Electronic properties of hydrogen-bonded complexes of benzene(HCN)(1-4): comparison with benzene(H2O)(1-4)

The electronic properties, specifically, the dipole and quadrupole moments and the ionization energies of benzene (Bz) and hydrogen cyanide (HCN), and the respective binding energies, of complexes of Bz(HCN)(1-4), have been studied through MP2 and OVGF calculations. The results are compared with the properties of benzene-water complexes, Bz(H(2)O)(1-4), with the purpose of analyzing the electronic properties of microsolvated benzene, with respect to the strength of the CH/π and OH/π hydrogen-bond (H-bond) interactions. The linear HCN chains have the singular ability to interact with the aromatic ring, preserving the symmetry of the latter. A blue shift of the first vertical ionization energies (IEs) of benzene is observed for the linear Bz(HCN)(1-4) clusters, which increases with the length of the chain. NBO analysis indicates that the increase of the IE with the number of HCN molecules is related to a strengthening of the CH/π H-bond, driven by cooperative effects, increasing the acidity of the hydrogen cyanide H atom involved in the π H-bond. The longer HCN chains (n ≥ 3), however, can bend to form CH/N H-bonds with the Bz H atoms. These cyclic structures are found to be slightly more stable than their linear counterparts. For the nonlinear Bz(HCN)(3-4) and Bz(H(2)O)(2-4) complexes, an increase of the binding energy with the number of solvent molecules and a decrease of the IE of benzene, relative to the values for the Bz(HCN) and Bz(H(2)O) complexes, respectively, are observed. Although a strengthening of the CH/π and OH/π H-bonds, with increasing n, also takes place for the Bz(H(2)O)(2-4) and Bz(HCN)(3-4) nonlinear complexes, Bz proton donor, CH/O, and CH/N interactions are at the origin of this decrease. Thus CH/π and OH/π H-bonds lead to higher IEs of Bz, whereas the weaker CH/N and CH/O H-bond interactions have the opposite effect. The present results emphasize the importance of both aromatic XH/π (X = C, O) and CH/X (X = N, O) interactions for understanding the structure and electronic properties of Bz(HCN)(n) and Bz(H(2)O)(n) complexes.     

Hydration and dissociation of hydrogen fluoric acid (HF)

The hydration and dissociation phenomena of HF(H(2)O)(n)() (n < or = 10) clusters have been studied by using both the density functional theory with the 6-311++G[sp] basis set and the M?ller-Plesset second-order perturbation theory with the aug-cc-pVDZ+(2s2p/2s) basis set. The structures for n > or = 8 are first reported here. The dissociated form of the hydrogen-fluoric acid in HF(H(2)O)(n) clusters is found to be less stable at 0 K than the undissociated form until n = 10. HF may not be dissociated at 0 K solely by water molecules because the HF H bond is stronger than the OH H bond, against the expectation that the dissociated HF(H(2)O)(n) would be more stable than the undissociated one in the presence of a number of water molecules. The dissociation would be possible for only a fraction of a number of hydrated HF clusters by the Boltzmann distribution at finite temperatures. This is in sharp contrast to other hydrogen halide acids (HCl, HBr, HI) showing the dissociation phenomena at 0 K for n > or = 4. The IR spectra of dissociated and undissociated structures of HF(H(2)O)(n) are compared. The structures and binding energies of HF(H(2)O)(n) are found to be similar to those of (H(2)O)(n+1). It is interesting that HF(H(2)O)(n=5,6,10) are slightly less stable compared with other sizes of clusters, just like the fact that (H(2)O)(n=6,7,11) are slightly less stable. The present study would be useful for the experimental/spectroscopic investigation of not only the dissociation phenomena of HF but also the similarity of the HF-water clusters to the water clusters.     

Metal complexes of tetrapodal ligands: synthesis, spectroscopic and thermal studies, and X-ray crystal structure studies of Na(I), Ca(II), Sr(II), and Ba(II) complexes of tetrapodal ligands N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine and N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine

Twelve complexes 1-12 of general category [M(ligand)(anion)(x)(water)(y)], where ligand = N,N,N',N'-tetrakis(2-hydroxypropyl/ethyl)ethylenediamine (HPEN/HEEN), anion = anions of picric acid (PIC), 3,5-dinitrobenzoic acid (DNB), 2,4-dinitrophenol (DNP), and o-nitrobenzoic acid (ONB), M = Ca(2+), Sr(2+), Ba(2+), or Na(+), x = 1 and 2, and y = 0-4, were synthesized. All of these complexes were characterized by elemental analysis, IR, (1)H and (13)C NMR, and thermal studies. X-ray crystal studies of these complexes 1-12, [Ca(HPEN)(H(2)O)(2)](PIC)(2).H(2)O (1), [Ca(HEEN)(PIC)](PIC) (2), Ba(HPEN)(PIC)(2) (3), [Na(HPEN)(PIC)](2) (4), Ca(HPEN)(H(2)O)(2)](DNB)(2).H(2)O (5),Ca(HEEN)(H(2)O)](DNB)(2).H(2)O (6), [Sr(HPEN)(H(2)O)(3)](DNB)(2) (7), [Ba(HPEN)(H(2)O)(2)](DNB)(2).H(2)O](2) (8), [[Ba(HEEN)(H(2)O)(2)](ONB)(2)](2) (9), [[Sr(HPEN)(H(2)O)(2)](DNP)(2)](2) (10), [[Ba(HPEN)(H(2)O)(2)](DNP)(2)](2) (11), and [Ca(HEEN)(DNP)](DNP) (H(2)O) (12), have been carried out at room temperature. Factors which influence the stability and the type of complex formed have been recognized as H-bonding interactions, presence/absence of solvent, nature of the anion, and nature of the cation. Both the ligands coordinate the metal ion through all the six available donor atoms. The complexes 1 and 5-11 have water molecules in the coordination sphere, and their crystal structures show that water is playing a dual character. It coordinates to the metal ion on one hand and strongly hydrogen bonds to the anion on the other. These strong hydrogen bonds stabilize the anion and decrease the cation-anion interactions by many times to an extent that the anions are completely excluded out of the coordination sphere and produce totally charge-separated complexes. In the absence of water molecules as in 2 and 3 the number of hydrogen bonds is reduced considerably. In both the complexes the anions case interact more strongly with the metal ion to give rise to a partially charge-separated 2 or tightly ion-paired 3 complex. High charge density Ca(2+) forms only monomeric complexes. It has more affinity toward stronger nucleophiles such as DNP and PIC with which it gives partially charge-separated eight-coordinated complexes. But with relatively weaker nucleophile like DNB, water replaces the anion and produces a seven coordinated totally charge-separated complex. Sr(2+) with lesser charge/radius ratio forms only charge-separated monomeric as well as dimeric complexes. Higher coordination number of Sr(2+) is achieved with coordinated water molecules which may be bridging or nonbridging in nature. All charge-separated complexes of the largest Ba(2+) are dimeric with bridging water molecules. Only one monomeric ion-paired complex was obtained with Ba(PIC)(2). Na(+) forms a unique dinuclear cryptand-like complex with HPEN behaving as a heptadentate chelating-cum-bridging ligand.     

Theoretical study of negatively charged Fe(-)-(H2O)(n ≤ 6) clusters

Interactions of a singly negatively charged iron atom with water molecules, Fe(-)-(H(2)O)(n≤6), in the gas phase were studied by means of density functional theory. All-electron calculations were performed using the B3LYP functional and the 6-311++G(2d,2p) basis set for the Fe, O, and H atoms. In the lowest total energy states of Fe(-)-(H(2)O)(n), the metal-hydrogen bonding is stronger than the metal-oxygen one, producing low-symmetry structures because the water molecules are directly attached to the metal by basically one of their hydrogen atoms, whereas the other ones are involved in a network of hydrogen bonds, which together with the Fe(δ-)-H(δ+) bonding accounts for the nascent hydration of the Fe(-) anion. For Fe(-)-(H(2)O)(3≤n), three-, four-, five-, and six-membered rings of water molecules are bonded to the metal, which is located at the surface of the cluster in such a way as to reduce the repulsion with the oxygen atoms. Nevertheless, internal isomers appear also, lying less than 3 or 5 kcal/mol for n = 2-3 or n = 4-6. These results are in contrast with those of classical TM(+)-(H(2)O)(n) complexes, where the direct TM(+)-O bonding usually produces high symmetry structures with the metal defining the center of the complex. They show also that the Fe(-) anions, as the TM(+) ions, have great capability for the adsorption of water molecules, forming Fe(-)-(H(2)O)(n) structures stabilized by Fe(δ-)-H(δ+) and H-bond interactions.     

Hydration profiles of aromatic amino acids: conformations and vibrations of L-phenylalanine-(H2O)n clusters

IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine-(H(2)O)(n) clusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of L-phenylalanine (L-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for L-Phe-(H(2)O)(1), and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In L-Phe-(H(2)O)(2), only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of L-Phe-(H(2)O)(3) the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the pi-system. Finally, in isolated L-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initio calculation.     

From discrete [Y(DMF)8][Cu4(mu3-I)2(mu-I)3I2] ion pairs to extended [Y(DMF)6(H2O)2][Cu7(mu4-I)3(mu3-I)2(mu-I)4(I)]1infinity and [Y(DMF)6(H2O)3][Cu(I)7Cu(II)2(mu3-I)8(mu-I)6]2infinity arrays by H-bond templating in a confined solvent-free environment

An ionic heterometallic species [Y(DMF)(8)][Cu(4)(micro(3)-I)(2)(micro-I)(3)I(2)](1) was isolated from a solution of CuI, NH(4)I and YI(3)(Pr(i)OH)(4) in DMF-isopropoxyethanol, and was converted in a confined environment by progressive substitution of the DMF ligands with water molecules first into a 1D zig-zag structure [Y(DMF)(6)(H(2)O)(2)][Cu(7)(micro(4)-I)(3)(micro(3)-I)(2)(micro-I)(4)(I)](1infinity)(2) and finally into a 2D sheet [Y(DMF)(6)(H(2)O)(3)][Cu(I)(7)Cu(II)(2)(micro(3)-I)(8)(micro-I)(6)](2infinity)(3) by H-bond templating.     

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.     

Theoretical study of the solvation of HgCl2, HgClOH, Hg(OH)2 and HgCl3(-): a density functional theory cluster approach

The determination of the solvation shell of Hg(II)-containing molecules and especially the interaction between Hg(II) and water molecules is the first requirement to understand the transmembrane passage of Hg into the cell. We report a systematic DFT study by stepwise solvation of HgCl(2) including up to 24 water molecules. In order to include pH and salinity effects, the solvation patterns of HgClOH, Hg(OH)(2) and HgCl(3)(-) were also studied using 24 water molecules. In all cases the hydrogen bond network is crucial to allow orbital-driven interactions between Hg(II) and the water molecules. DFT Born-Oppenheimer molecular dynamics simulations starting from the stable HgCl(2)-(H(2)O)(24) structure revealed that an HgCl(2)-(H(2)O)(3) trigonal bipyramid effective solute appears and then the remaining 21 water molecules build a complete first solvation shell, in the form of a water-clathrate. In the HgCl(2), HgClOH, Hg(OH)(2)-(H(2)O)(24) optimized structures Hg also directly interacts with 3 water molecules from an orbital point of view (three Hg-O donor-acceptor type bonds). All the other interactions are through hydrogen bonding. The cluster-derived solvation energies of HgCl(2), HgClOH and Hg(OH)(2) are estimated to be -34.4, -40.1 and -47.2 kcal mol(-1), respectively.     

Site-specific addition of D(2)O to the (H(2)O)(6)(-) "hydrated electron" cluster: isomer interconversion and substitution at the double H-bond acceptor (AA) electron-binding site

We report the results of an experimental study designed to establish whether, once formed, one of the isomer classes of the hydrated electron clusters, (H(2)O)(n)(-), can interconvert with others when a water molecule is added by condensation. This is accomplished in an Ar-cluster mediated approach where a single intact D(2)O molecule is collisionally incorporated into argon-solvated water hexamer anions, creating the isotopically labeled D(2)O.(H(2)O)(6)(-).Ar(n) heptamer anion. Photoelectron and infrared predissociation spectroscopies are employed both to characterize the isomers generated in the condensation event and to track the position that the D(2)O label adopts within these isomeric structures. Despite the fact that the water hexamer anion precursor clusters initially exist in the isomer I form, incorporation of D(2)O produces mostly isomers I' and II in the labeled heptamer, which bind the electron more (I') or less (II) strongly than does the isomer I class. Isomers I and I' are known to feature electron binding primarily onto a single water molecule that resides in an AA (A = H-bond acceptor) site in the network. Surprisingly, the D(2)O molecule can displace this special electron-binding H(2)O molecule such that the anionic cluster retains the high binding arrangement. In the more weakly binding isomer II clusters, the D(2)O molecule fractionates preferentially to sites that give rise to the vibrational signature of isomer II.     

Two polymorphs of bis(2‐carbamoylguanidinium) fluorophosphonate dihydrate

Two polymorphs of bis(2‐carbamoylguanidinium) fluorophosphonate dihydrate, 2C₂H₇N₄O⁺·FO₃P²⁻·2H₂O, are presented. Polymorph (I), crystallizing in the space group Pnma, is slightly less densely packed than polymorph (II), which crystallizes in Pbca. In (I), the fluorophosphonate anion is situated on a crystallographic mirror plane and the O atom of the water molecule is disordered over two positions, in contrast with its H atoms. The hydrogen‐bond patterns in both polymorphs share similar features. There are O—H...O and N—H...O hydrogen bonds in both structures. The water molecules donate their H atoms to the O atoms of the fluorophosphonates exclusively. The water molecules and the fluorophosphonates participate in the formation of R⁴₄(10) graph‐set motifs. These motifs extend along the a axis in each structure. The water molecules are also acceptors of either one [in (I) and (II)] or two [in (II)] N—H...O hydrogen bonds. The water molecules are significant building elements in the formation of a three‐dimensional hydrogen‐bond network in both structures. Despite these similarities, there are substantial differences between the hydrogen‐bond networks of (I) and (II). The N—H...O and O—H...O hydrogen bonds in (I) are stronger and weaker, respectively, than those in (II). Moreover, in (I), the shortest N—H...O hydrogen bonds are shorter than the shortest O—H...O hydrogen bonds, which is an unusual feature. The properties of the hydrogen‐bond network in (II) can be related to an unusually long P—O bond length for an unhydrogenated fluorophosphonate anion that is present in this structure. In both structures, the N—H...F interactions are far weaker than the N—H...O hydrogen bonds. It follows from the structure analysis that (II) seems to be thermodynamically more stable than (I).     

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.     

Structure, stability and spectral signatures of monoprotic carborane acid-water clusters (CBW(n), where n = 1-6)

The gas phase structure, stability, spectra, and proton transfer properties of monoprotic carborane acid-water clusters [CB(11)F(m)H(11-m)(OH(2))(1)]-(H(2)O)(n) (where m = 0, 5, and 10; n = 1-6) have been calculated using density functional theory (DFT) with the Becke's three-parameter hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP) using 6-31+G* basis set. Results reveal that Eigen cation defects are found in CBW(n) (where n = 2-6) clusters and these clusters are significantly more stable than the non-Eigen geometry. In addition to the conventional hydrogen bond (H-bond) the role of dihydrogen bond (DHB) and halogen bond (XB) in the stabilization of these clusters can be observed from the molecular graphs derived from the atoms in molecules (AIM) analysis. Spectral information shows the features of Eigen cation and proton oscillation involved in the proton transfer process. The dissociation of proton from the perfluoro derivatives with two water molecules is more favorable when compared to the other derivatives.     

Interaction of Co(m)O(-) (m = 1-3) with water: anion photoelectron spectroscopy and density functional calculations

We investigated the reactions between cobalt-oxides and water molecules using photoelectron spectroscopy and density functional calculations. It has been confirmed by both experimental observation and theoretical calculations that dihydroxide anions, Co(m)(OH)(2)(-) (m = 1-3), were formed when Co(m)O(-) clusters interact with the first water molecule. Addition of more water molecules produced solvated dihydroxide anions, Co(m)(OH)(2)(H(2)O)(n)(-) (m = 1-3). Hydrated dihydroxide anions, Co(m)(OH)(2)(H(2)O)(n)(-), are more stable than their corresponding hydrated metal-oxide anions, Co(m)O(H(2)O)(n+1)(-).     

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.     

OH(3) (-) and O(2)H(5) (-) double Rydberg anions: predictions and comparisons with NH(4) (-) and N(2)H(7) (-)

A low barrier in the reaction pathway between the double Rydberg isomer of OH(3) (-) and a hydride-water complex indicates that the former species is more difficult to isolate and characterize through anion photoelectron spectroscopy than the well known double Rydberg anion (DRA), tetrahedral NH(4) (-). Electron propagator calculations of vertical electron detachment energies (VEDEs) and isosurface plots of the electron localization function disclose that the transition state's electronic structure more closely resembles that of the DRA than that of the hydride-water complex. Possible stabilization of the OH(3) (-) DRA through hydrogen bonding or ion-dipole interactions is examined through calculations on O(2)H(5) (-) species. Three O(2)H(5) (-) minima with H(-)(H(2)O)(2), hydrogen-bridged, and DRA-molecule structures resemble previously discovered N(2)H(7) (-) species and have well separated VEDEs that may be observable in anion photoelectron spectra.     

Clusters of hydrated methane sulfonic acid CH3SO3H.(H2O)n (n = 1-5): a theoretical study

Ab initio and density functional methods have been used to examine the structures and energetics of the hydrated clusters of methane sulfonic acid (MSA), CH3SO3H.(H2O)n (n = 1-5). For small clusters with one or two water molecules, the most stable clusters have strong cyclic hydrogen bonds between the proton of OH group in MSA and the water molecules. With three or more water molecules, the proton transfer from MSA to water becomes possible, forming ion-pair structures between CH3SO3- and H3O+ moieties. For MSA.(H2O)3, the energy difference between the most stable ion pair and neutral structures are less than 1 kJ/mol, thus coexistence of neutral and ion-pair isomers are expected. For larger clusters with four and five water molecules, the ion-pair isomers are more stable (>10 kJ/mol) than the neutral ones; thus, proton transfer takes place. The ion-pair clusters can have direct hydrogen bond between CH3SO3- and H3O+ or indirect one through water molecule. For MSA.(H2O)5, the energy difference between ion pairs with direct and indirect hydrogen bonds are less than 1 kJ/mol; namely, the charge separation and acid ionization is energetically possible. The calculated IR spectra of stable isomers of MSA.(H2O)n clusters clearly demonstrate the significant red shift of OH stretching of MSA and hydrogen-bonded OH stretching of water molecules as the size of cluster increases.     

A first principles molecular dynamics study of excess electron and lithium atom solvation in water-ammonia mixed clusters: structural, spectral, and dynamical behaviors of [(H2O)5NH3]- and Li(H2O)5NH3 at finite temperature

First principles molecular dynamics simulations are carried out to investigate the solvation of an excess electron and a lithium atom in mixed water-ammonia cluster (H(2)O)(5)NH(3) at a finite temperature of 150 K. Both [(H(2)O)(5)NH(3)](-) and Li(H(2)O)(5)NH(3) clusters are seen to display substantial hydrogen bond dynamics due to thermal motion leading to many different isomeric structures. Also, the structures of these two clusters are found to be very different from each other and also very different from the corresponding neutral cluster without any excess electron or the metal atom. Spontaneous ionization of Li atom occurs in the case of Li(H(2)O)(5)NH(3). The spatial distribution of the singly occupied molecular orbital shows where and how the excess (or free) electron is primarily localized in these clusters. The populations of single acceptor (A), double acceptor (AA), and free (NIL) type water and ammonia molecules are found to be significantly high. The dangling hydrogens of these type of water or ammonia molecules are found to primarily capture the free electron. It is also found that the free electron binding motifs evolve with time due to thermal fluctuations and the vertical detachment energy of [(H(2)O)(5)NH(3)](-) and vertical ionization energy of Li(H(2)O)(5)NH(3) also change with time along the simulation trajectories. Assignments of the observed peaks in the vibrational power spectra are done and we found a one to one correlation between the time-averaged populations of water and ammonia molecules at different H-bonding sites with the various peaks of power spectra. The frequency-time correlation functions of OH stretch vibrational frequencies of these clusters are also calculated and their decay profiles are analyzed in terms of the dynamics of hydrogen bonded and dangling OH modes. It is found that the hydrogen bond lifetimes in these clusters are almost five to six times longer than that of pure liquid water at room temperature.     

Relaxation dynamics in (HF)x(H2O)1-x solutions

The high-frequency dynamics of (HF)(x)(H(2)O)(1-x) solutions has been investigated by inelastic x-ray scattering. The measurements have been performed as a function of the concentration in the range x = 0.20-0.73 at fixed temperature T = 283 K. The results have been compared with similar data in pure water (x = 0) and pure hydrogen fluoride (x = 1). A viscoelastic analysis of the data highlights the presence of a relaxation process characterized by a relaxation time and a strength directly related to the presence of a hydrogen-bond network in the system. The comparison with the data on water and hydrogen fluoride shows that the structural relaxation time continuously decreases at increasing concentration of hydrogen fluoride passing from the value for water to the one for hydrogen fluoride tau(alphaHF), which is three times smaller. This is the consequence of a gradual decreasing number of constraints of the hydrogen-bond networks in passing from one liquid to the other.     

Change of electrical conductivities between hydrated and dehydrated samples of honeycomb sheet structures with mixed oxidation state paddlewheel dirhodium complexes and halide ions

A series of mixed oxidation state compounds, [{Rh(2)(acam)(4)}(3)(μ(3)-X)(2)]·nH(2)O (Hacam = acetamide; X = Cl, n = 4 (1·4H(2)O); X = Br, n = 10 (2·10H(2)O); X = I, n = 10 (3·10H(2)O)) and [{Rh(2)(pram)(4)}(3)(μ(3)-X)(2)]·6H(2)O (Hpram = propionamide; X = Cl (4·6H(2)O), Br (5·6H(2)O), I (6·6H(2)O)) were synthesized and their X-ray structures were determined. In the crystal structure of all of these complexes, dirhodium complexes and halide ions construct 2-D honeycomb sheet arrangements in which the walls consist of Rh(2) units and halide ions lie at the corners. Complexes 1·4H(2)O, 4·6H(2)O, 5·6H(2)O and 6·6H(2)O have three independent Rh(2) units, in which there are two Rh(2)(5+) and one Rh(2)(4+). In these structures, the water molecules hydrogen bond to O atoms and from the N atoms of the amidate ligands. The number of hydrogen bonds from water molecules to the Rh(2)(4+) unit is greater than that to the Rh(2)(5+) units. This suggests that there exists pinning of the oxidation states by water molecules. In the structures of 2·10H(2)O and 3·10H(2)O, all of the Rh(2) units are crystallographically equivalent. In these structures, eight of the 10 water molecules form a honeycomb-like network between the {Rh(2)(acam)(4)}(3)X(2) honeycomb sheets. The former four structures show very low electrical conductivities of ca. 10(-8) S cm(-1) (room temperature, pellets) and the latter structures have the higher values of ca. 10(-4) S cm(-1). In the former complexes, improvement of the values to 10(-6) S cm(-1) was observed, caused by loss of pinning water.     

Molecular dynamics of hydrogen bonds in protein-D2O: the solvent isotope effect

We suggest that the H-bond in proteins not only mirrors the motion of hydrogen in its own atomistic setting but also finds its origin in the collective environment of the hydrogen bond in a global lattice of surrounding H2O molecules. This water lattice is being perturbed in its optimal entropic configuration by the motion of the H-bond. Furthermore, bonding interaction with the lattice drop the H-bond energy from some 5 kcal/mol for the pure protein in the absence of H2O, to some 1.6 kcal/mol in the presence of the H2O medium. This low value here is determined in a computer experiment involving MD calculations and is a value close to the generally accepted value for biological systems. In accordance with these computer experiments under ambient conditions, the H-bond energy is seriously depressed, hence confirming the subtle effect of the H2O medium directly interacting with the H-bond and permitting a strong fluxional behavior. Furthermore, water produces a very large change in the entropy of activation due to the hydrogen bond breakage, which affects the rate by as much as 2 orders of magnitude. We also observe that there is an entire ensemble of H-bond structures, rather than a single transition state, all of which contribute to this H-bond. Here the model is tested by changing to D2O as the surrounding medium resulting in a substantial solvent isotope effect. This demonstrates the important influence of the environment on the individual hydrogen bond.