Defect chemistry of uranium oxides

Uranium oxides are known as nonstoichiometric compounds whose composition changes according to external conditions such as temperature and oxygen partial pressure. The change of composition caused by the formation of defect structure results in a change of their properties. In this paper, the compositional changes of UO₂ and doped UO₂ [(U, M)O₂; M=La, Ti, Pu, Th, Nb, Cr, etc.] and also those of other uranium oxides (U₄O₉, U₃O₈) are shown against oxygen partial pressure. From the results of doped UO₂, it is concluded that the valence control rule holds to a first approximation. The defect structures are estimated both from log x vs. log Po₂ (x: deviation from the stoichiometric composition and Po₂: oxygen partial pressure) and log vs. log Po₂ (: electrical conductivity) relations. The defect structures of UO₂ and doped UO₂ are derived based on the Willis model for UO_2+x. The detect structure of U₄O₉ phase is similar to that of UO_2+x, but the defect structures of U₃O₈ phase are complicated due to the existence of many higher-order phase transitions. The thermodynamic data such as the partial molar enthalpy and entropy and the heat capacity are important to characterize the defect structure. The high temperature heat capacities of UO₂ doped with Gd show pronounced increases at high temperatures the onset temperature decreases as the dopant content increases. The increase of heat capacity is interpreted to be due to the formation of lattice defects. The heat capacity measurements on U₄O₉ and U₃O₈ clucidate the presence of the phase transition. The mechanisms of these phase transitions are discussed.     

The role of amine sulfates in hydrothermal uranium chemistry

A series of novel uranium sulfates containing organic structure directing cations has been synthesized from amine sulfate precursors under hydrothermal conditions. The amine sulfates act as a soluble source of the protonated amines and sulfate ions at low temperature and provide a reaction pathway in which no amine decomposition is observed. The protonated amines act as both space fillers and hydrogen-bond donors in the three-dimensional structure. The factors governing the formation of the observed hydrogen-bonding networks were probed through the use of bond valence sums, which allow the quantification of residual negative charge and determination of the relative nucleophilicity of each oxide ligand. The hydrogen bonding in these new compounds is dependent upon two factors. First, the oxide ligands with the highest nucleophilicities are preferential acceptors with respect to their less nucleophilic counterparts. Second, geometric constraints that result from the formation of multiple hydrogen bonds from a single ammonium center can dictate the donation to oxides with smaller negative charges. Crystal data for [N4C6H12][SO4]2 x 2H2O, a = 7.2651(2) A, b = 7.3012(2) A, c = 8.3877(3) A, alpha = 90.260(1) degrees, beta = 100.323(1) degrees, gamma = 113.0294(15) degrees, triclinic, P-1 (No. 2), Z = 1; for [N4C6H22][UO2(H2O)(SO4)2]2 x 6H2O, a = 6.7318(1) A, b = 9.2975(1) A, c = 13.1457(3) A, alpha = 72.3395(6) degrees, beta = 89.1401(7) degrees, gamma = 70.0267(12) degrees, triclinic, P-1 (No. 2), Z = 1; for [N4C6H22][UO2(SO4)2)2, a = 9.3771(2) A, b = 12.9523(3) A, c = 18.9065(6) A, orthorhombic, Pbca (No. 61), Z = 4; for [N5C8H28]2[(UO2)5(H2O)5(SO4)10] x H2O, a = 7.76380(5) A, b = 14.16890(5) A, c = 56.46930(5) A, orthorhombic, Pbnm (No. 62), Z = 4.     

The chemistry of uranium and related radionuclides in Lake Ontario waters

The chemistry of uranium and its eight associated daughters has been considered in Lake Ontario water along with that of selected transuranic elements. These transuranics arise mainly from fallout, and to a lesser extent from nuclear fuel reprocessing. Most exist as free ions or oxides of various kinds, uranium probably exists as a strong uranyl carbonate complex.     

The vitality of uranium molecular chemistry at the dawn of the XXIst century

The intent of this Dalton Perspective is to highlight the recent advances in uranium molecular chemistry, with the results reported during the 2000-2006 period. This discipline is currently witnessing an impressive development, together with the theoretical chemistry and solid-state chemistry of the f-elements, and its face has profoundly changed, revealing unsuspected structural and reactivity features. This progress required and was facilitated by the use of new precursors. Studies of low-valent compounds gave a better insight into lanthanide(III)/actinide(III) differentiation and led to the discovery of unusual reactions, including activation of small molecules. A number of tetravalent uranium complexes, in particular polynuclear compounds, have been synthesized, which exhibit exciting structures and physicochemical properties. The potential of uranium(III) and uranium(IV) complexes in catalysis has been confirmed. The uranyl complexes, from mononuclear species to supramolecular assemblies, reveal a variety of novel structures, changing the generally accepted ideas on the coordination geometry and the stability of the UO2(2+) ion.     

Structural chemistry and magnetic behavior of binary uranium silicides

Binary uranium silicides have been thoroughly reinvestigated with respect to crystal chemistry and magnetic properties; for most compounds, magnetic studies have been carried out at low temperatures for the first time (2 to 300 K, 0 to 5 T). Formation of all known binary silicides has been confirmed: U₃Si, U₃Si₂ (U₃Si₂-type), USi (USi-type), U₃Si₅ (defect AlB₂-type), USi_1.88 (defect ThSi₂-type) and USi₃ (Cu₃Au-type). At the composition U₃Si_∼5 three different phases have been observed: the defect AlB₂-type as well as a phase separation into two orthorhombically distorted Alb₂-type related phases. USi_2−x with the tetragonal defect ThSi₂-type structure at its silicon poor phase boundary was found to be in equilibrium with USi_2−x of the orthorhombic defect GdSi₂ type. Precise U-U distances have been derived from X-ray single crystal counter data for U₃Si₂, USi, for the AlB₂-type subcell of U₃Si₅, and for USi_1.84 (defect ThSi₂-type). From susceptibility measurements, a band type paramagnetism was observed for U₃Si₂, whereas a temperature independent paramagnetism was confirmed for USi₃. Curie-Weiss paramagnetism was encountered within the investigated temperature range for U₃Si₅ and USi_1.88. A modified Curie-Weiss law was revealed for USi with the USi-type, whereas ferromagnetic ordering at T_c = 125 K was observed for the oxygen stabilized “USi” with the FeB-type. Magnetism of the uranium silicides is discussed as a function of the distant dependent 5f-5f electron overlap and d-f electron hybridization. No superconductivity was observed above 2 K.     

Neptunium thiophosphate chemistry: intermediate behavior between uranium and plutonium

Black crystals of Np(PS(4)), Np(P(2)S(6))(2), K(11)Np(7)(PS(4))(13), and Rb(11)Np(7)(PS(4))(13) have been synthesized by the reactions of Np, P(2)S(5), and S at 1173 and 973 K; Np, K(2)S, P, and S at 773 K; and Np, Rb(2)S(3), P, and S at 823 K, respectively. The structures of these compounds have been characterized by single-crystal X-ray diffraction methods. Np(PS(4)) adopts a three-dimensional structure with Np atoms coordinated to eight S atoms from four bidentate PS(4)(3-) ligands in a distorted square antiprismatic arrangement. Np(PS(4)) is isostructural to Ln(PS(4)) (Ln = La-Nd, Sm, Gd-Er). The structure of Np(P(2)S(6))(2) is constructed from three interpenetrating diamond-type frameworks with Np atoms coordinated to eight S atoms from four bidentate P(2)S(6)(2-) ligands in a distorted square antiprismatic geometry. The centrosymmetric P(2)S(6)(2-) anion comprises two PS(2) groups connected by two bridging S centers. Np(P(2)S(6))(2) is isostructural to U(P(2)S(6))(2). A(11)Np(7)(PS(4))(13) (A = K, Rb) adopts a three-dimensional channel structure built from interlocking [Np(7)(PS(4))(13)](11-)-screw helices with A cations residing in the channels. The structure of A(11)Np(7)(PS(4))(13) includes four crystallographically independent Np atoms. Three are connected to eight S atoms in bicapped trigonal prisms. The other Np atom is connected to nine S atoms in a tricapped trigonal prism. A(11)Np(7)(PS(4))(13) is isostructural to A(11)U(7)(PS(4))(13). From Np-S bond distances and charge-balance, we infer that Np is trivalent in Np(PS(4)) and tetravalent in Np(P(2)S(6))(2) and A(11)Np(7)(PS(4))(13). Np exhibits a behavior intermediate between U and Pu in its thiophosphate chemistry.     

The importance of a single methyl group in determining the reaction chemistry of pentamethylcyclopentadienyl cyclooctatetraenyl uranium metallocenes

The steric factors that allow trivalent [(C(5)Me(5))(3)U] (1) to function as a three-electron reductant with C(8)H(8) to form tetravalent [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-C(8)H(8))] (2) have been explored by examining the synthesis and reactivity of the intermediate, "[(C(5)Me(5))(2)(C(8)H(8))U]" (3), and the slightly less crowded analogues, [(C(5)Me(5))(C(5)Me(4)H)(C(8)H(8))U] and [(C(5)Me(4)H)(2)(C(8)H(8))U], that have, successively one less methyl group. The reaction of [{(C(5)Me(5))(C(8)H(8))U(μ-OTf)}(2)] (4; OTf=OSO(2) CF(3)) with two equivalents of KC(5)Me(5) in THF gave ring-opening to "[(C(5)Me(5))(C(8)H(8))U{O(CH(2))(4)(C(5) Me(5))}]" consistent with in situ formation of 3. Reaction of 4 with two and four equivalents of KC(5)Me(4)H generates two equivalents of [(C(5)Me(5))(C(5)Me(4)H)(C(8)H(8))U] (5) and [(C(5)Me(4)H)(2)(C(8)H(8))U] (6), respectively, which in contrast to 3 were isolable. Tetravalent 5 reduces phenazine and PhEEPh (E=S, Se, and Te) to form the tetravalent uranium reduction products, [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-C(12)H(8)N(2))] (7), [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-SPh)(2)] (8), [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-SePh)(2)] (9), and [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-TePh)(2)] (10), consistent with sterically induced reduction. In contrast, the less sterically crowded 6 does not react with these substrates.     

The solid state chemistry of uranium: Part IV. The thermal decomposition and kinetics of tetrachlorobis(N,N, N1, N1-tetramethylurea) uranium(IV)

The thermal decomposition of UCl₄2tmu in an oxygen atmosphere was studied. Decomposition of single crystals begins around 180° C and approximates to UCl₄2tmu_(s) + O_2(g) → UO₂Cl₂tmu_(s) + tmu_(g) + gases and is exothermic (ΔH = −270 ± 5 kJ mol⁻¹). The apparent activation energy for the initial stages (nucleation process) of the reaction was estimated as 362 kJ mol⁻¹. The growth period is described by a one-dimensional diffusion process and the decay period by the contracting-area model.     

The uranium content of sea-water

A review of the literature on the uranium content of sea-water.     

鞘氨醇的化学

动植物细胞中鞘脂及其代谢产物鞘氨醇等具有多种生理作用，是生命体传递信息的化学信使。鞘氨醇等的合成在有机合成界形成了一个经久不衰的热潮，应用各种新技术的合成方法不断出现。本文主要综述了鞘氨醇等衍生脂质的化学研究，特别是它们的化学合成进展。     

The chemistry of indole

In the reaction of 2-aminoindoles with -diketones, depending on the basicity of the medium, cyclization may take place with the participation of position 1 or 3 and the formation of the corresponding -carbolines, pyrimidoindoles, or mixtures of them.For Communication (XXXVI), see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1043–1046, August, 1973.     

The chemistry of pyrazolidine

The conditions for the thin-layer chromatographic separation of some groups of 3,5-dioxopyrazolidines on alumina have been established. It is shown that the chromatographic behavior of these compounds depends mainly on their ability to undergo keto-enol and lactim-lactam tautomerism.For Part XII, see [4].     

The question mark at uranium

Being excerpts from pages 187, 203, 204, 207, 208, 209, 210 and 211 of Uncle Tungsten, extracted by Michael Laing with the consent of the author, Professor Oliver Sacks, and Picador Publishers.     

The chemistry of ethyleneimine

The reaction of N-aminoethyleneimine (I) with substituted benzenesulfonyl chlorides (II) has been studied. The main products of the heterolysis of the ring are substituted N-(-chloroethyl)benzenesulfonohydrazides (V). The structure of V has been shown by IR, UV, and PMR spectroscopy, and also by independent synthesis.For part III, see [23].     

The chemistry of ethyleimine

The reaction of N-aminoethylenimine with aliphatic ketones leading to the formation of their ethylenehydrazones has been studied.For Part V, see [1].