Search

The crystal structures and magnetic properties of the nitronyl nitroxide radicals 4,5-dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (1), 4,5-dihydro-2,4,4,5,5-pentamethyl-3-oxido(1H-imidazol-1-yloxyl) (2), 2-(4-chlorophenyl)-4,5-dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (3), and 4,5-dihydro-2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (4) are reported. Compound 1 has two polymorphic forms: the α phase is monoclinic (P21/n space group), with a single molecule in the asymmetric unit, and the β phase is monoclinic (P21/c space group), with four molecules in the asymmetric unit. In the two polymorphs, the molecules are arranged in dimers formed by hydrogen bonds of the type C&bond;H···O&bond;N. The crystal structure of 3 contains layers of antiparallel ribbons of molecules. Compound 4 crystallizes with solvent molecules, and an intramolecular hydrogen bond is formed between the 2-OH group of the phenyl ring and the nitroxide O-atom. Compound 4 also loses the two O-atoms of the nitroxide moiety upon heating to 90°. Magnetic measurements showed that both α and β polymorphs of 1 exhibit antiferromagnetic coupling. The best fit to the experimental data was obtained using Bleany&bond;Bower's singlet-triplet model (H=−2JSaSb): J=−11.2 K for the α phase and J=−15.0 K for the β phase. Compounds 3 and 4 show no evidence for spin coupling.

Three groups of singlet ground state [TCNE]22−(TCNE=tetracyanoethylene) dimers with characteristic intradimer CC separations (r) and dihedral angles (d) [i.e., group St(r∼1.6 Å; d=180°), Lt(r∼3.5 Å; d=180°), and Lc(r∼2.9 Å; d=∼0°); notation: S/L: short/long bond length; subscript t/c: trans/cis, respectively] are experimentally characterized. The Stgroup is comprised of σ‐dimers of [TCNE].−and octacyanobutanediide, [C4(CN)8]2−, which have a typical, albeit long, sp3–sp3σ bond (r∼1.6 Å) between each [TCNE].−moiety and characteristic νCN, νCC, and δCCNIR absorptions. The L groups are structurally characterized as π‐dimers of [TCNE].−that are either eclipsed with r∼2.9 Å (Lc) and the nitriles bend away from the nominal TCNE plane away from the center of the dimer by 5.0° (∼sp2.17) or are noneclipsed with r∼3.5 Å (Lt) and the nitriles bend toward the center of the dimer by 1.9° (∼sp2.06). Ab initio computations on isolated dimers were used to study the formation and stability of these exceptionally long CC (≥2.9 Å) bonding interactions as well as the process of π‐[TCNE]22−dimer formation for the Lcand Ltgroups. The results of these computational studies show that the ground‐state potential curve is that of a closed‐shell/open‐shell singlet, depending on the distance. The short Stgroup (r∼1.6 Å) of dimers in this surface are true minimum‐energy structures; however, the Ltand Lcgroups are unstable, although two different nonphysical minima are found when imposing a double occupancy of the orbitals. These minima are metastable relative to dissociation into the isolated [TCNE].−units. Consequently, the existence of dimer dianions in crystals is due to cation⋅⋅⋅[TCNE]−interactions, which provide the electrostatic stabilization necessary to overcome the intradimer electrostatic repulsion. This cation‐mediated π*–π* [TCNE]−⋅⋅⋅[TCNE]−interaction complies with Pauling's definition of a chemical bond. This bonding interaction involves the π* orbitals of each fragment, and arise from the overlap of the b2gSOMO on each of the two [TCNE].−s to form a filled b2u[TCNE]22−orbital. Although a π dimer typically forms, if the fragments are close enough a σ dimer can form. Due to the presence of cation‐mediated intradimer CC bonding interactions the Lcgroup of π‐[TCNE]22−dimers exhibits experimentally observable νCNIR absorptions at 2191±2 (m), 2173±3 (s), and 2162±3 cm−1(s) and νCCat 1364±3 cm−1(s) as well as a new UV‐Vis feature in the range of 15 000 to 18 200 cm−1(549 to 667 nm) and averaging 16 825±1180 cm−1(594 nm) assigned to the predicted new intradimer 1A1g→ 1B1utransition and is purple on reflected light. Upon cooling to 77 K in 2‐methyl tetrahydrofuran, this new band occurs at 18 940 cm−1(528 nm) for {[Et4N]+}2[TCNE]22−, and the yellow solution turns deep red. Group Ltis characterized by νCNabsorptions at 2215±2, 2197±3, and 2180±4 cm−1and νCCat 1209±9 cm−1(w), while group SThas νCNbands at 2215±4, 2157±3, and 2107±4 cm−1and νCCat 1385±1 cm−1(vs).

Neutron flux from linear accelerators is conventionally monitored using ionization chambers containing one or more foils thinly coated with a fissionable or fissile material. Due to the long pulse rise times resulting from the ionization mechanism, fission chambers are prone to pulse pile-up in high-neutron-flux environments. In addition, their relatively low efficiencies result in extremely long counting times in low-flux environments. To ameliorate these effects, a novel type of neutron flux monitor, consisting of fissionable material loaded in a liquid scintillator, has been developed, characterized, and tested in the beam line at the Los Alamos Neutron Science Center. This is a rugged, cost-efficient detector with high efficiency, a short signal rise time, and the ability to be used in low neutron-flux environments. Compared with a conventional fission chamber, the fissionable scintillator displays a significantly higher event rate. Related research on nanocomposite scintillators for gamma-ray detection suggests the possibility of extending this approach by synthesizing fissionable material nanoparticles and loading them into an organic scintillator. We will present results of the design and characterization process and an analysis of the results of the beam line experiments.

The low melting point, negligible vapor pressure, good solubility, and thermal and chemical stability make ionic liquids useful materials for a wide variety of applications. Polyoxometalates are early transition metal oxygen clusters that can be synthesized in many different sizes and with a variety of heterometals. The most attractive feature of POMs is that their physical properties, in particular electrical, magnetic, and optical properties, can be easily modified following known procedures. POM ionic liquids can be obtained by selecting the appropriate cation. Different alkyl ammonium and alkyl phosphonium salts are being used to produce new POM ionic liquids together with organic or inorganic luminescent centers to design light emitting materials. Ammonium and phosphonium cations with activated, polymerizable groups are being used to further polymerize the ionic liquid into transparent, solid materials with high metal densities.

A room temperature fulleride-based ionic liquid has been developed and characterized. Utilizing the trihexyl (tetradecyl) phosphonium cation, the ionic liquid [PC6C6C6C14]3 [C60]Cl was synthesized, and distinctly shows spectroscopic characteristics of the [C60]2- anion. The glass transition of the ionic liquid occurs well below room temperature at -56 {degree sign} C, but it still retains an extremely high viscosity at room temperature. Attempts were made to obtain the pure fulleride salt [PR4]3[C60], but nearly always results in the binary fulleride-chloride salt.

For Abstract see ChemInform Abstract in Full Text.

The cover picture showsthe b2gHOMO for [TCNE].−, the structure (top, middle) and bonding of the π‐[TCNE]22−dimer and its b2uHOMO and b1gLUMO. At the bottom of the picture solutions of [Et4N]2[TCNE]2in dichloromethane solution at room temperature, which is yellow due to the presence of [TCNE].−, and at −90 °C, which is red due to shifting of the 2 [TCNE].−⇌ [TCNE]22−equilibrium to the right, thus forming [TCNE]22−in solution. For more details see the article by J. S. Miller et al. on p. 4894 ff.