The hydrated proton H+(H2O)n. IV. A high-precision neutron diffraction study of the diaquohydrogen ion, (H2OHOH2)+, in trans-dichlorobis(ethylenediamine)cobalt(III) chloride hydrochloride dihydrate

A neutron-diffraction study of hemimorphite

The hydrogen atoms in hemimorphite, Zn4Si2O7(OH)2 · H2O, have been located and its crystal structure refined using 415 three-dimensional singlecrystal neutron-diffraction data. The mineral is orthorhombic, space group Imm2, with a = 8.367(5), b = 10.730(6), c = 5.115(3) Å, and Z = 2. The structure consists of three-membered rings of corner-sharing Zn(OH)O3 ( × 2) and SiO4 tetrahedra arranged in compact sheets parallel to (010). Three oxygen atoms in each tetrahedron are bonded to two zinc atoms and one silicon atom, while a fourth oxygen atom forms a bridging bond to an equivalent cation in an adjacent sheet. The water molecules are oriented parallel to (010) inside large cavities between the tetrahedral sheets and are held in place by hydrogen bonds to and from the hydroxyl groups of the Zn – OH – Zn bridging linkages. Mulliken population analyses calculated using constant bond lengths and the observed angles within and between the tetrahedra allow a rationalization of the bond-length variations in the SiO4 group, but are less successful in the case of Zn(OH)O3. Detailed analysis of observed bond length and calculated overlappopulation variations in a variety of tetrahedral oxyanions suggests that the poorer agreement in the case of the Zn tetrahedron is more a function of the larger overall size of the group than of the relative ionic character of the bonds.

Nucleophilicity-periodic trends and connection to basicity

The potential energy profiles of 18 identity S(N)2 reactions have been estimated by using G2-type quantum-chemical calculations. The reactions are: X- + CH3-X --> X-CH3 + X- and XH + CH3-XH+ --> +HX-CH3 + XH (X = NH2, OH, F, PH2, SH, Cl, AsH2, SeH, Br). Despite the charge difference, the barrier heights and the geometrical requirements upon going from the reactant to the transition structure are surprisingly similar for X- and XH. The barrier heights decrease on going from left to right in the periodic table, and increasing ionization energy (of X- and XH) is correlated with decreasing barrier. The observed trends are explained in terms of substrates with stronger electrostatic character giving rise to lower energetic barriers due to decreased electron repulsion in the transition structure. On the basis of this study, the relationship between the kinetic concept of nucleophilicity and the thermodynamic concept of basicity has been analyzed and clarified. Since the trends in intrinsic nucleophilicity (only defined for identity reactions) and basicity are opposite, overall nucleophilicity (defined for any reaction) will be determined by the relative contribution of the two factors. Only for strongly exothermic reactions will basicity and nucleophilicity be matching.