2500-3900 cm(-1) region. We concentrate on the sharp free OH stretching bands to deduce the local H-bonding configurations of water molecules on the cluster surface. As in the spectra of the protonated water clusters, the free OH bands in NH4+ (H2O)n evolve from a quartet at small sizes (n<7), to a doublet around n=9, and then to a single peak at the n=20 magic number cluster, before the doublet re-emerges at larger sizes. This spectral simplification at the magic number cluster mirrors that found earlier in the H+(H2O)n clusters. We characterize the likely structures at play for the n=19 and 20 clusters with electronic structure calculations. The most stable form of the n=20 cluster is predicted to have a surface-solvated NH4+ ion that lies considerably lower in energy than isomers with the NH4+ in the interior.     

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.     

Microsolvation of Co2+ and Ni2+ by acetonitrile and water: photodissociation dynamics of M(2+)(CH3CN)n(H2O)m

The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560-660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M(2+)(CH(3)CN)(n) (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H(+)(CH(3)CN), is observed as a minor channel for M(2+)(CH(3)CN)(2)(H(2)O)(2) and M(2+)(CH(3)CN)(2)(H(2)O) but is not seen in M(2+)(CH(3)CN)(3)(H(2)O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M(2+)(H(2)O)(4). Measurements of the photodissociation yield reveal that four-coordinate Co(2+) clusters dissociate more readily than Ni(2+) clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni(2+) clusters. For the two-coordinate clusters, dissociation is via electron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co(2+)(CH(3)CN)(2)(H(2)O) is calculated to occur via a (CH(3)CN)Co(2+)-OH(-)-H(+)(NCCH(3)) salt-bridge transition state, reducing kinetic energy release in the dissociation.