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.     

Conformation of 1,2-Dimethoxyethane in Water

To understand the conformation of 1,2-dimethoxyethane (DME) in water, a system of two kinds of molecules, DME and H_2O, was focused. The interaction of various conformers of DME with water was studied by means of ab initio molecular orbital calculation with 6-31G (d) basis set. It is shown that there are two forms of interactions between the two molecules in the system, the close touched (H_2O attaches to the two oxygen atoms of DME) and the open touched (H_2O attaches to one oxygen atom of DME) structures. The conformation of DME is remarkably influenced by the interactions. Instead the ttt conformer is preferred in the gas state, with a close touched H_2O the tgt conformer becomes the most stable one. The obtained hydration energies show that the stabilized order of DME conformers by water is tgt>tgg′>ttt.     

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

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

Resonant 2-photon ionization study of the conformation and the binding of water molecules to 2-phenylethanethiol (PhCH2CH2SH)

The structures of 2-phenylethanethiol (PET, PhCH(2)CH(2)SH) and its 1:1 water clusters have been studied using resonant two-photon ionization spectroscopy including band contour analysis and UV-UV holeburning, combined with extensive ab initio calculations on ground and excited states. The most populated conformer, labeled Ggpi, has a gauche arrangement about the SCCC and HSCC bonds that permits a stabilizing SH...dpi type of hydrogen bond. The other observed conformer, Ag, is anti with respect to the SCCC bond. In the dominant 1:1 water cluster, a water molecule binds to the Ggpi conformer via an OH...S hydrogen bond and two significant CH...O interactions. There is also evidence for water binding to conformer Ag with a similar arrangement, and for a second Ggpi cluster where water inserts between the SH and the aromatic ring. The additional interactions to the water molecules result in net D(e) binding energies approximately double those resulting from a single thiol-water hydrogen bond. The (1)(pi,pi(*)) excited state lifetimes in the bare molecules are very short because of internal conversion to a dissociative (1)(n,pi(*)) state related to the thiol. In the dominant Gw(1) cluster, the lifetime is significantly increased from <1 to approximately 4 ns. Hydrogen bonding to the thiol, which raises the energy of the dissociative (1)(n,pi(*)) state, accounts for this behavior.     

Dissociation of hydrogen fluoride in HF(H(2)O)(7)

We have previously demonstrated that H-bond arrangement has a significant influence on the energetics, structure and chemistry of water clusters. In this work, the effect of H-bond orientation on the dissociation of hydrogen fluoride with seven water molecules is studied by means of graph theory and high level ab initio methods. It is found that cubic structures of HF(H(2)O)(7) are more stable than structures of other topologies reported in the literature. Electronic calculations on all possible H-bond orientations of cubie-HF(H(2)O)(7) show that ionized structures are energetically more favorable than nonionized ones. This is an indication that seven water molecules might be capable of ionizing hydrogen fluoride.     

Microhydration of protonated glycine: an ab initio family tree

The incremental hydration of the glycine cation is investigated using an ab initio approach fully correcting for basis set superposition errors and explicitly incorporating electron-correlation effects. Structures with zero to four surrounding water molecules have been determined. It is demonstrated that the successive aggregates follow a Darwinian family tree, the most stable complexes systematically belonging to the same branch of the tree. In strong contrast with neutral glycine, the direct hydrogen bonding to the glycine cation is favored over bridging water structures. The agreement between experimental and theoretical hydration enthalpies and Gibbs free energies is impressive, as ab initio estimates almost systematically fit the experimental error bars. For GlyH(+)-(H2O) and GlyH(+)-(H2O)3, we show that two structures are generated by the experimental setup. The present approach also resolves most of the previous theory/experiment discrepancies and provides patterns for the evolution of the vibrational spectra: a decrease of the hydrogen-bond stretching frequency indicating second-shell water molecules. Additionally, the impact of bulk solvent solvation is investigated, as four discrete water molecules still do not fully hydrate the protonated glycine.     

Electron hydration dynamics in water clusters: A direct ab initio molecular dynamics approach

Electron attachment dynamics of excess electron in water cluster (H2O)n (n = 2 and 3) have been investigated by means of full-dimensional direct ab initio molecular dynamics (MD) method at the MP26-311++G(d,p) level. It was found that the hydrogen bond breaking due to the excess electron is an important process in the first stage of electron capture in water trimer. Time scale of electron localization and hydrogen bond breaking were determined by the direct ab initio MD simulation. The initial process of hydration in water cluster is clearly visualized in the present study. In n = 3, an excess electron is first trapped around the cyclic water trimer with a triangular form, where the excess electron is equivalently distributed on the three water molecules at time zero. After 50 fs, the excess electron is concentrated into two water molecules, while the potential energy of the system decreases by -1.5 kcal/mol from the vertical point. After 100 fs, the excess electron is localized in one of the water molecules and the potential energy decreases by -5.3 kcal/mol, but the triangular form still remained. After that, one of the hydrogen bonds in the triangular form is gradually broken by the excess electron, while the structure becomes linear at 100-300 fs after electron capture. The time scale of hydrogen bond breaking due to the excess electron is calculated to be about 300 fs. Finally, a dipole bound state is formed by the linear form of three water molecules. In the case of n = 2, the dipole bound anion is formed directly. The mechanism of electron hydration dynamics was discussed on the basis of theoretical results.     

Opposing Electronic and Nuclear Quantum Effects on Hydrogen Bonds in H2O and D2O

The effect of extending the O−H bond length(s) in water on the hydrogen-bonding strength has been investigated using static ab initio molecular orbital calculations. The “polar flattening” effect that causes a slight σ-hole to form on hydrogen atoms is strengthened when the bond is stretched, so that the σ-hole becomes more positive and hydrogen bonding stronger. In opposition to this electronic effect, path-integral ab initio molecular-dynamics simulations show that the nuclear quantum effect weakens the hydrogen bond in the water dimer. Thus, static electronic effects strengthen the hydrogen bond in H₂O relative to D₂O, whereas nuclear quantum effects weaken it. These quantum fluctuations are stronger for the water dimer than in bulk water.     

苯四甲酸桥连的镍配合物[Ni(btec)(H2O)4][Ni(imi)2(H2O)4]·2H2O的合成及结构

At room temperature, a coordination polymer [Ni(btec)(H2O)4][Ni(imi)2(H2O)4]·2H2O was synthesized by reaction of NiCl2·6H2O, 1,2,4,5-benzenetetracarboxylic dianhydridc and imidazole in a water/THF solution. The structure was determined by X-ray diffraction crystal structure analysis. It crystallizes in triclinic P1 space group with the crystal cell parameters of a=0.67542 (2) nm, b=1.000 14 (1) nm, c=1.09088 (3) nm, α=74.140(2)°, β= 74.388(1)°, y=73.239(2)°, and V=0.664 09(3) nm3, Z=1. The crystal structure shows that Ni1 atom is coordinated by four water and two imidazole molecules, while Ni2 is coordinated by four water molecules and two carboxyl oxygen atoms. The 1,2,4,5-benzenetetracarboxylate ions bridge Ni2 coordination centers to form one-dimensional chain structure. Moreover, the chains are further linked together by hydrogen bonds to form a two-dimensional network. CCDC: 295873.     

Excited-state proton transfer through water bridges and structure of hydrogen-bonded complexes in 1H-pyrrolo[3,2-h]quinoline: adiabatic time-dependent density functional theory study

Proton transfer reaction is studied for 1H-pyrrolo[3,2-h]quinoline-water complexes (PQ-(H(2)O)(n), n = 0-2) in the ground and the lowest excited singlet states at the density functional theory (DFT) level. Cyclic hydrogen-bonded complexes are considered, in which water molecules form a bridge connecting the proton donor (pyrrole NH group) and acceptor (quinoline nitrogen) atoms. To understand the effect of the structure and length of water bridges on the excited-state tautomerization in PQ, the potential energy profile of the lowest excited singlet state is calculated adiabatically by the time-dependent DFT (TDDFT) method. The S(0) --> S(1) excitation of PQ is accompanied by significant intramolecular transfer of electron density from the pyrrole ring to the quinoline fragment, so that the acidity of the N-H group and the basicity of the nitrogen atom of the quinoline moiety are increased. These excited-state acid-base changes introduce a driving force for the proton transfer reaction. The adiabatic TDDFT calculations demonstrate, however, that the phototautomerization requires a large activation energy in the isolated PQ molecule due to a high energy barrier separating the normal form and the tautomer. In the 1:1 cyclic PQ-H(2)O complex, the energy barrier is dramatically reduced, so that upon excitation of this complex the tautomerization can occur rapidly in one step as concerted asynchronous movements of the two protons assisted by the water molecule. In the PQ-(H(2)O)(2) solvate two water molecules form a cyclic bridge with sterically strained and unfavorable hydrogen bonds. As a result, some extra activation energy is needed for initiating the proton dislocation along the longer hydrogen-bond network. The full tautomerization in this complex is still possible; however, the cooperative proton transfer is found to be highly asynchronous. Large relaxation and reorganization of the hydrogen-bonded water bridge in PQ-(H(2)O)(2) are required during the proton translocation from the pyrrole NH group to the quinoline nitrogen; this may block the complete tautomerization in this type of solvate.     

Water superstructures within organic arrays; hydrogen-bonded water sheets, chains and clusters

A strategy for encouraging the formation of extended water arrays is presented, in which molecules that contain a 1,4-dihydroquinoxaline-2,3-dione core are used as supramolecular hosts for the accommodation of guest water molecules and arrays. These molecules were selected as they contain a hydrophilic oxalamide-based "terminus" that allows water molecules to hydrogen-bond to the host organic molecules as well as to each other. The host molecules also contain a hydrophobic "end" based upon an aromatic ring, which serves to encourage the formation of discrete water clusters in preference to three-dimensional networks, as the water molecules cannot form strong hydrogen bonds with this part of the molecule. A systematic study of several hydrated structures of four organic molecules based on 1,4-dihydroquinoxaline-2,3-dione (qd) is discussed. The organic molecules, qd, 6-methyl-1,4-dihydroquinoxaline-2,3-dione (mqd), 6,7-dimethyl-1,4-dihydroquinoxaline-2,3-dione (dmqd) and 1,4-dihydrobenzo[g]quinoxaline-2,3-dione (Phqd), act as supramolecular crystal hosts for the clusters of water, with zero-, one- and two-dimensional arrays of water being observed. The hydrogen bonding in the structures, both within the water clusters and between the clusters and organic molecules, is examined. In particular, the structure of dmqd6 H2O contains a two-dimensional water sheet composed of pentagonal and octagonal units. Phqd3 H2O forms a hydrophilic extended structure encouraging the formation of one-dimensional chains consisting entirely of water. Both qd2 H2O and dmqd2 H2O can be considered to form one-dimensional chains, but only by utilising bridging carbonyl groups of the oxalamide moieties to form the extended array; if only the water is considered, zero-dimensional water tetramers are observed. The remaining hydrated structures, [Na+dmqd-]dmqdH2O, dmqd1/3H2O and mqd1/2H2O, all contain discrete water molecules but do not form extended water structures.     

Characterization of hydrated Na+(phenol) and K+(phenol) complexes using infrared spectroscopy

Hydrated alkali metal ion-phenol complexes were studied to model these species in aqueous solution for M=Na and K. IR predissociation spectroscopy in the O-H stretch region was used to analyze the structures of M+(Phenol)(H2O)n cluster ions, for n = 1-4. The onset of hydrogen bonding was observed to occur at n=4. Ab initio calculations were used to qualitatively explore the types of hydrogen-bonded structures of the M+(Phenol)(H2O)4 isomers. By combining the ab initio calculations and IR spectra, several different structures were identified for each metal ion. In contrast to benzene, detailed in a previous study of Na+(Benzene)n(H2O)m [J. Chem. Phys. 110, 8429 (1999)], phenol is able to bind directly to Na+ even in the presence of four waters. This is likely the result of the sigma-type interaction between the phenol oxygen and the ion. With K+, the dominant isomers are those in which the phenol O-H group is involved in a hydrogen bond with the water molecules, while with Na+, the dominant isomers are those in which the phenol O-H group is free and the water molecules are hydrogen-bonded to each other. Spectra and ab initio calculations for the M+(Phenol)Ar cluster ions for M=Na and K are reported to characterize the free phenol O-H stretch in the M+(Phenol) complex. While pi-type configurations were observed for binary M+(Phenol) complexes, sigma-type configurations appear to dominate the hydrated cluster ions.     

Implementation of dynamical nucleation theory effective fragment potentials method for modeling aerosol chemistry

In this work, the dynamical nucleation theory (DNT) model using the ab initio based effective fragment potential (EFP) is implemented for evaluating the evaporation rate constant and molecular properties of molecular clusters. Predicting the nucleation rates of aerosol particles in different chemical environments is a key step toward understanding the dynamics of complex aerosol chemistry. Therefore, molecular scale models of nanoclusters are required to understand the macroscopic nucleation process. On the basis of variational transition state theory, DNT provides an efficient approach to predict nucleation kinetics. While most DNT Monte Carlo simulations use analytic potentials to model critical sized clusters, or use ab initio potentials to model very small clusters, the DNTEFP Monte Carlo method presented here can treat up to critical sized clusters using the effective fragment potential (EFP), a rigorous nonempirical intermolecular model potential based on ab initio electronic structure theory calculations, improvable in a systematic manner. The DNTEFP method is applied to study the evaporation rates, energetics, and structure factors of multicomponent clusters containing water and isoprene. The most probable topology of the transition state characterizing the evaporation of one water molecule from a water hexamer at 243 K is predicted to be a conformer that contains six hydrogen bonds, with a topology that corresponds to two water molecules stacked on top of a quadrangular (H(2)O)(4) cluster. For the water hexamer in the presence of isoprene, an increase in the cluster size and a 3-fold increase in the evaporation rate are predicted relative to the reaction in which one water molecule evaporates from a water hexamer cluster.     

Carbohydrate amino acids: the intrinsic conformational preference for a beta-turn-type structure in a carbopeptoid building block

Infrared ion-dip spectroscopy coupled with DFT and ab initio calculations are used to establish the intrinsic conformational preference of the basic structural unit of a peptide mimic, a cis-tetrahydrofuran-based "carbopeptoid" (amide-sugar-amide), isolated at low temperature in the gas phase. The carbopeptoid units form a beta-turn-type structure, stabilized by an intramolecular NH --> O=C hydrogen bond across the sugar ring, thus forming a 10-membered, C10 turn. Despite the clear preference for C10 beta-turn structures in the basic unit, however, the presence of multiple hydrogen-bond donating and accepting groups also generates a rich conformational landscape, and alternative structures may be populated in related molecules. Calculations on an extended, carbopeptoid dimer unit, which includes an alternating amide-sugar-amide-sugar-amide chain, identify conformers exhibiting alternative hydrogen-bonding arrangements that are somewhat more stable than the lowest-energy double beta-turn forming conformer.     

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.     

Entropy-driven rearrangement of the water network at the hydrated amide group of the trans-formanilide-water cluster in the gas phase

Photoionization-induced rearrangement of the water network in the trans-formanilide 1:4 cluster, FA-(H(2)O)(4), has been investigated by using IR-photodissociation spectroscopy and quantum chemical calculations. The IR spectrum of FA-(H(2)O)(4) in the S(0) state shows that the observed cluster has a cyclic hydrogen-bonded structure where the CO group and the NH group of FA are bridged with four water molecules, consistent with the reported structure [E. G. Robertson, Chem. Phys. Lett., 2000, 325, 299]. However, the corresponding cyclic hydrogen-bonded structure in the D(0) state of [FA-(H(2)O)(4)](+) is a minor product arising from photoionization via the S(1)-S(0) origin of FA-(H(2)O)(4). The dominant product has an extended H-bonded structure, where the intermolecular hydrogen bond between the hydrogen of the OH group of a water molecule and the CO group is dissociated. This is the first observation of a photoionization-induced rearrangement of the water network in [FA-(H(2)O)(4)](+). Through DFT calculations, we conclude that the rearrangement occurs due to entropic effects.     

Theoretical analysis of reaction kinetics with singlet oxygen molecules

A comparative analysis of predictive ability of three approaches to estimate the rate constants of reactions of H(2), H, H(2)O and CH(4) with electronically excited O(2)(a(1)Δ(g)) and O(2)(b(1)Σ(g)(+)) molecules is conducted. The first approach is based on a detailed ab initio study of potential energy surfaces. The second one is known as the "bond energy-bond order" method, and the third approach is a modification of the updated method of vibronic terms that makes it possible to evaluate the activation energy of reactions involving electronically excited species. The comparison showed that the estimates of the energy barrier by the updated method of vibronic terms for some reactions can be in good agreement with ab initio calculations and available experimental data. It was revealed that reactions of O(2)(b(1)Σ(g)(+)) molecules with H(2), H(2)O and CH(4) molecules and with the H atom result in the formation of electronically excited species. The reactivity of O(2)(b(1)Σ(g)(+)) molecules is smaller than that of O(2)(a(1)Δ(g)) ones, but much higher as compared to the reactivity of ground state O(2) molecules. For each reaction under study involving oxygen molecules in the excited electronic states O(2)(a(1)Δ(g)) and O(2)(b(1)Σ(g)(+)) the recommended temperature-dependent rate constants are presented.     

Modeling the interactions of phthalocyanines in water: from the Cu(II)-tetrasulphonate to the metal-free phthalocyanine

A quantum and statistical study on the effects of the ions Cu(2+) and SO(3)(-) in the solvent structure around the metal-free phthalocyanine (H(2)Pc) is presented. We developed an ab initio interaction potential for the system CuPc-H(2)O based on quantum chemical calculations and studied its transferability to the H(2)Pc-H(2)O and [CuPc(SO(3))(4)](4-)-H(2)O interactions. The use of the molecular dynamics technique allows the determination of energetic and structural properties of CuPc, H(2)Pc, and [CuPc(SO(3))(4)](4-) in water and the understanding of the keys for the different behaviors of the three phthalocyanine (Pc) derivatives in water. The inclusion of the Cu(2+) cation in the Pc structure reinforces the appearance of two axial water molecules and second-shell water molecules in the solvent structure, whereas the presence of SO(3)(-) anions implies a well defined hydration shell of about eight water molecules around them making the macrocycle soluble in water. Debye-Waller factors for axial water molecules have been obtained in order to examine the potential sensitivity of the extended x-ray absorption fine structure technique to detect the axial water molecules.     

Microsolvation of formamide: a rotational study

Microsolvated formamide clusters have been generated in a supersonic jet expansion and characterized using Fourier transform microwave spectroscopy. Three conformers of the monohydrated cluster and one of the dihydrated complex have been observed. Seven monosubstituted isotopic species have been measured for the most stable conformer of formamide...H(2)O, which adopts a closed planar ring structure stabilized by two intermolecular hydrogen bonds (N-H...O(H)-H...O=C). The two higher energy forms of formamide...H(2)O have been observed for the first time. The second most stable conformer is stabilized by a O-H...O=C and a weak C-H...O hydrogen bond, while, in the less stable form, water accepts a hydrogen bond from the anti hydrogen of the amino group. For formamide...(H(2)O)(2), the parent and nine monosubstituted isotopic species have been observed. In this cluster the two water molecules close a cycle with the amide group through three intermolecular hydrogen bonds (N-H...O(H)-H...O(H)-H...O=C), the nonbonded hydrogen atoms of water adopting an up-down configuration. Substitution (r(s)) and effective (r(0)) structures have been determined for formamide, the most stable form of formamide...H(2)O and formamide...(H(2)O)(2). The results on monohydrated formamide clusters can help to explain the observed preferences of bound water in proteins. Clear evidence of sigma-bond cooperativity effects emerges when comparing the structures of the mono- and dihydrated formamide clusters. No detectable structural changes due to pi-bond cooperativity are observed on formamide upon hydration.     

Ab initio molecular dynamics free-energy study of microhydration effects on the neutral-zwitterion equilibrium of phenylalanine.

The structures and relative energies of the most stable conformers of both naked and microsolvated phenylalanine, Phe.(H(2)O)(n)(n=0-3), are calculated by density functional theory. For selected structures, coordination-constrained ab initio molecular dynamics simulations determine the proton-transfer mechanism connecting neutral and zwitterionic forms of Phe. The associated free-energy profiles are calculated by thermodynamic integration. While no zwitterionic free-energy minimum is found for naked Phe, microsolvation is found to stabilize the zwitterionic form. For cluster sizes n > or = 3, the proton-transfer equilibrium shifts towards the zwitterionic structure for specific proton-transfer pathways. The energetically most favourable interconversion path between the neutral and zwitterionic forms is through a H(2)O bridge with free-energy barriers as low as 14.4 kJ mol(-1) for Phe.(H(2)O)(3). The free energy required for breaking a carboxylic OH bond involved in intramolecular hydrogen bonding is typically lower than in the water-assisted case. However, the resulting zwitterion turns out to be unstable with respect to the backward proton-transfer reaction.