Electronic Structure Trends in N‐Heterocyclic Carbenes (NHCs) with Varying Number of Nitrogen Atoms and NHC?Transition‐Metal Bond Properties

Carbenes derived from five‐membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N‐heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug‐cc‐pVTZ level. The examined parameters include the energies of the σ‐lone pair at C_carbene and the π‐HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet–triplet energy gap was used as a measure of the stability of the N‐heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the p_π population and the natural charge at C_carbene with NHC structure. Additionally, the transition metal? NHC bond in L‐AuCl and L‐TiCl₄ and the nature of the orbital interactions between the NHC and the transition‐metal fragment were analysed in detail by the extended transition state–natural orbitals for chemical valence (ETS–NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHC? gold and the NHC? titanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.     

Synthesis and properties of N,N′-dialkylimidazolium bis(nonafluorobutane-1-sulfonyl)imides: a new subfamily of ionic liquids

A series of N,N′-dialkylimidazolium bis(nonafluorobutane-1-sulfonyl)imides was synthesized in high yields by quaternization of imidazole derivatives with various readily available alkylating reagents, followed by anion exchange with highly stable and non-hygroscopic potassium bis(nonafluorobutane-1-sulfonyl)imide. The latter was obtained by an improved method starting from ammonium chloride and nonafluorobutane-1-sulfonyl fluoride. The quaternary imidazolium salts thus obtained constitute a new subfamily of thermally stable and remarkably hydrophobic ionic liquids with melting points in the range 0-40 °C and solubilities in water and organic solvents (aromatic hydrocarbons, dialkyl ethers) in the range of 0.5-1.5 wt%. The ionic liquids can be easily purified from ionic byproducts (e.g., halogenide salts) by aqueous extraction followed by thorough drying in a high vacuum without loss of yield. Due to the above features, these new ionic fluids may be considered as promising recyclable media in repeated catalytic processes.     

Solvent-controlled selective synthesis of a trans-configured benzimidazoline-2-ylidene palladium(II) complex and investigations of its Heck-type catalytic activity

Reaction of N,N′-dimethylbenzimidazolyl iodide (A) with Pd(OAc)₂ in DMSO gives selectively trans-bis(N,N′-dimethylbenzimidazoline-2-ylidene) palladium(II) diiodide (trans-2) in 77% yield. The selective formation of the trans-coordination isomer and thus the cis-trans rearrangement is driven by the insolubility of trans-2 in DMSO. X-ray single-crystal diffraction analysis and ¹³C NMR spectroscopy confirm the trans-geometry of the square planar Pd(II) complex. Catalytic studies show that cis-1 and trans-2 are highly efficient in the Mizoroki-Heck coupling reaction of aryl bromides and activated aryl chlorides both in DMF and [N(n-C₄H₉)₄]Br as ionic liquid. The catalytic activities of Pd(II) complexes with N-heterocyclic carbene ligands derived from benzimidazole are comparable to their imidazole-derived analogues.     

Temperature dependence of collective states in the random-phase approximation

We investigate the temperature dependence of collective states in the framework of the random-phase approximation at finite temperature. We show that sum rules can be extended to collective energies at finite temperature. Numerical methods are developed to solve the RPA equations at finite temperature. Results are presented and discussed in the case of ⁴⁰Ca for isovector dipole and isoscalar octupole vibrations, using oscillator wave functions and a zero-range force. We show that the broadening of giant dipole resonances observed experimentally, appears as a natural consequence of the structure of the RPA equations. Comparison is made with the schematic model for which the temperature dependence of collective states can be worked out analytically.     

Microscopic structure of Er-related optically active centers in crystalline silicon

A successful observation and analysis of the Zeeman effect on the lambda approximately 1.54 microm photoluminescence band in Er-doped crystalline MBE-grown silicon are presented. The symmetry of the dominant optically active centers is conclusively established as orthorhombic I(C(2v)) with g axially approximately 18.39 and g radially approximately 0. In this way the long standing puzzle as regards the paramagnetism of optically active Er-related centers in silicon is settled. Preferential generation of a single type of an optically active Er-related center confirmed in this study is essential for photonic applications of Si:Er.     

SCF CI calculations of the K-shell ionization potential of carbon in methane and in the fluoromethanes

The vertical ionization potentials of the K-shell of carbon have been calculated in CH₄, CH₃F, CH₂F₂, CHF₃ and CF₄. The ab initio SCF CI approach has been used for the first three compounds with an attempt to rationalize the choice of the atomic orbital basis sets. The ionization potentials of CHF₃ and CF₄ have been extrapolated from the proceeding results. The effect of the CI cannot be neglected in either of these compounds, but it can in CH₄.     

Optimum coasting flight in a horizontal plane

Busemann's concept of optimum altitude for rectilinear coasting flight has been extended to flight in a horizontal plane. The reachable domain of a vehicle in coasting flight in a horizontal plane, starting from an initial velocity, is obtained. It is shown that the area covered first increases with the altitude and then decreases to zero when the ceiling is reached. In particular, there exists an altitude for maximum longitudinal range and another altitude for maximum lateral range. Optimum variations in the lift coefficient and the bank angle to reach the boundary of the footprint are discussed.     

Intermediate-thrust arcs and their optimality in a central,time-invariant force field

This paper presents the general equations of the intermediate-thrust arcs in a general, time-invariant, central force field. Two families of planar arcs, namely, the family of Lawden's spirals in the equatorial plane of an oblate planet and the family of intermediate-thrust arcs in a gravitational field of the form /r ⁿ , have been considered in detail. The Kelley-Contensou condition has been used to test their optimality condition. It is shown that, in the first case, there exist portions of the arcs at a finite distance satisfying the condition, while, in the second case, the entire family satisfies the condition forn 3. Hence, in a perturbed Newtonian gravitational force field, the intermediate-thrust arcs, under certain favorable conditions, can be part of an optimal trajectory.     

Analytical solutions of a class of optimum orbit modifications

This paper presents the complete analytical solution of several fundamental problems in orbital correction. The initial orbit is represented by a given point in the phase space, while the final orbit is constrained to stay in a given curve which can be bounded, unbounded, or composed of a finite number of segments of different curves. The inclusion of atmospheric maneuver as part of the optimum process is discussed; its analytical treatment can be carried out by modifying the final state to include the set of orbits having their perigee at the boundary of the atmosphere.The selection of the apogee and perigee distances as state variables gives a symmetric form to the problem and results in a linear differential equation of the first order for the ratio of the adjoint variables. The introduction of a curve of comparison, called the separatrix, facilitates the discussion of the existence of a corner on an optimal trajectory.This work was supported by NASA Contract No. NASr 54(06).     

Ultrasmall reduced graphene oxide with high near-infrared absorbance for photothermal therapy

We developed nanosized, reduced graphene oxide (nano-rGO) sheets with high near-infrared (NIR) light absorbance and biocompatibility for potential photothermal therapy. The single-layered nano-rGO sheets were ～20 nm in average lateral dimension, functionalized noncovalently by amphiphilic PEGylated polymer chains to render stability in biological solutions and exhibited 6-fold higher NIR absorption than nonreduced, covalently PEGylated nano-GO. Attaching a targeting peptide bearing the Arg-Gly-Asp (RGD) motif to nano-rGO afforded selective cellular uptake in U87MG cancer cells and highly effective photoablation of cells in vitro. In the absence of any NIR irradiation, nano-rGO exhibited little toxicity in vitro at concentrations well above the doses needed for photothermal heating. This work established nano-rGO as a novel photothermal agent due to its small size, high photothermal efficiency, and low cost as compared to other NIR photothermal agents including gold nanomaterials and carbon nanotubes.