Kenji Waizumi

Structural Studies on Saturated Aqueous Solutions of Manganese(II), Cobalt(II), and Nickel(II) Chlorides by X-ray Diffraction

As a part of our studies on crystallization processes of electrolytes, the structure of aqueous solutions of MCl2 (M = Mn, Co, Ni) equilibrated with hydrate crystals, MCl2 · mH2O (m = 6, 4, 2), was investigated by means of X-ray diffraction at 25, 40, 55, and 70°C. The complexes formed in MnCl2 solutions, were found to be mixed–ligand chloroaqua octahedral complexes of M2+ ions with the Mn—O and Mn—Cl distances of about 220 and 251 pm, respectively. The average number of Mn—Cl and Mn—O interactions increased from 1.2 to 1.9 and decreased from 4.8 to 4.1, respectively, with changing MnCl2 solutions from Mn25 (MnCl2 solution at 25°C) to Mn70 (MnCl2 solution at 70°C). In the octahedral species of Co2+, the Co—O and Co—Cl distances were found to be about 211 and 240 pm, respectively. With an increase in the saturated concentration by changing temperature from 25 to 70°C, the average coordination number of the Co—Cl contact per Co2+ increased from 0.5 to 1.2, and the average number of Co—O interactions decreased from 5.5 to 4.8. The structural analysis was carried out by taking into consideration the existence of the tetrahedral species in the solutions saturated at 40, 55, and 70°C, on the assumption of the existence of [CoCl4]2−. The Co—Cl distance was found to be 228 pm, while the number of Co—Cl interactions in the [CoCl4] complex was calculated to be 3.7 by the least-squares calculations. The Ni—O and Ni—Cl distances were estimated to be about 206 and 237 pm, respectively. The frequency factor n of the Ni—O and Ni—Cl interactions decreased monotonously from 5.6 to 5.0 and increased from 0.4 to 1.0, respectively, with increasing NiCl2 concentration. The n values of the Co—Cl and Ni—Cl interactions of the octahedral complexes increased sharply with concentration at higher concentrations. Comparing structures of the complexes in the saturated solutions and the hydrate crystals of these metal ions, we discussed a role of the complexing species on crystallization of the hydrates.

A molecular approach to the formation of KCl and MgCl+ ion-pairs in aqueous solution by density functional calculations

Abstract Full geometry optimizations have been carried out on molecular models of [M(H 2 O) 6 ] n + , [M(H 2 O) 6 …H 2 O] n + and [M(H 2 O) 6 …Cl] ( n −1)+ (M=K and Mg, n =1 for K and 2 for Mg) using the density functional methods. The optimized geometries of [K(H 2 O) 6 ] + and [Mg(H 2 O) 6 ] 2+ were a regular octahedron. In the optimizations of [M(H 2 O) 6 …H 2 O] n + and [M(H 2 O) 6 …Cl] ( n −1)+ , the octahedral structures of [M(H 2 O) 6 ] n + units were completely broken for K + complexes and almost kept for Mg 2+ complexes. The results have been discussed in connection with the formation of KCl and MgCl + ion-pairs in aqueous solutions.

Atomic force microscopy studies on growing surfaces of bovine insulin crystals

The morphology and the growth mechanism of the {100} surfaces of 2Zn-insulin crystals were investigated by in situ and ex situ atomic force microscopy (AFM). The dominant growth mechanism of the insulin crystals is the formation and lateral expansion of two-dimensional (2D) nuclei on the growing surfaces. Several types of dislocation spirals, such as conventional single, double and triple spirals as well as spirals generated by a planar defect, were also observed on the surfaces. Besides those, growth by three-dimensional (3D) insulin islands with mutilayer stacks, which suddenly appeared on the surface, occasionally occurred. Every step was separated from each other with a height of about 3.5 nm, which corresponds to that of a single layer composed of insulin hexamers. An anisotropy of step velocity was revealed from a series of sequential images of growing crystal surfaces. The step velocity of the multilayer stacks was about 50% of that of a single step, which implies that only little of the diffusion fields exist. On the other hand, the degeneration behavior of the multilayer steps provided evidence for the Schwoebel effect, i.e. surface diffusion slightly operates as the mechanism of solute transport towards the steps. Some topics, such as the growth rate controlling factor, aggregation phenomena and the occurrence of point and planar defects, are also discussed on the basis of the observations.

Ab initio density functional studies on the stability of tetrathiocyanato complexes of Zn(II), Cd(II) and Hg(II)

Abstract The most stable structures and formation energies of [M(NCS)4]2−, [M(NCS)2(SCN)2]2−, [M(SCN)4]2− (M=Zn(II), Cd(II), Hg(II)) and [Cd(NCS)3(SCN)]2− have been calculated by the ab initio density functional method. The complex anions [M(NCS)4]2− were optimized to regular tetrahedral geometry with a linear M–N–C, and the [M(SCN)4]2− complex anions to a twisted tetrahedral geometry. The geometry optimizations of [M(NCS)2(SCN)2]2− and [Cd(NCS)3(SCN)]2− contained both linear M–N–C and bent M–S–C bonds formed in the optimized complexes The formation energies of [M(NCS)4]2−, [M(NCS)2(SCN)2]2− and [M(SCN)4]2− are −2701, −2625 and −2557 kJ mol−1 for Zn(II), −2378, −2317 and −2262 kJ mol−1 for Cd(II), and −3803, −4082 and −4333 kJ mol−1 for Hg(II), respectively, and that of [Cd(NCS)3(SCN)]2− is −2342 kJ mol−1. A comparison of the formation energies indicated that both in water and in dimethyl sulfoxide [Zn(NCS)4]2− and [Hg(SCN)4]2− are the most stable complexes among the respective coordination isomers. However, the complex anions [Cd(NCS)3(SCN)]2− and [Cd(NCS)2(SCN)2]2− in dimethylformamide and in water, respectively, were less stable than [Cd(NCS)4]2−. The tetrathiocynato metal complexes with several coordination modes of SCN− in solution were discussed on the basis of their formation energies.

Structural rigidity of first hydration spheres of Na+ and Ca2+ in cluster models. Full geometry optimizations of [M(H2O)6]n+, [M(H2O)6⋯H2O]n+ and [M(H2O)6⋯Cl](n−1)+ (M = Na and Ca, n = 1 for Na and 2 for Ca) by density functional calculations

The intrinsic structural rigidity of hexaaqua complexes of Na+ and Ca2+ has been examined on the basis of full geometry optimizations on cluster models of [M(H2O)6]n+, [M(H2O)6⋯H2O]n+ and [M(H2O)6⋯Cl](n−1)+ (M = Na and Ca, n = 1 for Na and 2 for Ca) by use of the ab initio density functional method with Gaussian-type basis sets. The optimized geometries of [Na(H2O)6]+ and [Ca(H2O)6]2+ were both a regular octahedron. In the optimization for adding a water molecule or a chloride anion to the [Na(H2O)6]+ model, [Na(H2O)6⋯H2O]+ and [Na(H2O)6⋯Cl], each octahedral [Na(H2O)6]+ unit was kept within six-coordination, although both structures were strongly distorted. On the other hand, in the [Ca(H2O)6⋯H2O]2+ and [Ca(H2O)6⋯Cl]+ system, the additional ligand, H2O and Cl−, was participated in the coordination to the Ca2+ ion and the coordination number of Ca2+ was changed from six to seven. The results were compared with those of the K+ and Mg2+ complexes previously reported, and the differences in the intrinsic structural rigidity of the hexaaqua complexes of Na+, K+, Mg2+ and Ca2+ were explained in terms of the charges and ionic radii of the cations. The formation of an Mn+ -Cl− ion-pair in aqueous solution was also discussed.

Structural and energetic studies on double salts of M(II)Mg2Cl6·12H2O (M Ca, Mn, Cd) by X-ray diffraction and density functional methods

The crystal structure of the double salt complex MnMg2Cl6·12H2O has been determined by a single crystal X-ray diffraction method. The phase is trigonal, space group P31c, with unit cell dimensions a = 9.953(3) and c = 11.467(3) A. The crystal structure consists of two kinds of well-separated octahedra, [Mg(H2O)6]2+ and [MnCl6]4−, which is isomorphous with the CdMg2Cl6·12H2O crystal. In order to examine the stability of the double salt crystals, full geometry optimizations have been carried out for several octahedral polyhedra of [M(H2O)6]2+ and [MCl6]4− (MMg2+, Ca2+, Mn2+, Cd2+) by the ab initio density functional method. Comparison of formation energies (ΔE) for [Mg(H2O)6]2+, [M(II)(H2O)6]2+, [MgCl6]4− and [M(II)Cl6]4− independently optimized {(ΔE([Mg(H2O)6]2+) + ΔE([M(II)Cl6]4−)) versus (ΔE([MgCl6]4−) + ΔE([M(II)(H2O6]2+)); M  Ca, Mn, Cd}, reveal that the former combination of polyhedra is significantly stable in comparison with the latter, indicating that the formation energies of the first coordination spheres play a decisive role in determining the constituent polyhedra of the double salt crystals.

CRYSTALLOGRAPHIC INVESTIGATIONS OF DIAQUATETRACHLORO-COBALT(II) AND NICKEL(II) COMPLEXES

Abstract The crystal structures of 2RbCl·CoCl2·2H2O (1) and 2RbCl·NiCl2·2H2O (2) have been determined by X-ray diffraction analyses. Both crystallize in triclinic space group P 1 with a = 5.596(3), b = 6.486(3), c = 7.009(3) A, α = 65.35(2), β = 87.68(2), γ = 84.58(3)°, Z = 1 for 1, and a = 5.571(2), b = 6.450(3), c = 6.968(2) A, α = 65.52(4), β = 87.57(3), γ = 84.22(3)°, Z = 1 for 2. The Co2+ and Ni2+ atoms are both coordinated in distorted trans-octahedral geometries by four Cl− and two water molecules, and Rb+ is surrounding by eight Cl− ions. The average interatomic distances between the first transition metal(II) and chloride ions in trans-[M(II)Cl4(OH2)2]2- (M(II) = Co, Ni) moieties are 2.479 and 2.437 A for 1 and 2, respectively, and the M(II)-O bond lengths are 2.107 and 2.070 A for 1 and 2, respectively. With the same M(II) cations, the M(II)-Cl distance is significantly longer than those of cis- and trans-[M(II)Cl2(OH2)4] octahedral moieties reported previously in M(II)Cl2 · 4H2O and M(II)Cl2 · ...