Excess electron and lithium atom solvation in water clusters at finite temperature: an ab initio molecular dynamics study of the structural, spectral, and dynamical behavior of (H2O)6- and Li(H2O)6

The roles of hydrogen bonds in the solvation of an excess electron and a lithium atom in water hexamer cluster at 150 K have been studied by means of ab initio molecular dynamics simulations. It is found that the hydrogen bonded structures of (H(2)O)(6)(-) and Li(H(2)O)(6) clusters are very different from each other and they dynamically evolve from one conformer to other along their simulation trajectories. The populations of the single acceptor, double acceptor, and free type water molecules are found to be significantly high unlike that in pure water clusters. Free hydrogens of these type of water molecules primarily capture the unbound electron density in these clusters. It is found that the binding motifs of the free electron evolve with time and the vertical detachment energy of (H(2)O)(6)(-) and vertical ionization energy of Li(H(2)O)(6) also change with time. Assignments of the observed peaks in vibrational power spectra are done, and we found direct correlations between the time-averaged population of water molecules in different hydrogen bonding states and the spectral features. The dynamical aspects of these clusters have also been studied through calculations of time correlations of instantaneous stretch frequencies of OH modes which are obtained from the simulation trajectories through a time series analysis.     

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.     

The solvation of two electrons in the gaseous clusters of Na- (NH3)n and Li- (NH3)n

Alkali metal ammonia clusters, in their cationic, neutral, and anionic form, are molecular models for the alkali-ammonia solutions, which have rich variation of phases with the solvated electrons playing an important role. With two s electrons, the Na(-)(NH(3))(n) and Li(-)(NH(3))(n) clusters are unique in that they capture the important aspect of the coupling between two solvated electrons. By first principles calculations, we demonstrate that the two electrons are detached from the metal by n = 10, which produces a cluster with a solvated electron pair in the vicinity of a solvated alkali cation. The coupling of the two electrons leads to either the singlet or triplet state, both of which are stable. They are also quite distinct from the hydrated anionic clusters Na(-)(H(2)O)(n) and Li(-)(H(2)O)(n), in that the solvated electrons are delocalized and widely distributed among the solvent ammonia molecules. The Na(-)(NH(3))(n) and Li(-)(NH(3))(n) series, therefore, provide another interesting type of molecular model for the investigation of solvated electron pairs.     

Microscopic solvation of a lithium atom in water-ammonia mixed clusters: solvent coordination and electron localization in presence of a counterion

The microsolvation structures and energetics of water-ammonia mixed clusters containing a lithium atom, i.e., Li(H(2)O)(n)(NH(3)), n = 1-5, are investigated by means of ab initio theoretical calculations. Several structural aspects such as the solvent coordination to the metal ion and binding motifs of the free valence electron of the metal are investigated. We also study the energetics aspects such as the dependence of vertical ionization energies on the cluster size, and all these structural and energetics aspects are compared to the corresponding results of previously studied anionic water-ammonia clusters without a metal ion. It is found that the Li-O and Li-N interactions play a very important role in stabilizing the lithium-water-ammonia clusters, and the presence of these metal ion-solvent interactions also affect the characteristics of electron solvation in these clusters. This is seen from the spatial distribution of the singly occupied molecular orbital (SOMO) which holds the ejected valence electron of the Li atom. For very small clusters, SOMO electron density is found to exist mainly at the vicinity of the Li atom, whereas for larger clusters, it is distributed outside the first solvation shell. The free dangling hydrogens of water and ammonia molecules are involved in capturing the SOMO electron density. In some of the conformers, OH{e}HO and OH{e}HN types of interactions are found to be present. The presence of the metal ion at the center of the cluster ensures that the ejected electron is solvated at a surface state only, whereas both surface and interiorlike states were found for the free electron in the corresponding anionic clusters without a metal ion. The vertical ionization energies of the present clusters are found to be higher than the vertical detachment energies of the corresponding anionic clusters which signify a relatively stronger binding of the free electron in the presence of the positive metal counterion. The shifts in different vibrational frequencies are also calculated for the larger clusters, and the results are discussed for some of the selective modes of water and ammonia molecules that are directly influenced by the location and hydrogen bonding state of these molecules in the clusters.     

Ab initio molecular dynamics simulations of an excited state of X(-)(H(2)O)(3) (X = Cl, I) complex

Upon excitation of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters, the electron transfers from the anionic precursor to the solvent, and then the excess electron is stabilized by polar solvent molecules. This process has been investigated using ab initio molecular dynamics (AIMD) simulations of excited states of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters. The AIMD simulation results of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) are compared, and they are found to be similar. Because the role of the halogen atom in the photoexcitation mechanism is controversial, we also carried out AIMD simulations for the ground-state bare excess electron -- water trimer [e(-)(H(2)O)(3)] at 300 K, the results of which are similar to those for the excited state of X(-)(H(2)O)(3) with zero kinetic energy at the initial excitation. This indicates that the rearrangement of the complex is closely related to that of e(-)(H(2)O)(3), whereas the role of the halide anion is not as important.     

Vibrational analysis of Ag3(PO2NH)3, Na3(PO2NH)3 x H2O, Na3(PO2NH)3 x 4H2O,

FT IR and FT Raman spectra of Ag3(PO2NH), (Compound 1), Na3(PO2NH)3 x H2O (Compound II), Na3(PO2NH)3 x 4H2O (Compound III), [C(NH2)3]3(PO2NH)3 x H2O (Compound IV) and (NH4)4(PO2NH)4 x 4H2O (Compound V) are recorded and analyzed on the basis of the anions, cations and water molecules present in each of them. The PO2NH- anion ring in compound I is distorted due to the influence of Ag+ cation. Wide variation in the hydrogen bond lengths in compound III is indicated by the splitting of the v2 and v3 modes of vibration of water molecules. The NH4 ion in compound V occupies lower site symmetry and exhibits hindered rotation in the lattice. The correlations between the symmetric and asymmetric stretching vibrations of P-N-P bridge and the P-N-P bond angle have also been discussed.     

Racemic monoperoxovanadium(V) complexes with achiral OO and ON donor set heteroligands: synthesis, crystal structure and stereochemistry of [NH3(CH2)2NH3][VO(O2)(ox)(pic)].2H2O and [NH3(CH2)2NH3][VO(O2)(ox)(pca)

Monoperoxovanadium(V) complexes, [NH3(CH2)2NH3][VO(O2)(ox)(pic)].2H2O (1) and [NH3(CH2)2NH3][VO(O2)(ox)(pca)] (2) [NH3(CH2)2NH3 = ethane-1,2-diammonium(2+), ox=oxalate(2-), pic=pyridine-2-carboxylate(1-), pca=pyrazine-2-carboxylate(1-)], were synthesized and characterized by X-ray analysis, IR and Raman spectroscopies. The five equatorial positions of the pentagonal bipyramid around the vanadium atoms are occupied by the eta2-peroxo ligand, two oxygen atoms of the ox, and the nitrogen atom of the pic or pca ligands, respectively. The oxo ligand and the oxygen atom of pic or pca are in the axial positions. Networks of X-HO (X=C, N or O) hydrogen bonds, and pi-pi interactions between aromatic rings in and anion-pi interactions in , determine the molecular packings and build up the supramolecular architecture. Three stereochemical rules for occupation of the donor sites in two-heteroligand [VO(O2)(L1)(L2)] complexes (L1, L2 are bidentate neutral or differently charged anionic heteroligands providing an OO, NN or ON donor set) are discussed. and crystallize as racemic compounds. The 51V NMR spectra proved that the parent complex anions of and partially decompose on dissolution in water to the monoperoxo-ox, -pic or -pca complexes.     

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.     

Isotope exchange and structural rearrangements in reactions between size-selected ionic water clusters, H3O(+)(H2O)(n) and NH4(+)(H2O)(n), and D2O

Hydrogen/deuterium exchange in reactions of H3O(+)(H2O)n and NH4(+)(H2O)n (1 < or = n < or = 30) with D2O has been studied experimentally at center-of-mass collisions energies of < or = 0.2 eV. For a given cluster size, the cross-sections for H3O(+)(H2O)n and NH4(+)(H2O)n are similar, indicating a structural resemblance and energetics of binding. For protonated pure water clusters, H3O(+)(H2O)n, reacting with D2O the main H/D exchange mechanism is found to be proton catalyzed. In addition the H/D scrambling becomes close to statistically randomized for the larger clusters. For NH4(+)(H2O)n clusters reacting with D2O, the main mechanism is a D2O/H2O swap reaction. The lifetimes of H3O(+)(H2O)n clusters have been estimated using RRKM theory and a plateau in lifetime vs. cluster size is found already at n = 10.     

Electron solvation in water-ammonia mixed clusters: Structure, energetics, and the nature of localization states of the excess electron

The structure and energetics of water-ammonia mixed clusters with an excess electron, [(H2O)n(NH3)m]- with m=1, n=2-6 and m=2, n=2, and also the corresponding neutral clusters are investigated in detail by means of ab initio quantum chemical calculations. The authors focus on the localization structure of the excess electron with respect to its surface versus interiorlike states, its binding to ammonia versus water molecules, the spatial and orientational arrangement of solvent molecules around the excess electron, the changes of the overall hydrogen-bonded structure of the clusters as compared to those of the neutral ones and associated dipole moment changes, vertical detachment energies of the anionic clusters, and also the vertical attachment energies of the neutral clusters. It is found that the hydrogen-bonded structure of the anionic clusters are very different from those of the neutral clusters unlike the case of water-ammonia dimer anion, and these changes in structural arrangements lead to drastically different dipole moments of the anionic and the neutral clusters. The spatial distribution of the singly occupied molecular orbital holding the excess electron shows only surface states for the smaller clusters. However, for n=5 and 6, both surface and interiorlike binding states are found to exist for the excess electron. For the surface states, the excess electron can be bound to the dangling hydrogens of either an ammonia or a water molecule with different degrees of stability and vertical detachment energies. The interiorlike states, wherever they exist, are found to have a higher vertical detachment energy than any of the surface states of the same cluster. Also, for interiorlike states, the ammonia molecule with its dangling hydrogens is always found to stay on top or on a far side of the charge density of the excess electron without participating in the hydrogen bond network of the cluster; the intermolecular hydrogen bonds are formed by the water molecules only which add to the overall stability of these anionic clusters.     

Sequential addition of H2O, CH3OH, and NH3 to Al3O3-: a theoretical study

Photoelectron spectra of two species, Al3O3(H2O)2- and Al3O3(CH3OH)2-, that are produced by the addition of two water or methanol molecules to Al3O3- are interpreted with density-functional geometry optimizations and electron propagator calculations of vertical electron detachment energies. In both cases, there is only one isomer that is responsible for the observed spectral features. A high barrier to the addition of a second molecule may impede the formation of Al3O3N2H6- clusters in an analogous experiment with NH3.     

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.     

A novel series of materials composed of arrays of vanadium oxide container molecules, [V18O42(X)] (X = H2O,Cl-, Br-): synthesis and characterization of [M2(H2N(CH2)2NH2)5][(M(H2N(CH2)2NH2)2]2V18O42(X)]. 9H2O (M = Zn,Cd)

The synthesis and characterization of the novel systems [Zn(2)(H(2)N(CH(2))(2)NH(2))(5)][(Zn(H(2)N(CH(2))(2)NH(2))(2))(2)V(18)O(42)(H(2)O)].9H(2)O (1), [Cd(2)(H(2)N(CH(2))(2)NH(2))(5)][(Cd(H(2)N(CH(2))(2)NH(2))(2))(2)V(18)O(42)(Br)].9H(2)O (2), and [Zn(2)(H(2)N(CH(2))(2)NH(2))(5)][(Zn(H(2)N(CH(2))(2)NH(2))(2))(2)V(18)O(42)(Cl)].9H(2)O (3) have been described. These materials represent a new class of solids that have been prepared by combining conventional coordination compounds with spherical polyoxovanadate clusters. The isomorphous structures of these hybrid solids consist of two-dimensional arrays of container cluster molecules [V(18)O(42)(X)] (X = H(2)O, Br-, Cl-) interlinked by the transition metal complex moieties [M(H(2)N(CH(2))(2)NH(2))(2)] (M = Zn, Cd). These compounds contain an unprecedented complex cation, [M(2)(H(2)N(CH(2))(2)NH(2))(5)](4+). Crystal data for 1: C(9)H(46)N(9)O(26)V(9)Zn(2), monoclinic space group P2(1)/m (No. 11), a = 12.3723(7) A, b = 20.9837(11) A, c = 15.8379(8) A, beta = 97.3320(10) degrees, Z = 4.     

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.     

Infrared predissociation spectroscopy of M+ (C6H6)(1-4)(H2O)(1-2)Ar(0-1) cluster ions, M = Li, Na

Infrared predissociation (IRPD) spectra of Li(+)(C(6)H(6))(1-4)(H(2)O)(1-2)Ar(0-1) and Na(+)(C(6)H(6))(2-4)(H(2)O)(1-2)Ar(1) are presented along with ab initio calculations. The results indicate that the global minimum energy structure for Li(+)(C(6)H(6))(2)(H(2)O)(2) has each water forming a π-hydrogen bond with the same benzene molecule. This bonding motif is preserved in Li(+)(C(6)H(6))(3-4)(H(2)O)(2)Ar(0-1) with the additional benzene ligands binding to the available free OH groups. Argon tagging allows high-energy Li(+)(C(6)H(6))(2-4)(H(2)O)(2)Ar isomers containing water-water hydrogen bonds to be trapped and detected. The monohydrated, Li(+) containing clusters contain benzene-water interactions with varying strength as indicated by shifts in OH stretching frequencies. The IRPD spectra of M(+)(C(6)H(6))(1-4)(H(2)O)(1-2)Ar are very different for lithium-bearing versus sodium-bearing cluster ions emphasizing the important role of ion size in determining the most favorable balance of competing noncovalent interactions.     

The double H‐atom acceptability of the P=O group in new XP(O)(NHCH2C6H4‐2‐Cl)2 phosphoramidates [X = C6H5O– and CF3C(O)NH–]: a database analysis of compounds having a P(O)(NHR) group

In the hydrogen‐bond patterns of phenyl bis(2‐chlorobenzylamido)phosphinate, C₂₀H₁₉Cl₂N₂O₂P, (I), and N,N′‐bis(2‐chlorobenzyl)‐N′′‐(2,2,2‐trifluoroacetyl)phosphoric triamide, C₁₆H₁₅Cl₂F₃N₃O₂P, (II), the O atoms of the related phosphoryl groups act as double H‐atom acceptors, so that the P=O...(H—N)₂ hydrogen bond in (I) and the P=O...(H—N_amide)₂ and C=O...H—N_C(O)NHP(O) hydrogen bonds in (II) are responsible for the aggregation of the molecules in the crystal packing. The presence of a double H‐atom acceptor centre is a result of the involvement of a greater number of H‐atom donor sites with a smaller number of H‐atom acceptor sites in the hydrogen‐bonding interactions. This article also reviews structures having a P(O)NH group, with the aim of finding similar three‐centre hydrogen bonds in the packing of phosphoramidate compounds. This analysis shows that the factors affecting the preference of the above‐mentioned O atom to act as a double H‐atom acceptor are: (i) a higher number of H‐atom donor sites relative to H‐atom acceptor centres in molecules with P(=O)(NH)₃, (N)P(=O)(NH)₂, C(=O)NHP(=O)(NH)₂ and (NH)₂P(=O)OP(=O)(NH)₂ groups, and (ii) the remarkable H‐atom acceptability of this atom relative to the other acceptor centre(s) in molecules containing an OP(=O)(NH)₂ group, with the explanation that the N atom bound to the P atom in almost all of the structures found does not take part in hydrogen bonding as an acceptor. Moreover, the differences in the H‐atom acceptability of the phosphoryl O atom relative to the O atom of the alkoxy or phenoxy groups in amidophosphoric acid esters may be illustrated by considering the molecular packing of compounds having (O)₂P(=O)(NH) and (O)P(=O)(NH)(N)groups, in which the unique N—H unit in the above‐mentioned molecules almost always selects the phosphoryl O atom as a partner in forming hydrogen‐bond interactions. The P atoms in (I) and (II) are in tetrahedral coordination environments, and the phosphoryl and carbonyl groups in (II) are anti with respect to each other (the P and C groups are separated by one N atom). In the crystal structures of (I) and (II), adjacent molecules are linked via the above‐mentioned hydrogen bonds into a linear arrangement parallel to [100] in both cases, in (I) by forming R₂²(8) rings and in (II) through a combination of R₂²(10) and R₂¹(6) rings.     

Charge‐transfer‐to‐solvent absorption spectra of I−(H2O)3–5 at a finite temperature via simulation

Ground‐state equilibrium Born–Oppenheimer molecular dynamics on I^?(H₂O)_3–5 clusters at ～200 K are performed to sample configurations for calculating the charge‐transfer‐to‐solvent (CTTS) absorption spectra for these clusters. When there are more water molecules in clusters, the calculated CTTS spectra are found to become more intense with the absorption maxima shifting to higher energies, which is in agreement with experimental results. In addition, compared with the findings for optimized structures, the absorption energies of the iodide 5p orbitals are red‐shifted at ～200 K because, on average, the distances between the iodide and the dangling hydrogen atoms are increased at finite temperatures which weakens the interactions between the iodide and water molecules in the clusters. Moreover, the number of ionic hydrogen bonds in the clusters are also reduced. However, it is found that all dangling hydrogen atoms must be considered to obtain a good correlation between the CTTS excitation energy and the average distance between the iodide and the dangling hydrogen atoms, which indicates the existence of the strong interactions of the CTTS electron with all of the dangling hydrogen atoms.     

Transition from hydrogen atom to hydride abstraction by Mn4O4(O2PPh2)6 versus [Mn4O4(O2PPh2)6]+: O-H bond dissociation energies and the formation of Mn4O3(OH)(O2PPh2)6

Synthesis, characterization, and reactions of the novel manganese-oxo cubane complex [Mn(4)O(4)(O(2)PPh(2))(6)](ClO(4)), 1+ (ClO(4)(-)), are described. Cation 1+ is composed of the [Mn(4)O(4)](7+) core surrounded by six bidentate phosphinate ligands. The proton-coupled electron transfer (pcet) reactions of phenothiazine (pzH), the cation radical (pzH(.+)(ClO(4)(-)), and the neutral pz* radical with 1+ are reported and compared to Mn(4)O(4)(O(2)PPh(2))(6) (1). Compound 1+ (ClO(4)(-)) reacts with excess pzH via four sequential reduction steps that transfer a total of five electrons and four protons to 1+. This reaction forms the doubly dehydrated manganese cluster Mn(4)O(2)(O(2)PPh(2))(6) (2) and two water molecules derived from the corner oxygen atoms. The first pcet step forms the novel complex Mn(4)O(3)(OH)(O(2)PPh(2))(6) (1H) and 1 equiv of the pz+ cation by net hydride transfer from pzH. Spectroscopic characterization of isolated 1H is reported. Reduction of 1 by pzH or a series of para-substituted phenols also produces 1H via net H atom transfer. A lower limit to the homolytic bond dissociation energy (BDE) (1H --> 1 + H) was estimated to be >94 kcal/mol using solution phase BDEs for pzH and para-substituted phenols. The heterolytic BDE was estimated for the hydride transfer reaction 1H --> 1+ + H(-) (BDE approximately 127 kcal/mol). These comparisons reveal the O-H bond in 1H to be among the strongest of any Mn-hydroxo complex measured thus far. In three successive H atom transfer steps, 1H abstracts three hydrogen atoms from three pzH molecules to form complex 2. Complex 2 is shown to be identical to the "pinned butterfly" cluster produced by the reaction of 1 with pzH (Ruettinger, W. F.; Dismukes, G. C. Inorg. Chem. 2000, 39, 1021-1027). The Mn oxidation states in 2 are formally Mn(4)(2II,2III), and no further reduction occurs in excess pzH. By contrast, outer-sphere electron-only reductants such as cobaltacene reduce both 1+ and 1 to the all Mn(II) oxidation level and cause cluster fragmentation. The reaction of pzH(.+) with 1+ produces 1H and the pz+ cation by net hydrogen atom transfer, and terminates at 1 equiv of pzH(.+) with no further reaction at excess. By contrast, pz* does not react with 1+ at all, indicating that reduction of 1+ by electron transfer to form pz+ does not occur without a proton (pcet to 1+ is thermodynamically required). Experimental free energy changes are shown to account for these pcet reactions and the absence of electron transfer for any of the phenothiazine series. Hydrogen atom abstraction from substrates by 1 versus hydride abstraction by 1(+ )()illustrates the transition to two-electron one-proton pcet chemistry in the [Mn(4)O(4)](7+) core that is understood on the basis of free energy consideration. This transition provides a concrete example of the predicted lowest-energy pathway for the oxidation of two water molecules to H(2)O(2) as an intermediate within the photosynthetic water-oxidizing enzyme (vs sequential one-electron/proton steps). The implications for the mechanism of photosynthetic water splitting are discussed.     

Ionization induced relaxation in solvation structure: a comparison between Na(H2O)n and Na(NH3)n

The constant ionization potential for hydrated sodium clusters Na(H2O)n just beyond n=4, as observed in photoionization experiments, has long been a puzzle in violation of the well-known (n+1)(-1/3) rule that governs the gradual transition in properties from clusters to the bulk. Based on first principles calculations, a link is identified between this puzzle and an important process in solution: the reorganization of the solvation structure after the removal of a charged particle. Na(H2O)n is a prototypical system with a solvated electron coexisting with a solvated sodium ion, and the cluster structure is determined by a balance among three factors: solute-solvent (Na+-H2O), solvent-solvent (H2O-H2O), and electron-solvent (OH{e}HO) interactions. Upon the removal of an electron by photoionization, extensive structural reorganization is induced to reorient OH{e}HO features in the neutral Na(H2O)n for better Na+-H2O and H2O-H2O interactions in the cationic Na+(H2O)n. The large amount of energy released, often reaching 1 eV or more, indicates that experimentally measured ion signals actually come from autoionization via vertical excitation to high Rydberg states below the vertical ionization potential, which induces extensive structural reorganization and the loss of a few solvent molecules. It provides a coherent explanation for all the peculiar features in the ionization experiments, not only for Na(H2O)n but also for Li(H2O)n and Cs(H2O)n. In addition, the contrast between Na(H2O)n and Na(NH3)n experiments is accounted for by the much smaller relaxation energy for Na(NH3)n, for which the structures and energetics are also elucidated.     

First characterization of the ammine-ammonium complex [(NH4(NH3)4)2(mu-NH3)2]2+ in the crystal structure of [NH4(NH3)4][B(C6H5)4].NH3 and the [NH4(NH3)4]+ complex in [NH4(NH3)4][Ca(NH3)7]As3S6.2NH3 and [NH4(NH3)4][Ba(NH3)8]As3S6.NH3

The compound [NH4(NH3)4][B(C6H5)4].NH3 (1) was prepared by the reaction of NaB(C(6)H(5))(4) with a proton-charged ion-exchange resin in liquid ammonia. [NH(4)(NH(3))(4)][Ca(NH(3))(7)]As(3)S(6).2NH(3) (2) and [NH4(NH3)4][Ba(NH3)8]As3S6.NH3 (3) were synthesized by reduction of As(4)S(4) with Ca and Ba in liquid ammonia. All ammoniates were characterized by low-temperature single-crystal X-ray structure analysis. They were found to contain the ammine-ammonium complex with the maximal possible number of coordinating ammonia molecules, the [NH4(NH3)4]+ ion. 1 contains a special dimer, the [(NH4(NH3)4)2(mu-NH3)2]2+ ion, which is formed by two[NH4(NH3)4]+ ions linked by two ammonia molecules. The H(3)N-H...N hydrogen bonds in all three compounds range from 1.82 to 2.20 A (DHA = Donor-H...Acceptor angles: 156-178 degrees). In 2 and 3, additional H(2)N-H...S bonds to the thioanions are observed, ranging between 2.49 and 3.00 A (DHA angles: 120-175 degrees). Two parallel phenyl rings of the [B(C(6)H(5))(4)](-) anion in 1 form a pi...pi hydrogen bond (C...C distance, 3.38 A; DHA angles, 82 degrees), leading to a dimeric [B(C6H5)4]2(2-) ion.