Synthesis and X－ray Structure of a Binuclear Nickel（Ⅱ） Complex of Nickel（Ⅱ） Perchlorate and 2，2＇－Bipyridyl with N，N＇－Bis（2－aminoethyl）oxamido Nickel（Ⅱ）

ＳｙｎｔｈｅｓｉｓａｎｄＸ－ｒａｙＳｔｒｕｃｔｕｒｅｏｆａＢｉｎｕｃｌｅａｒＮｉｃｋｅｌ（Ⅱ）ＣｏｍｐｌｅｘｏｆＮｉｃｋｅｌ（Ⅱ）Ｐｅｒｃｈｌｏｒａｔｅａｎｄ２，２＇－ＢｉｐｙｒｉｄｙｌｗｉｔｈＮ，Ｎ＇－Ｂｉｓ（２－ａｍｉｎｏｅｔｈｙｌ）ｏｘａｍｉｄ...

Synthesis,Crystal Structure and Theoretical Calculations of a Nickel(Ⅱ) Coordination Polymer Assembled by 4,4'-Oxydibenzoic Acid and 1,3-Bis(imidazol-1-ylmethyl)-benzene Ligands

A new metal-organic coordination polymer [Ni（oba）（mbix）（H2O）]n·n H2O（H2oba = 4,4‘-oxydibenzoic acid,mbix = 1,3-bis（imidazol-1-ylmethyl）-benzene） 1 has been hydrothermally synthesized and structurally characterized by elemental analysis,IR,TG,UV and single-crystal X-ray diffraction.Green crystals crystallize in the triclinic system,space group P1 with a = 10.1915（18）,b = 11.415（2）,c = 12.314（2）A,α = 74.263（3）,β = 84.545（2）,γ = 74.369（3）o,V = 1327.6（4） A,C28H26N4 Ni O7,Mr = 589.24,Dc = 1.474 g/cm3,F（000） = 612,Z = 2,μ（Mo Kα） = 0.786 mm-1,the final R = 0.0332 and w R = 0.0869 for 4622 observed reflections（I 〉 2σ（I））.The structure of 1 exhibits a one-dimensional chain-like structure.In addition,natural bond orbital（NBO） analysis was performed by the PBE0/LANL2 DZ method in Gaussian 03 Program.The calculation results show obvious covalent interaction between the coordinated atoms and Ni（Ⅱ） ion.

Determination of Trace Amounts of Nickel (Ⅱ) by Graphite Furnace Atomic Absorption Spectrometry Coupled with Cloud Point Extraction

A new method based on the cloud point extraction（CPE） for separation and preconcentration of nickel（Ⅱ） and its subsequent determination by graphite furnace atomic absorption spectrometry（GFAAS） was proposed, 8-hydroxyquinoline and Triton X-100 were used as the ligand and surfactant respectively. Nickel（Ⅱ） can form a hy-drophobic complex with 8-hydroxyquinoline, the complex can be extracted into the small volume surfactant rich phase at the cloud point temperature（CPT） for GFAAS determination. The factors affecting the cloud point extraction, such as pH, ligand concentration, surfactant concentration, and the incubation time were optimized. Under the optimal conditions, a detection limit of 12 ng/L and a relative standard deviation（RSD） of 2.9% were obtained for Ni（Ⅱ） determination. The enrichment factor was found to be 25. The proposed method was successfully applied to the determination of nickel（Ⅱ） in certified reference material and different types of water samples and the recovery was in a range of 95%―103%.

Synthesis, Crystal Structure and Thermal Stability of a Nickel(Ⅱ) Complex with Bicycle[2.2.1]-2-hepten-5,6-dicarboxylic Acid

The nickel complex {Ni（2,2-bipy）（H2O）3[C8H11O2（COO）]}（H2O）3 with bicycle-[2.2.1]-2-hepten-5,6-dicarboxylic acid [C7H8（COOH）2] and 2,2＇-bipyridine （bipy） as ligands has been synthesized and characterized. It crystallizes in monoclinic, space group P , with a = 0.74975（3）, b = 1.20309（4）, c = 1.30593（4） nm, α = 109.861（2）, β = 98.519（2）, γ = 90.575（2）o, V = 1.09337（7） nm3, Dc = 1.552 g/cm3, Z = 2, F（000） = 524, the final GOOF = 1.064, R = 0.0397 and wR = 0.1171. The crystal structure shows that the nickel ion is coordinated with four oxygen atoms from one bicycle[2.2.1]-2-hepten-5,6-dicarboxylic acid molecule and three water molecules and two nitrogen atoms from the 2,2′-bipyridine molecule, forming a distorted octahedral coordination geometry. The result of TG analysis shows that the title complex is stable under 200.0 ℃.

Hydrothermal Synthesis and Crystal Structure of a New One-dimensional Nickel(Ⅱ) Complex with 2-Pyridinecarboxyic Acid and 4,4′-Bipyridine Ligands

A new metal-organic coordination polymer [Ni[（2-pya）2（4,4′-bipy）]n·6nH2O （2-pya = 2-pyridinecarboxylic acid, 4,4′-bipy = 4,4′-pyridine） 1 has been hydrothermally synthesized and structurally characterized by elemental analysis, IR spectrum, TG and single-crystal X-ray diffraction. The complex crystallizes in tetragonal, space group I4 1/α with a = b = 22.5642（19）, c = 10.7118（18） A, V = 5453.8（11） A^3, C22H28N4NiO10, Mr= 567.19, Dc = 1.382 g/cm^3, μ（MoKα） = 0.769 mm^-1, F（000） =2368, Z = 8, the final R = 0.0572 and wR = 0.1254 for 1401 observed reflections （I 〉 2σ（I））. It exhibits a one-dimensional chain-like structure by mixed ligands of 2-pyridinedicarboxylic acid and 4,4′-pyridine.

2-AMINOMETHYLPYRIDINE NICKEL(Ⅱ) COMPLEXES—SYNTHESIS,MOLECULAR STRUCTURE AND CATALYSIS OF ETHYLENE POLYMERIZATION

A series of new nickel(Ⅱ)complexes with 2-aminomethylpyridine ligands,(2-PyCH_2NHAr)_2NiBr_2(Ar=2,6- dimethylphenyl 2a;2,6-diisopropylphenyl 2b,2,6-difluorophenyl 2c),have been synthesized and used as catalyst precursors for ethylene polymerization in the presence of methylaluminoxane(MAO).The catalysts containing ortho-alkyl-substituents afford high molecular weight branched polyethylenes as well as a certain amount of oligomers.Enhancing the steric bulk of the alkyl substituent of the catalyst resulted in...

Simple, Selective, and Sensitive Spectrophotometric Method for Determination of Trace Amounts of Nickel(Ⅱ), Copper (Ⅱ), Cobalt (Ⅱ), and Iron (Ⅲ) with a Novel Reagent 2-Pyridine Carboxaldehyde Isonicotinyl Hydrazone

A selective and sensitive reagent of 2-pyridine carboxaldehyde isonicotinyl hydrazone（2-PYAINH） was synthesized and studied for the spectrophotometric determination of nickel, copper, cobalt, and iron in detail. At a pH value of 7.0, 9,0, 9.0, and 8.0, respectively, which greatly increased the selectivity; nickel, copper, cobalt, and iron reacted with 2-PYAINH to form a 1：2 yellow-orange, 1：2 yellow-green, 1：2 yellow and 1：1 yellow complexes, with absorption peaks at 363, 352, 346, and 359 nm, respectively. Under the optimal conditions, Beer＇s law was obeyed over the ranges of 0.01-1.4, 0.01-1.5, 0.01-2.7, and 0.01-5.4 mg/L respectively. The apparent molar absorptivity and Sandell＇s sensitivities were 8.4×10^4, 5.2×10^4, 7.1×10^4, and 3.9×10^4 L·mol^-l·cm^-1, respectively, and 0.00069, 0.0012, 0.00078, and 0.0014 μg·cm2, respectively. The detection limits were found to be 0.001, 0.002, 0.003, and 0.01 mg/L, respectively. The detailed study of various interfering ions to make the method more sensitive was carried out and selective and several real samples were analyzed with satisfactory results.

Synthesis,Crystal Structure and Fluorescence Property of a New Trinuclear Nickel(Ⅱ) Complex Bridged by 2,6-Pyridine Dicarboxylic Acid

A new trinuclear nickel complex,[Ni3（pdc）3（2,2＇-bipy）3（H2O）2]·2H2O（H2pdc = pyridine 2,6-dicarboxylic acid,2,2＇-bipy = 2,2＇-bipyridine） has been hydrothermally synthesized and characterized by IR,elemental analysis and X-ray diffraction methods.Crystal data for this complex：monoclinic,space group P21/n,a = 21.206（4）,b = 10.002（2）,c = 28.066（6）,β = 108.18（3）°,C51H41N9Ni3O16,Mr = 1212.06,V = 5.656（2） nm3,Dc = 1.423 g·cm-3,μ（MoKα） = 1.062 mm-1,Z = 4,F（000） = 2488,GOOF = 1.034,the final R = 0.0543 and wR = 0.1237 for 6149 observed reflections with Ⅰ 〉 2σ（Ⅰ）.In the complex,three nickel（Ⅱ） ions are bridged by the pyridine 2,6-dicarboxylic acid groups,and all nickel（Ⅱ） ions are seven-coordinated by nitrogen atoms of 2,2＇-bipyridine and pyridine 2,6-dicarboxylic acid and oxygen atoms from pyridine 2,6-dicarboxylic and water to adopt a severely distorted pentagonal bipyramidal geometry.The emission and excitation peaks of the complex in ethanol solutions are located at 336 and 316 nm,respectively.

Hydrothermal Synthesis, Crystal Structure and Properties of One Binuclear Nickel(Ⅱ) Complex with 3-(2-Pyridiyl)-1H-1,2,4-triazole

A new binuclear nickel（Ⅱ） complex [Ni_2（C_7 N_4 H_5）_3（C_9 H_9 O_2）]n has been hydrothermally synthesized with nickel sulfate, 3-（pyridin-2-yl）-1H-1,2,4-triazole and 3,5-dimethylbenzoic acid（3,5-DMBA）. It crystallizes in the triclinic space group P1 with a = 11.5769（7）, b = 12.3115（7）, c = 12.6431（7） A, α = 81.4860（10）o, β = 64.2830（10）o, γ = 63.7130（10）o, V = 1453.45（15） A^3, D_c = 1.604 g/cm^3, Z = 2, F（000） = 720, the final GOOF = 1.048, R = 0.0285 and wR = 0.0628. The crystal structure shows that the whole molecule consists of two nickel ions bridged by three μ_2-η~1：η~0-3-（pyridin-2-yl）-1,2,4-triazole anions. The coordination environment of Ni（Ⅱ） ion is NiO_2 N_4 and NiN_5, giving a distorted octahedral geometry and square pyramidal geometry respectively. The luminescence and thermal properties of the complex were investigated.

Chiral Resolution of β-dl-Phenylalanine Using Chiral Macrocyclic Nickel(Ⅱ) Complexes

The reactions of chiral four-coordinated nickel（Ⅱ） complex [Ni（RR-L）]（ClO4）2/[Ni（SS- L）]（ClO4）2 with β-dl-phenylalanine in acetonitrile/water gave a six-coordinated enantiomer of [Ni（RR-L）（β-d-HPhe）]（ClO4）2（1） and [Ni（SS-L）（β-l-HPhe）]（ClO4）2（2）, respectively（L = 5,5,7, 12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, HPhe = phenylalanine）. Single-crystal X-ray diffraction analyses revealed that the nickel（Ⅱ） atom displays a distorted octahedral coordination geometry by coordinating with four nitrogen atoms of L in a folded configuration, and two carboxylate oxygen atoms of β-d/l-HPhe in cis-position in both complexes. The monomers of [Ni（RR-L）（β-d-HPhe）]2＋ and [Ni（SS-L）（β-l-HPhe）]2＋ are connected through intermolecular hydrogen bonds to generate one-dimensional zigzag chains, respectively. The homochiral natures of 1 and 2 are confirmed by the results of CD spectroscopy.

Electrochemical and Spectroscopic Studies on the Interaction between Tetracoordinate Macrocyclic Copper（Ⅱ）, Nickel（Ⅱ） and Cobalt（Ⅲ） Complexes with DNA

Mononuclear copper（Ⅱ）, nickel（Ⅱ） and cobalt（Ⅲ） tetracoordinate macrocyclic complexes were synthesized and spectroscopically characterized. The crystal structure of the three compounds were determined by X-ray crystallography. The electrochemical experimental results indicate that the three complexes could interact with DNA mainly by electrostatic interaction. The interaction of tetracoordinate macrocyclic cobalt（Ⅲ） complex with DNA was studied by cyclic voltammetry and UV-vis spectroscopy. The experimental results reveal that tetracoordinate macrocyc- lic cobalt（Ⅲ） complex could interact with DNA by electrostatic interaction to form a 1 ： 1 DNA association complex with a binding constant of 7.50 ×10^3 L·mol^-1.

Nickel(Ⅱ) removal from water using silica-based hybrid adsorbents:Fabrication and adsorption kinetics

A series of novel silica-based hybrid adsorbents were prepared by the crosslinking reaction of N-[3- （trimethoxysilyl）propyl] ethylene diamine （TMSPEDA） with epichlorohydrin （ECH） via a sol-gel process. Fourier transform infrared （FTIR） spectra confirmed that the reaction occurred. TGA curves showed that the thermal stability of these hybrid adsorbents reached as high as 180 ℃. As a typical example, the adsorption performance of nickel（U） ions onto an adsorbent （the volume ratio of TMSPEDA and ECH was 4：1 ） was explored. It was found that the adsorption of nickel（Ⅱ） ions onto this adsorbent followed the Lagergren pseudo-second-order kinetic model. The investigation of the adsorption mechanism demonstrated that nickel（Ⅱ） adsorption was chiefly controlled by diffusion-chemisorption, suggesting that more diffusion processes were involved in the adsorption of nickel（Ⅱ） ions onto this type of adsorbents. Desorption experiment indicates that these hybrid adsorbents can be regenerated. These findings reveal that this type of silica-based hybrid adsorbent is promising in the separation and recovery of nickel（Ⅱ） ions from Ni-containing wastewater or contaminated water.

Syntheses, Crystal Structures and Photoluminescence Properties of Cadmium(Ⅱ) and Nickel(Ⅱ) Complexes with 2-(2-Benzimidazolyl)quinoline

Two novel complexes, [Cd2（BMQU）2Cl4] （1） and [Ni（BMQU）2HPO4]·1.5H2O （2） （BMQU = 2-（2-benzimidazolyl）quinoline）, were synthesized by the hydrothermal method and characterized by elemental analysis, IR, and TG-DTG. The crystal structures were determined by single-crystal X-ray diffraction. Both 1 and 2 crystallize in the triclinic system, space group P . The data for 1： a = 0.8342（7）, b = 0.9226（9）, c = 1.0646（8） nm, α = 90.819（2）, β = 97.466（2）, γ = 98.280（2）°. The Cd（Ⅱ） is coordinated with three chlorine atoms and two nitrogen atoms of a BMQU molecule, generating a distorted square-pyramidal geometry. The dinuclear Cd（Ⅱ） complex is formed by two chlorine bridge bonds, and the one-dimensional chain structure is constructed with the hydrogen bond N-H…Cl and π-π stacking interaction. The data for 2： a = 1.2251（1）, b = 1.2451（1）, c = 1.2868（1） nm, α = 107.510（2）, β = 98.630（1）, γ = 109.921（2）°. The Ni（Ⅱ） is coordinated with four nitrogen atoms of two BMQU molecules and two oxygen atoms of a HPO42-, forming a distorted-octahedral geometry. The two-dimensional layer structure is formed by the hydrogen bonds and π-π stacking interaction between neighboring molecules. Complex 1 shows a strong blue fluorescence emission （λmax= 456 nm） at solid state.

Topography Structure and Scanning Tunneling Spectrum of Nickel( Ⅱ )-tetraphenylporphyrin Molecules on Au(111)

Scanning tunneling microscopy （STM） and scanning tunneling spectroscopy （STS） were performed on monolayer film of NiTPP supported on Au（111） under ultrahigh vacuum （UHV） conditions. The constant current STM images show remarkable bias dependence. High resolution STM data clearly show the individual NiTPP molecules and allow easy differentiation between NiTPP and CoTPP reported before. Scanning tunneling spectra, as a function of molecule-tip separation, were acquired over a range of tip motion of 0.42 nm. Spectra do not show the variation in band splitting with tip distance. It appears for molecules such as NiTPP that the average potential at the molecule is essentially the same at the same metal substrate. For molecules of the height of NiTPP, the scanning tunneling spectra should give reliable occupied and unoccupied orbital energies over a wide range of tip-molecule distances.

Synthesis,Crystal Structure and Magnetic Properties of an Oxalato-bridged Dinickel(Ⅱ) Complex with Nitronyl Nitroxide Radicals

A new oxalato-bridged dinickel（Ⅱ） complex [Ni2（NIT-MeImz）4（C2O4）]（Ac）2·8H2O has been synthesized and characterized structurally as well as magnetically.It crystallizes in monoclinic,space group C2/c with a=24.608（4）,b=21.399（3）,c=17.992（3）,β=131.680（2）°,V=7076（2）3,C50H86N16Ni2O24,Mr=1412.77,Z=4,Dc=1.326 g/cm3,μ（MoKα）=0.614 mm-1,F（000）=2984,R=0.0751 and wR=0.1791 for 2956 observed reflections with Ⅰ＆gt;2σ（Ⅰ）.X-ray analysis reveals that the crystal structure consists of [Ni2（NIT-MeImz）4（C2O4）]2＋ moiety and two Ac^- anions. Each nickel（H） ion is six-coordinated with a distorted octahedral geometry： two nitrogen atoms and two oxygen atoms from NIT-Melmz together with two other oxygen atoms from the oxalato ion. The 2-D net structure is formed and arranged through intermolecular H-bonding interactions. Magnetic measurements show antiferromagnetic interactions for this complex.

Crystal Structure, Spectroscopic Characterization, and Electrochemical and Thermal Stability Properties of a Dinuclear Nickel(Ⅱ) Complex

A new nickel（Ⅱ） complex Ni2（L）2（2,2＇-bipy）2·5.5H2O with methy-bicycle[2.2.1]hept-5-ene-2,3-dicarboxylic acid（H2L） and 2,2？-bipyridine（2,2＇-bipy） as ligands has been synthesized in the mixed solvent DMF and water（v：v = 5：2）. It crystallizes in the triclinic space group P1 with a = 10.414（2）, b = 12.884（3）, c = 16.176（4） A, α = 70.715（5）, β = 80.599（5）, γ = 77.383（6）°, V = 1989.4（8） A^3, Dc = 1.531 g/cm^3, Z = 2, F（000） = 958, GOOF = 1.028, the final R = 0.0808 and w R = 0.2036. The crystal structure shows that the whole molecule consists of two independent dinuclear units, in which two nickel ions are bridged by two μ2-η1：η0 3-carboxylate groups of L2- anions. The coordination environment of Ni（Ⅱ） ion is Ni N2O3, giving a distorted square pyramidal geometry. The thermal stability and electrochemical properties of the complex were investigated.

Syntheses and Crystal Structures of Two Nickel(Ⅱ) Complexes [Ni(dmpzm)_2(L)]Cl·nH_2O (L=HCO_2,OAc;n=2,3;dmpzm=1,1'-Methylenebis(3,5-dimethyl-1H-pyrazole))

Reactions of NiCl2-6H2O with dmpzm and formic or acetic acid at PH = 7 produced the two title compounds, [Ni（dmpzm）2（HCO2）]Cl·2H2O 1 and [Ni（dmpzm）2（OAc）]C2·H2O 2 （dmpzm = 1,1 ＇-methylenebis（3,5-dimethyl-lH-pyrazole））. 1 and 2 were characterized by elemental analysis, IR spectra, and thermogravimetric analysis. 1 crystallizes in the orthorhombic system, space group Pcca with a = 18.700（2）, b = 8.6843（9）, c = 18.098（2）A, V= 2939.1（5）A^3, Z = 4, Dc = 1.319 g/cm^3, T = 193 K, C23H37CINsNiO4, Mr = 583.75, F（000） = 1232, p（MoKa） = 0.792 cm^-1, S = 1.079, R = 0.0772 and wR = 0.1958 for 2265 observed reflections with I〉 2σ（I）. 2 crystallizes in the orthorhombic system, space group Pcca with a = 17.011（3）, b = 18.630（4）, c = 19.300（4）A, V = 6117（2）A^3, Z = 8, Dc = 1.337 g/cm^3, T= 193（2） K, C24H41C1N8NiO5, Mr = 615.79, F（000） = 2608, μ（MoKa） = 0.768 cm^-1, S = 1.059, R = 0.0518 and wR = 0.1109 for 5124 observed reflections with I 〉 2σ（I）. The Ni atoms in 1 or 2 are chelated by two dmpzm ligands and one formate or acetate anion, forming slightly distorted octahedral geometries. In the crystals of 1 or 2, intermolecular hydrogen bonding interactions result in the formation of 2D hydrogen-bonded frameworks.

Syntheses and Structures of Two Macrocyclic Nickel(Ⅱ) Complexes Bridging Benzene Polycarboxylic Acid

Two complexes with formulas [Ni L][（Ni L）（IPA）2]·8H2O（1） and [（Ni L）（H2O）2] [（Ni L）（PMA）]·4H2O（2）（L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane,IPA2– = isophthalic anion,PMA4– = 1,2,4,5-benzenetetracarboxylic anion） were synthesized and characterized by elemental analyses and IR spectra.The crystal structures were determined by X-ray diffraction.In complex 1,the [Ni L]^2＋ bridged IPA^2– to give [（Ni L）（IPA）2]^2– monomer,and the [Ni L]^2＋ bridged PMA4– to form a one-dimensional chain [（Ni L）（PMA）]n^2n– in complex 2.The [Ni L]^2＋ and [（Ni L）（IPA）2]^2–/[（Ni L）（PMA）]^2– are connected through intermolecular hydrogen bonding to generate three-dimensional supramolecular structures.

Synthesis, Characterization, Crystal Structure and Antibacterial Activities of a Nickel(Ⅱ) Complex with 2'-(2-Thienylidene)-hydroxybenzoylhydrazide

In the ethanol solvent, a nickel（Ⅱ） complex Ni（C12H10N2O2S）2 upon reaction of 2＇-（2-thienylidene）-hydroxybenzoylhydrazide with nickel acetate was synthesized, and its structure was characterized by IR, UV, elemental analysis and X-ray diffraction analysis. The crystal belongs to monoclinic system, space group C2/c with a = 22.052（3）, b = 5.9681（6）, c = 18.522（2）A , β = 110.679（4）°, V = 2280.6（4）A^3, Z = 4, Mr = 551.27, μ = 1.606 mm^-1, Dc = 1.075 g/cm^3, F（000） = 1136 and Rint = 0.0556. The nickel（Ⅱ ） atom in the compound is four-coordinated with two nitrogen atoms from amide and two oxygen atoms from keto group. The biological activities have been measured, show- ing the compound exhibits better anti-bacterial activity than the ligand.

Two Nickel(Ⅱ) Complexes Constructed by Novel Bis(3-methoxy-2-pyridyl)ether-6,6′-dicarboxylic Acid Ligand:Lattice Water Control of Structure

Under given conditions, two complexes of [Ni（H20）2BEDA]·2H2O 1 and [Ni（Py）2- BEDA]·6H2O 2 （BEDA = bis（3-methoxy-2-pyridyl）ether-6,6′-dicarboxylic acid） have been synthesized and characterized by elemental analysis and X-ray single-crystal diffraction. Crystal data for 1： C14H18NiN2O11, monoelinic C2/c, a = 14.3844（17）, b = 12.9900（15）, c = 9.6309（11） A, β = 104.3350（10）°, V= 1743.5（4） A3, Z = 4, Dc= 1.711 g/cm^3, F（000） = 928, μ = 1.179 mm^-1, Mr= 449.01, the final R = 0.0228 and wR = 0.0625. Crystal data for 2： C24H32NiN4O13, triclinic P1, a = 9.423（2）, b = 11.863（3）, c = 13.089（3） A, α = 91.511（3）, β = 92.465（3）, γ = 100.696（2）°, V = 1435.6（6） A^3, Z = 2, Dc= 1.488 g/cm^3, F（000） = 672, μ = 0.748 mm^-1, Mr= 643.25, the final R = 0.0400 and wR = 0.0975. Interestingly, in the two complexes, lattice water molecules dominate its crystal structures. Therefore, extensive intermolecular hydrogen bonds assemble 1 and 2 into 2D extended sheets and a 3D open framework, respectively. Furthermore, water molecules present in 2 are associated to form water clusters.