The synthesis and conformational analysis of the first series of peptoid oligomers composed of consecutive
N
-(alkylamino)glycine units is investigated. We demonstrate that
N
-(methylamino)glycine ...homooligomers can be readily synthesized in solution using
N
-Boc-
N
-methylhydrazine as a peptoid submonomer and stepwise or segment coupling methodologies. Their structures were analyzed in solution by 1D and 2D NMR, in the solid state by X-ray crystallography (dimer
2
), and implicit solvent QM geometry optimizations.
N
-(Methylamino)peptoids were found to preferentially adopt
trans
amide bonds with the side chain N–H bonds oriented approximately perpendicular to the amide plane. This orientation is conducive to local backbone stabilization through intra-residue hydrogen bonds but also to intermolecular associations. The high capacity of
N
-(methylamino)peptoids to establish intermolecular hydrogen bonds was notably deduced from pronounced concentration-dependent N–H chemical shift variation in
1
H NMR and the antiparallel arrangement of mirror image molecules held together via two hydrogen bonds in the crystal lattice of dimer
2
.
Strategies for interpreting mass spectrometric and Raman spectroscopic data have been developed to study the structure and reactivity of uranyl peroxide cage clusters in aqueous solution. We ...demonstrate the efficacy of these methods using the three best-characterized uranyl peroxide clusters, {U24}, {U28}, and {U60}. Specifically, we show a correlation between uranyl–peroxo–uranyl dihedral bond angles and the position of the Raman band of the symmetric stretching mode of the peroxo ligand, develop methods for the assignment of the ESI mass spectra of uranyl peroxide cage clusters, and show that these methods are generally applicable for detecting these clusters in the solid state and solution and for extracting information about their bonding and composition without crystallization.
Three uranium(VI)-bearing materials were synthesized hydrothermally using the organic ligand 4,4′-biphenyldicarboxylic acid: (UO2)(C14O4H8) (1); (UO2)2(C14O4H8)2(OH)·(NH4)(H2O) (2); ...(UO2)2(C14O4H8)(OH)2 (3). Compound 1 was formed after 1 day at 180 °C in an acidic environment (pHi = 4.03), and compounds 2 and 3 coformed after 3 days under basic conditions (pHi = 7.95). Coformation of all three compounds was observed at higher pHi (9.00). Ex situ Raman spectra of single crystals of 1–3 were collected and analyzed for signature peaks. In situ hydrothermal Raman data were also obtained and compared to the ex situ Raman spectra of the title compounds in an effort to acquire formation mechanism details. At pHi = 4.00, the formation of 1 was suggested by in situ Raman spectra. At an increased pHi (7.90), the in situ data implied the formation of compounds 1 and 3. The most basic conditions (pHi = 9.00) yielded a complex mixture of phases consistent with that of increased uranyl hydrolysis.
Six new uranium phosphites, phosphates, and mixed phosphate–phosphite compounds were hydrothermally synthesized, with an additional uranyl phosphite synthesized at room temperature. These compounds ...can contain UVI or UIV, and two are mixed-valent UVI/UIV compounds. There appears to be a strong correlation between the starting pH and reaction duration and the products that form. In general, phosphites are more likely to form at shorter reaction times, while phosphates form at extended reaction times. Additionally, reduction of uranium from UVI to UIV happens much more readily at lower pH and can be slowed with an increase in the initial pH of the reaction mixture. Here we explore the in situ hydrothermal redox reactions of uranyl nitrate with phosphorous acid and alkali-metal carbonates. The resulting products reveal the evolution of compounds formed as these hydrothermal redox reactions proceed forward with time.
A new strontium uranyl oxyfluoride, (UO
)
F
Sr
(H
O)
(NO
)·H
O, was synthesized under hydrothermal conditions. The single-crystal X-ray structure was determined. This compound crystallizes in the ...triclinic space group P1̅ (No. 2), with unit cell parameters a = 10.7925(16) Å, b = 10.9183(16) Å, c = 13.231(2) Å, α = 92.570(8)°, β = 109.147(8)°, γ = 92.778(8)°, V = 1468.1(4) Å
, and Z = 2. The structure is built from uranyl-containing Formula: see text chains of tetrameric units of corner-sharing UO
F
pentagonal bipyramids. These chains are linked through trimeric strontium units to form strontium-uranyl oxyfluoride layers further assembled by nitrate groups. The interlayer space is occupied by free water molecules. This compound was characterized by spectroscopic methods, especially
F NMR highlighting the many different fluoride sites. Structural relationships with other uranyl oxyfluorides were investigated through the different F/O ratios, the structural building unit, and the structural arrangement.
A new strontium uranyl oxyfluoride, (UO2)4F13Sr3(H2O)8(NO3)·H2O, was synthesized under hydrothermal conditions. The single-crystal X-ray structure was determined. This compound crystallizes in the ...triclinic space group P1̅ (No. 2), with unit cell parameters a = 10.7925(16) Å, b = 10.9183(16) Å, c = 13.231(2) Å, α = 92.570(8)°, β = 109.147(8)°, γ = 92.778(8)°, V = 1468.1(4) Å3, and Z = 2. The structure is built from uranyl-containing ( UO 2 ) 4 F 13 ∞ 1 chains of tetrameric units of corner-sharing UO2F5 pentagonal bipyramids. These chains are linked through trimeric strontium units to form strontium–uranyl oxyfluoride layers further assembled by nitrate groups. The interlayer space is occupied by free water molecules. This compound was characterized by spectroscopic methods, especially 19F NMR highlighting the many different fluoride sites. Structural relationships with other uranyl oxyfluorides were investigated through the different F/O ratios, the structural building unit, and the structural arrangement.
The compound Na4(UO2)(S2)3(CH3OH)8 was synthesized at room temperature in an oxygen-free environment. It contains a rare example of the (UO2)(S2)34– complex in which a uranyl ion is coordinated by ...three bidentate persulfide groups. We examined the possible linkage of these units to form nanoscale cage clusters analogous to those formed from uranyl peroxide polyhedra. Quantum chemical calculations at the density functional and multiconfigurational wave function levels show that the uranyl–persulfide–uranyl, U–(S2)–U, dihedral angles of model clusters are bent due to partial covalent interactions. We propose that this bent interaction will favor assembly of uranyl ions through persulfide bridges into curved structures, potentially similar to the family of nanoscale cage clusters built from uranyl peroxide polyhedra. However, the U–(S2)–U dihedral angles predicted for several model structures may be too tight for them to self-assemble into cage clusters with fullerene topologies in the absence of other uranyl-ion bridges that adopt a flatter configuration. Assembly of species such as (UO2)(S2)(SH)44– or (UO2)(S2)(C2O4)44– into fullerene topologies with ∼60 vertices may be favored by use of large counterions.
In order to synthesize chemical filters for the selective removal of volatile fluorides, commercial magnesium fluoride MgF2 with high specific surface area (HSA) was investigated. The amount of -OH ...groups substituting fluorine is not negligible, partly due to the high surface area, but also due to the synthesis route. These hydroxyl groups induce a Lewis basicity on the surface of metal fluorides. The amount of these Lewis basic sites has been tailored using fluorination with F2 gas. The sorption of VOF3, used as model gas, onto these fluorides was investigated. The versatility of surface chemistry as a function of a number of Lewis basic sites opens the way to filter selectivity mixture of volatile fluorides depending on their Lewis acidity. HSA MgF2 acts as a stable matrix towards the gas to be purified, and the selectivity may be achieved by a higher Lewis acidity of the gaseous impurity.
Two chiral cage clusters built from uranyl polyhedra and (HPO3)2– groups have been synthesized in pure yield and characterized structurally and spectroscopically in the solid state and aqueous ...solution. Synthesis reactions under ambient conditions in mildly acidic aqueous solutions gave clusters U 22 PO 3 and U 28 PO 3 that contain belts of four uranyl peroxide pentagonal and hexagonal bipyramids, in contrast to earlier reported uranyl peroxide cage clusters that are built from four-, five-, and six-membered rings of uranyl hexagonal bipyramids. U 22 PO 3 and U 28 PO 3 are also the first chiral uranyl-based cage clusters, the first that contain uranyl pentagonal bipyramids that contain no peroxide ligands, and the first that incorporate (HPO3)2– bridges between uranyl ions. They are built from 22 uranyl polyhedra and 20 (HPO3)2– groups, or 28 uranyl polyhedra and 24 (HPO3)2– groups, with the outer and inner surfaces of the cages passivated by the O atoms of uranyl ions. Small-angle X-ray scattering (SAXS) profiles demonstrated that U 22 PO 3 clusters formed in solution within 1 h after mixing of reactants, and remained in solution for 2 weeks prior to crystallization. Time-resolved electrospray ionization mass spectrometry and SAXS demonstrated that U 28 PO 3 clusters formed in solution within 1 h of mixing the reactants, and remained in solution 1 month before crystallization. Crystallization of U 22 PO 3 and U 28 PO 3 is accelerated by addition of KNO3. Clusters of U 22 PO 3 with and without encapsulated cations exhibit markedly different aqueous solubility, reflecting the importance of cluster surface charge in fostering linkages through counterions to form a stable solid.
The boric acid flux reaction of NpO2(ClO4)2 with NaClO4 affords Na(NpO2)4B15O24(OH)5(H2O)(ClO4)·0.75H2O (NaNpBO-1). NaNpBO-1 possesses a layered structure consisting of double neptunyl(VI) borate ...sheets bridged by another NpVI site through cation–cation interactions. The sole presence of NpVI in NaNpBO-1 is supported by absorption and vibrational spectroscopy.
