The low solar neutrino flux: A search for a resonance in 6Be

A study of neutron time-of-flight spectra from the ⁴He(³He, n)⁶ reaction at E_3He = 38.61 MeV, sets a one standard deviation upper limit of 7 μb/sr for the cross section to a possible narrow level near the ³He + ³He threshold.     

Hydrogen-bonded anionic rosette networks assembled with guanidinium and C3-symmetric oxoanion building blocks

Two premeditated anionic rosette-layer architectures have been constructed using guanidinium and ubiquitous C3-symmetric oxoanions that carry unequal charges, employing bulky organic cations as interlayer templates. The undulate guanidinium-carbonate network occurs in 4[(C2H5)4N+].8[C(NH2)3+].3(CO3)2-.3(C2O4)2-.2H2O, in which the disordered (C2H5)4N+ guests are accommodated in pouches and channels within a complex three-dimensional hydrogen-bonded host framework. In [(C2H5)4N+]2.[C(NH2)3+].[1,3,5-C3H3(COO-)3].6H2O, ordered (C2H5)4N+ cations are sandwiched between planar guanidinium-trimesate host layers whose honeycomb cavities are tightly fitted with flattened-chair (H2O)6 clusters of symmetry 2.     

The 18O(p, α)15N cross section at low energies

The ¹⁸O(p, α)¹⁵N reaction cross section has been measured over the energy range 661 keV > E_c.m > 223 keV. The S-function was extrapolated to energies of astrophysical interest using the R-matrix theory. The S-factor, S₀, is estimated to be 46 MeV · b which is a factor of 3 larger than the value used in a recent tabulation of nuclear reaction rates. The effects of broad levels near the proton threshold are discussed.     

Semi-empirical molecular orbital conformational study of benzannelated cyclooctatetraenes and related compounds, and x-ray crystal structure of benzocyclooctatetraene

Semi-empirical molecular orbital conformational analyses of benzocyclooctatetraene, 1, dibenzo [a, c] cyclooetatetraene, 2 dibenzo [a, e] cyclooctatetraene, 3 tribenzo[a, c, e]-cyclooctatetraene, 4 tetrabenzo[a, c, e, g] cyclooetatetraene, 5 octafluorocyclooetatetraene, 6A, and cyclooctatetraene, 6B, have been done at the MNDO and MINDO/3 (excluding 4 and 5) levels of approximation. Comparison of these results with the X-ray structural data for 1 (as determined in the present work and the other benzannelated cyclooctatetraenes showed that, while MNDO is vastly superior to MINDO/3 both fail to account for the subtle effects of benzannelation on the conformation of the cyclooetatetraene ring. The experimental data indicate that the tub conformation of cyclooetatetraene is only slightly perturbed by the successive fusion of one and two non-adjacent benzene rings, but steric repulsion between adjacent annelated benzene rings, when present, predominates and gives rise to more folded configurations. Compound 1 crystallizes in space group P2₁/n with a = 1 1.856(2 B = 6.327(1 c = 12.470(3 Å, β = 110.03(2)°, and Z = 4. The structure has been solved by direct methods and refined to R = 0.049 for 1435 observed Mo-K diffractometer data.     

Synthesis, structure and dioxygen reactivity of a bis(micro-iodo)dicopper(I) complex supported by the [N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N,N-di-(2-pyridylmethyl)]amine ligand

The air-sensitive bis(micro-iodo)dicopper(I) complex 1 supported by [N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N,N-di-(2-pyridylmethyl)]amine (L) has been prepared by treating copper(I) iodide with L in anhydrous THF. Compound 1 crystallizes as a dimer in space group C2/c. Each copper(I) center has distorted tetrahedral N2I2 coordination geometry with Cu-N(pyridyl) distances 2.061(3) and 2.063(3) A, Cu-I distances 2.6162(5) and 2.7817(5) and a Cu...Cu distance of 2.9086(8) A. Complex 1 is rapidly oxidized by dioxygen in CH2Cl2 with a 1 : 1 stoichiometry giving the bis(micro-iodo)peroxodicopper(II) complex [Cu(L)(micro-I)]2O2 (2). The reaction of 1 with dioxygen has been characterized by UV-vis, mass spectrometry, EPR and Cu K-edge X-ray absorption spectroscopy at low temperature (193 K) and above. The mass spectrometry and low temperature EPR measurements suggested an equilibrium between the bis(micro-iodo)peroxodicopper(II) complex 2 and its dimer, namely, the tetranuclear (peroxodicopper(II))2 complex [Cu(L)(micro-I)]4O4 (2'). Complex 2 undergoes an effective oxo-transfer reaction converting PPh3 into O=PPh3 under anaerobic conditions. At sufficiently high concentration of PPh3, the oxygen atom transfer from 2 to PPh3 was followed by the formation of [Cu(PPh3)3I]. The dioxygen reactivity of 1 was compared with that known for other halo(amine)copper(I) dimers.     

Polyhedral C2@Agn cages distorted by ancillary pyridine N-oxide ligands in silver-acetylenediide complexes

Reactions of the pyridine N-oxide ligands L, L2 and L3 with the silver acetylenediide-containing system under hydrothermal conditions gave rise to four silver-acetylenediide complexes bearing interesting C2@Agn motifs: (Ag2C2)2(AgCF3CO2)8(L1)3.5 (1), (Ag2C2)2(AgCF3CO2)8(L2)2 (2), (Ag2C2)(AgCF3CO2)4(L3) (3) and [(Ag7(C2)(CF3SO3)3(L3)2(H2O)2] x 2CF3SO3 (4) (L = nicotinic acid N-oxide, L(1) = pyridine N-oxide, L2 = 1,2-bis(4-pyridyl)ethane N,N'-dioxide, L3 = 1,3-bis(4-pyridyl)propane N,N'-dioxide), which exhibit new distorted polyhedral C2@Agn cage motifs. Complex 1 has a pair of acetylenediide dianions encapsulated in a Ag(14) aggregate composed of three polyhedral parts, whereas 2 contains an irregular (C2)2@Ag13 double cage. In 3, the basic building unit is a centrosymmetric (C2)2@Ag12 double cage with each component single cage taking the shape of a highly distorted triangulated dodecahedron with one missing vertex. As to complex 4, the core is a C2@Ag7 single cage in the form of a slightly distorted monocapped trigonal prism with four cleaved edges that include all three vertical sides. Furthermore, in the silver-rich environment, the pyO-type ligands are induced to exhibit unprecedented coordination modes, such as the mu(5)-O,O,O,O',O' ligation mode of L2 in 2 and the mu4-O,O,O',O' mode of L3 in 3 and 4.     

Pyroelectric properties of BiFeO3 ceramics prepared by a modified solid-state-reaction method

BiFeO₃ (BFO) ceramics were prepared by a modified solid-state-reaction method which adopts a higher heating/cooling rate during the sintering process than usually used. It was found that the calcination temperature T _cal (from 400 to 750°C) does not influence the BFO phase formation, while the sintering temperature T _sin (from 815 to 845°C) dominates the phase purity. The optimum sintering temperature was in the range from 825 to 835°C. The optimized samples exhibit saturated ferroelectric hysteresis loops with a remnant polarization of 13.2 μC/cm². The measured piezoelectric coefficient d ₃₃ was 45 pC/N. No remnant magnetization was observed in all of the samples. The pyroelectric properties were studied as a function of temperature and frequency. A pyroelectric coefficient as high as 90 μC/m² K was obtained at room temperature in the optimized sample. An abrupt decrease of the pyroelectric coefficient was observed at temperatures between 70 and 80°C. On the basis of our results, BFO may have the potential for pyroelectric applications.     

1D silver cluster-assembled materials act as a platform for selectively erasable photoluminescent switch of acetonitrile

A new 1D silver cluster-assembled complex Ag_(10)bpy-CH_3CN exhibiting intense photoluminescence was reported. Upon the loss of coordinated acetonitrile, Ag_(10)bpy-CH_3CN form Ag_(10)bpy with Ag_(10)S_6 cores slightly distorted, being accompanied with photoluminescence quenching. Inverse process can be realized by treating Ag_(10)bpy with acetonitrile and shows a selectively erasable photoluminescent switch.