Mechanical properties and fracture behaviors of C/C composites with PyC/TaC/PyC,PyC/SiC/TaC/PyC multi-interlayers

Carbon/carbon (C/C) composites with PyC/TaC/PyC or PyC/SiC/TaC/PyC multi-interlayers were prepared by isothermal chemical vapor infiltration, followed by Furan resin impregnation and carbonization. Microstructures, mechanical properties including flexural strength, ductile displacement, and fracture behaviors of composites were studied. Furthermore, composites were heat treated at 2000 °C to study the effects of heat treatment on mechanical properties and fracture behaviors. PyC/TaC/PyC and PyC/SiC/TaC/PyC multi-interlayers have been deposited uniformly in C/C composites. With the introduction of PyC/TaC/PyC multi-interlayers in C/C composites, the flexural strength decreases; however, the ductile displacement increases. The fracture behavior changes from brittleness (0% TaC) to pseudo-ductility (5% TaC) and high toughness (10% TaC). When PyC/SiC/TaC/PyC multi-interlayers are introduced in C/C composites, the flexural strength is improved remarkably from 270 MPa to 522 MPa, but the ductile displacement decreases obviously from 0.49 mm to 0.24 mm, and the fracture behavior becomes brittle again. After heat treatment at 2000 °C, the flexural strength decreases, but the ductile displacement increases and pseudo-ductility or high toughness can be obtained.     

New photophysical insights in noncovalent interaction between fulleropyrrolidine and a series of zincphthalocyanines

The present article reports, for the first time, the photophysical aspects of noncovalent interaction of a fullerene derivative, namely, C(60) pyrrolidine tris-acid ethyl ester (PyC(60)) with a series of zincphthalocyanines, for example, underivatized zincphthalocyanine (1), zinc-1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (2), and zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (3) in toluene. Ground state electronic interaction of PyC(60) with 1, 2 and 3 has been evidenced from the observation of well-defined charge transfer (CT) absorption bands in the visible region. Utilizing the CT transition energy, vertical electron affinity (E(A)(v)) of PyC(60) is determined. Steady state fluorescence experiment enables us to determine the value of binding constant (K) in the magnitude of 2.60 × 10(4) dm(3)·mol(-1), 2.20 × 10(4) dm(3)·mol(-1), and 1.27 × 10(4) dm(3)·mol(-1) for the noncovalent complexes of PyC(60) with 1, 2, and 3, respectively. K values of PyC(60)-ZnPc complexes suggest that PyC(60) is incapable of discriminating between 1, 2, and 3 in solution. Lifetime experiment signifies the importance of static quenching phenomenon for our presently investigated supramolecules and it yields larger magnitude of charge separated rate constant for the PyC(60)-1 species in toluene. Photoinduced energy transfer between PyC(60) and ZnPc derivatives, namely, 1, 2, and 3, in toluene, has been evidenced with nanosecond laser photolysis method by observing the transient absorption bands in the visible region; transient absorption studies establish that energy transfer from (T)PyC(60)* to the ZnPc occurs predominantly, as confirmed by the consecutive appearance of the triplet states of PyC(60). Theoretical calculations at semiempirical level (PM3) evoke the single projection geometric structures for the PyC(60)-ZnPc systems in vacuo, which also proves that interaction between PyC(60) and ZnPc is governed by the electrostatic mechanism rather than dispersive forces associated with π-π interaction.     

Disorder-order phase change of omega-(N-pyrrolyl)alkanethiol self-assembled monolayers on gold induced by STM scans and thermal activation

Molecular ordering of pyrrolyl-terminated alkanethiol self-assembled monolayers (PyC(n)SH SAMs) on Au(111) substrates (n = 11 or 12) was investigated by scanning tunneling microscopy (STM) and various spectroscopic methods. The SAMs, which were in a disordered state when formed at room temperature, could be ordered either globally by thermal annealing at 70 degrees C, or locally via stimulation with repetitive STM scans. The ordered phase was characterized by small domains of molecular rows formed along 112[combining macron] directional set with an inter-row corrugation period close to 1.44 nm, in which defects were abundant. Based on the experimental results, the molecular arrangement in the ordered PyC(n)SH SAM was proposed to be a (5x radical3)rect structure with a molecular deficiency >or=10%. While mechanical interactions between molecules and scanning probe tips had been pointed out as the major cause of scan-induced phase transformations in other SAM systems, electronic or electrostatic factors were thought to affect considerably the scan-induced ordering process in this SAM system. From comparison of surface molecular coverage between disordered and thermally ordered SAMs of PyC(12)SH, it was inferred that the disorder could be ascribed to both kinetic and thermodynamic factors. The kinetic barrier to the ordered phase was supposed to result from strong dipole-dipole interactions among the pyrrolyl endgroups.     

Organic mediator-induced structural transformation in superlattices of monolayer-protected gold nanoparticles

N-acetylglutathione (NAG)-protected gold nanoparticles self-assemble into three-dimensional (3D) face-centered cubic (fcc)-type superlattices at an air/water interface under highly acidic conditions. To prepare the well-defined superlattices, 1month's incubation is at least necessary since the size growth of the as-prepared nanoparticles is essential. Addition of 4-pyridinecarboxyic acid (PyC), a bifunctional hydrogen-bonding mediator, promotes the formation of the superlattices, which are created for about 2weeks' storage. Interestingly, PyC-induced nanoparticle superlattices are in a body-centered tetragonal (bct) structure. The fcc-to-bct phase transformation would be due to stronger interaction between NAG and PyC than that between NAG molecules on the gold nanoparticle surfaces.     

Enhanced Electron Transfer Ability via Coordination in Block Copolymer/Porphyrin/Fullerene Micelle

Inspired by structures of antenna-reaction centers in photosynthesis,the complex micelle was prepared from zinc tetra-phenyl porphyrin (ZnTPP),fullerene derivative (PyC60) and poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-bPCL).The core-shell structure made the hydrophobic donor-acceptor system work in aqueous.In micellar core,coordination interaction occurred between ZnTPP and PyC60 molecules which ensured the enhanced energy migration from the donor to the acceptor.The enhanced interaction between porphyrin and fullerene was confirmed by absorption,steady-state fluorescence and transient fluorescence.The generation of singlet oxygen and superoxide radical was detected by iodide method and reduction of nitro blue tetrazolium,respectively,which confirmed that electron transfer reaction in the complex micellar core occurred.Moreover,the complex micelle exhibited effective electron transfer performance in photodebromination of 2,3-dibromo-3-phenylpropionic acid.The complex micellar structure endowed the donor-acceptor system with improved stability under irradiation.This strategy could be helpful for designing new electron transfer platform and artificial photosynthetic system.     

Influence of the apical ligand in the thermotropic mesomorphism of cationic copper-based surfactants

A new pyridine-based bidentate ligand L (PyC18) was used to develop copper-containing surfactants that exhibit mesomorphism. Complexes [(L (PyC18)) 2Cu (II)Y]Y were synthesized, where Y is an anionic ligand bromo ( 1), nitrato ( 2), or perchlorato ( 3). The nature of these apical ligands determines the mesogenic behavior of 1- 3: The smallest bromo-substituted species 1 shows a metastable liquid crystalline phase at 110 degrees C, the nitrato-substituted 2 increases the transition temperature to 136 degrees C, and the bulky perchlorato-substituted 3 shows reversible mesophases at 153 degrees C. The behavior of these complexes shows similarities and suggests that at low temperatures the crystals of these compounds are bilayered structures with interdigitated alkyl tails. At higher temperatures the tails undergo rapid conformational changes that force these layers to swell until the opposing alkyl chains are separated from each other, and the mesophase is a monolayer smectic A. Small changes in the geometry of cationic mesogens can be imposed by the presence of apically coordinated anions, allowing for tuning in the properties of the resulting mesophases.     

菌紫质分子沉积膜的制备

菌紫质分子沉积膜的制备李正强，王力彦，张希，李伯符，沈家骢（吉林大学分子光谱与分子结构开放实验室，长春，１３００２３）胡坤生，王敖金（中国科学院北京生物物理研究所，北京）关键词菌紫质，分子沉积膜，功能结构细菌视紫红质（Ｂａｃｔｅｒｉｏｒｈｏｄｏｐｓｉ...     

3D coordination networks based on supramolecular chains as building units: synthesis and crystal structures of two silver(I) pyridyldiethynides

Two silver(I) pyridyldiethynides, [Ag2(3,5-C2PyC2).4CF3CO2Ag.4H2O] ( A) and [Ag 2(3,5-C2PyC2).3AgNO3.H2O](B), were synthesized by reactions of 3,5-diethynylpyridine with silver trifluoroacetate and silver nitrate in high yield, respectively. X-ray crystallographic studies revealed that in A pyridyldiethynide groups connect Ag 11 cluster units to generate 1D supramolecular chains as bridging ligands, where each ethynide group interacts with four silver atoms. These supramolecular chains bearing pyridyl groups are linked by silver ions to form wavelike layers, which are further connected by trifluoroacetate ligands to afford a 3D coordination network. However, B exhibits a different structural feature, where two ethynide groups in one pyridyldiethynide ligand coordinate to three and four silver atoms, respectively. These silver ethynide cluster units are linked through silver-ethynide and argentophilic interactions, leading to a double silver chain by sharing silver atoms in these units. In B, the silver double chains are further connected by bridging pyridyldiethynide groups to generate 2D networks, which interact through the Ag-N coordination bonds between silver atoms and pyridyl groups in the adjacent layers to generate a 3D coordination network. In these two compounds, trifluoroacetate and nitrate groups exhibit different bonding modes, indicating that the counterion is an important factor influencing the structures of supramolecular chains and coordination networks.     

Interparticle spacing control in the superlattices of carboxylic acid-capped gold nanoparticles by hydrogen-bonding mediation

We have demonstrated that carboxylic acid-capped gold nanoparticles were self-assembled to form two-dimensional (2D) and/or three-dimensional (3D) superlattices at an air/water interface in the presence of a bifunctional hydrogen-bonding mediator such as 4-pyridinecarboxylic acid (PyC) or trans-3-(3-pyridyl)acrylic acid (PyA). Transmission electron microscopy revealed a hexagonal close-packed arrangement of nanoparticles in the superlattice with an extension of interparticle spacing. In the 2D superlattices, larger particles produced a higher-quality assembly having long-range translational ordering. Attenuated total reflectance IR (ATR-IR) spectroscopy revealed the presence of hydrogen bonds between the mediator used and the capping agents of carboxylic acid on nanoparticle surfaces. Since the experimentally obtained interparticle separation distance agreed approximately with that obtained by the geometrical model calculations, we conclude that the hydrogen-bonding mediation controlled the interparticle spacing or structure by monomolecular incorporation between adjacent nanoparticles in the superlattices.     

Are Organotin Reagents Derived from Bis(trimethylsilyl)picoline Suitable Precursors for the Preparation of Cyclometallated Complexes?

The organotin reagents [2‐PyC(SiMe₃)₂SnR₃] (R = Me, ⁿBu) were prepared in good yields from the reaction between the lithium salt of 2‐bis(trimethylsilyl)picoline and the corresponding trialkyltin chlorides. Reactions of these organotin reagents were carried out with various Pd and Pt complexes including [MCl₂(cod)] and [MCl₂(PhCN)₂] (M = Pd, Pt). The results show that a Me group is transferred to the metal atom, rather than the 2‐bis(trimethylsilyl)picolyl group. The mechanism for this reaction is discussed and reasons why Me group transfer occurs based on DFT computed structural data are given.     

Thin film pyrolytic carbon electrodes: A new class of carbon electrode for electroanalytical sensing applications

This communication describes the electrochemical properties of thin pyrolytic carbon (PyC) films created using a reliable, non-catalytic chemical vapour deposition (CVD) process. After deposition, the electron transfer characteristics of the films are optimised using a simple oxygen plasma treatment. The redox probes Ru(NH₃)₆^3+/2+, Fe(CN)₆^3?/4? and Fe^3+/2+ are employed to demonstrate that the resulting material is endowed with a large electrochemical surface area and outstanding electron transfer properties. Atomic force microscopy (AFM), Raman and X-ray photoelectron spectroscopy (XPS) are used to elucidate the morphology and chemical composition of the electrode surfaces. This material represents a new class of carbon electrode, and its large densities of edge-plane sites and oxygenated functionalities make it an ideal candidate for electrochemical sensor applications.     

Half‐sandwich ruthenium,rhodium and iridium complexes featuring oxime ligands: Structural studies and preliminary investigation of in vitro and in vivo anti‐tumour activities

Half‐sandwich ruthenium, rhodium and iridium complexes ( 1 – 12 ) were synthesized with aldoxime ( L1 ), ketoxime ( L2 ) and amidoxime ( L3 ) ligands. Ligands have the general formula [PyC(R)NOH], where R = H ( L1 ), R = CH₃ ( L2 ) and R = NH₂ ( L3 ). Reaction of [{(arene)MCl₂}₂] (arene = p ‐cymene, benzene, Cp*; M = Ru, Rh, Ir) with ligands L1 – L3 in 1:2 metal precursor‐to‐ligand ratio yielded complexes such as [{(arene)MLκ²_(N∩N)Cl}]PF₆. All the ligands act as bidentate chelating nitrogen donors in κ²_(N∩N) fashion while forming complexes. In vitro anti‐tumour activity of complexes 2 and 10 against HT‐29 (human colorectal cancer), BE (human colorectal cancer) and MIA PaCa‐2 (human pancreatic cancer) cell lines and non‐cancer cell line ARPE‐19 (human retinal epithelial cells) revealed a comparable activity although complex 2 demonstrated greater selectivity for MIA PaCa‐2 cells than cisplatin. Further studies demonstrated that complexes 3 , 6 , 9 and 12 induced significant apoptosis in Dalton's ascites lymphoma (DL) cells. In vivo anti‐tumour activity of complex 2 on DL‐bearing mice revealed a statistically significant anti‐tumour activity (P  = 0.0052). Complexes 1 – 12 exhibit HOMO–LUMO energy gaps from 3.31 to 3.68 eV. Time‐dependent density functional theory calculations explain the nature of electronic transitions and were in good agreement with experiments.     

Synthesis, structure, chemical stability, and electrical properties of Nb-, Zr-, and Nb-codoped BaCeO3 perovskites

We report the effect of donor-doped perovskite-type BaCeO(3) on the chemical stability in CO(2) and boiling H(2)O and electrical transport properties in various gas atmospheres that include ambient air, N(2), H(2), and wet and dry H(2). Formation of perovskite-like BaCe(1-x)Nb(x)O(3±δ) and BaCe(0.9-x)Zr(x)Nb(0.1)O(3±δ) (x = 0.1; 0.2) was confirmed using powder X-ray diffraction (XRD) and electron diffraction (ED). The lattice constant was found to decrease with increasing Nb in BaCe(1-x)Nb(x)O(3±δ), which is consistent with Shannon's ionic radius trend. Like BaCeO(3), BaCe(1-x)Nb(x)O(3±δ) was found to be chemically unstable in 50% CO(2) at 700 °C, while Zr doping for Ce improves the structural stability of BaCe(1-x)Nb(x)O(3±δ). AC impedance spectroscopy was used to estimate electrical conductivity, and it was found to vary with the atmospheric conditions and showed mixed ionic and electronic conduction in H(2)-containing atmosphere. Arrhenius-like behavior was observed for BaCe(0.9-x)Zr(x)Nb(0.1)O(3±δ) at 400-700 °C, while Zr-free BaCe(1-x)Nb(x)O(3±δ) exhibits non-Arrhenius behavior at the same temperature range. Among the perovskite-type oxides investigated in the present work, BaCe(0.8)Zr(0.1)Nb(0.1)O(3±δ) showed the highest bulk electrical conductivity of 1.3 × 10(-3) S cm(-1) in wet H(2) at 500 °C, which is comparable to CO(2) and H(2)O unstable high-temperature Y-doped BaCeO(3) proton conductors.     

Intermetallic compounds of the heaviest elements and their homologs: the electronic structure and bonding of MM', where M=Ge, Sn, Pb, and element 114, and M'=Ni, Pd, Pt, Cu, Ag, Au, Sn, Pb, and element 114

Fully relativistic (four-component) density-functional theory calculations were performed for intermetallic dimers MM', where M=Ge, Sn, Pb, and element 114, and MM'=group 10 elements (Ni, Pd, and Pt) and group 11 elements (Cu, Ag, and Au). PbM and 114M, where M are group 14 elements, were also considered. The results have shown that trends in spectroscopic properties-atomization energies D(e), vibrational frequencies omega(e), and bond lengths R(e), as a function of MM', are similar for compounds of Ge, Sn, Pb, and element 114, except for D(e) of PbNi and 114Ni. They were shown to be determined by trends in the energies and space distribution of the valence ns(MM')atomic orbitals (AOs). According to the results, element 114 should form the weakest bonding with Ni and Ag, while the strongest with Pt due to the largest involvement of the 5d(Pt) AOs. In turn, trends in the spectroscopic properties of MM' as a function of M were shown to be determined by the behavior of the np(1/2)(M) AOs. Overall, D(e) of the element 114 dimers are about 1 eV smaller and R(e) are about 0.2 a.u. larger than those of the corresponding Pb compounds. Such a decrease in bonding of the element 114 dimers is caused by the large SO splitting of the 7p orbitals and a decreasing contribution of the relativistically stabilized 7p(1/2)(114) AO. On the basis of the calculated D(e) for the dimers, adsorption enthalpies of element 114 on the corresponding metal surfaces were estimated: They were shown to be about 100-150 kJ/mol smaller than those of Pb.     

Cubane, cuneane, and their carboxylates: a calorimetric, crystallographic, calculational, and conceptual coinvestigation

[reaction: see text] This study is a multinational, multidisciplinary contribution to the thermochemistry of dimethyl1,4-cubanedicarboxylate and the corresponding isomeric, cuneane derivative and provides both structural and thermochemical information regarding the rearrangement of dimethyl 1,4-cubanedicarboxylate to dimethyl 2,6-cuneanedicarboxylate. The enthalpies of formation in the condensed phase at T = 298.15 K of dimethyl 1,4-cubanedicarboxylate (dimethyl pentacyclo[4.2.0.0.(2,5)0.(3,8)0(4,7)]octane-1,4-dicarboxylate) and dimethyl 2,6-cuneanedicarboxylate (dimethyl pentacyclo[3.3.0.0.(2,4)0.(3,7)0(6,8)]octane-2,6-dicarboxylate) have been determined by combustion calorimetry, delta(f) H(o)m (cr)/kJ x mol(-1) = -232.62 +/- 5.84 and -413.02 +/- 5.16, respectively. The enthalpies of sublimation have been evaluated by combining vaporization enthalpies evaluated by correlation-gas chromatography and fusion enthalpies measured by differential scanning calorimetry and adjusted to T = 298.15 K, delta(cr) (g)Hm (298.15 K)/kJ x mol(-1) = 117.2 +/- 3.9 and 106.8 +/- 3.0, respectively. Combination of these two enthalpies resulted in delta(f) H(o)m (g., 298.15 K)/kJ x mol(-1) of -115.4 +/- 7.0 for dimethyl 1,4-cubanedicarboxylate and -306.2 +/- 6.0 for dimethyl 2,6-cuneanedicarboxylate. These measurements, accompanied by quantum chemical calculations, resulted in values of delta(f) Hm (g, 298.15 K) = 613.0 +/- 9.5 kJ x mol(-1) for cubane and 436.4 +/- 8.8 kJ x mol(-1) for cuneane. From these enthalpies of formation, strain enthalpies of 681.0 +/- 9.8 and 504.4 +/- 9.1 kJ x mol(-1) were calculated for cubane and cuneane by means of isodesmic reactions, respectively. Crystals of dimethyl 2,6-cuneanedicarboxylate are disordered; the substitution pattern and structure have been confirmed by determination of the X-ray crystal structure of the corresponding diacid.     

Synthetic, spectroscopic, and structural studies on organoimido molybdenum, tungsten, and rhenium phthalocyanines

Unprecedented imido phthalocyaninato complexes of pentavalent refractory metals [PcM(NR)Cl] (M = Mo, W, Re; R = tBu: 1, 3, 6, Mes: 2, 4, 7 or Ts: 5) have been synthesized by reductive cyclotetramerization of phthalonitrile in the presence of appropriate bis(imido) complexes of Mo, W and Re as templates. While d(1) Mo(V) and W(V) species 1-5 show distinctive EPR spectra corresponding to metal centered radicals with hyperfine coupling of two magnetically non-equivalent nitrogen atoms (4 equatorial and 1 axial N), corresponding d(2) Re(V) compounds 6 and 7 are diamagnetic. [PcMo(NtBu)Cl] 1 crystallizes from 1-chloronaphthalene in the tetragonal space group P4/n. The molecular structure reveals, that the metal center is located above the plane of the equatorial N4 and displaced towards the axial π-donor ligand. Due to the thermodynamic trans effect the Mo-Cl bond trans to the imido group is elongated to about 2.600(2) ?.     

Syntheses, Crystal and Band Structures, and Magnetic and Optical Properties of New CsLnCdTe(3) (Ln = La, Pr, Nd, Sm, Gd-Tm, and Lu)

A series of new quaternary semiconductor materials CsLnCdTe(3) (Ln = La, Pr, Nd, Sm, Gd-Tm, and Lu) was obtained from high-temperature solid-state reactions by the reactive halide flux method. These compounds belong to the layered KZrCuS(3) structure type and crystallize in the orthorhombic space group Cmcm (No. 63). Their structure features two-dimensional infinity(2)[LnCdTe(3)-] layers of edge- and vertex-sharing LnTe(6) octahedra with Cd atoms filling the tetrahedral interstices, which stack along b-axis. The Cs atoms are located between the infinity(2)[LnCdTe(3)-] layers and are surrounded by eight Te atoms to form a CsTe(8) bicapped trigonal prism. Such Te layers are more flexible than the Se analogues in the isostructural CsLnMSe(3) to accommodate nearly the entire Ln series. Theoretical studies performed on CsTmCdTe(3) show that the material is a direct band gap semiconductor and agrees with the result from a single-crystal optical absorption measurement. Magnetic susceptibility measurements show that the CsLnCdTe(3) (Ln = Pr, Nd, Gd, Dy, Tm) materials exhibit temperature-dependent paramagnetism and obey the Curie-Weiss law, whereas CsSmCdTe(3) does not.     

Thermodynamic, kinetic, and computational study of heavier chalcogen (S, Se, and Te) terminal multiple bonds to molybdenum, carbon, and phosphorus

Enthalpies of chalcogen atom transfer to Mo(N[t-Bu]Ar)3, where Ar = 3,5-C6H3Me2, and to IPr (defined as bis-(2,6-isopropylphenyl)imidazol-2-ylidene) have been measured by solution calorimetry leading to bond energy estimates (kcal/mol) for EMo(N[t-Bu]Ar)3 (E = S, 115; Se, 87; Te, 64) and EIPr (E = S, 102; Se, 77; Te, 53). The enthalpy of S-atom transfer to PMo(N[ t-Bu]Ar) 3 generating SPMo(N[t-Bu]Ar)3 has been measured, yielding a value of only 78 kcal/mol. The kinetics of combination of Mo(N[t-Bu]Ar)3 with SMo(N[t-Bu]Ar)3 yielding (mu-S)[Mo(N[t-Bu]Ar)3]2 have been studied, and yield activation parameters Delta H (double dagger) = 4.7 +/- 1 kcal/mol and Delta S (double dagger) = -33 +/- 5 eu. Equilibrium studies for the same reaction yielded thermochemical parameters Delta H degrees = -18.6 +/- 3.2 kcal/mol and Delta S degrees = -56.2 +/- 10.5 eu. The large negative entropy of formation of (mu-S)[Mo(N[t-Bu]Ar)3]2 is interpreted in terms of the crowded molecular structure of this complex as revealed by X-ray crystallography. The crystal structure of Te-atom transfer agent TePCy3 is also reported. Quantum chemical calculations were used to make bond energy predictions as well as to probe terminal chalcogen bonding in terms of an energy partitioning analysis.     

Syntheses, characterization, and X-ray crystal structures of beta-diketiminate group 13 hydrides, chlorides, and fluorides

A series of organometallic compounds of group 13 metals supported by the sterically encumbered beta-diketiminate ligand containing hydrides, fluorides, chlorides, and bromide have been synthesized and structurally characterized. The synthetic strategy applied utilizes halide metathesis and reduction of metal chlorides to the corresponding hydrides. Thus, the reaction of LLi.OEt2 with MeMCl2 affords LM(Me)Cl (M = Al (1), Ga (2), In (3)) and LGaBr2 (4) with GaBr3. Reduction of LGa(Me)Cl with LiH.BEt3 leads to the formation of LGa(Me)H (10). Synthesis of LGaH(2) (12) has been accomplished by reacting LGaI2 (8) with LiH.BEt3. LAl(Me)Cl (1) and LAlH2 (6) have been converted to LAl(Me)F (5) and LAlF2 (7), respectively. The former was obtained in a reaction of LAl(Me)Cl with Me3SnF while the latter was isolated in a reaction of LAlH2 with BF3.OEt2. Similarly reaction of LGaI2 (8) with Me3SnF affords LGaF2 (9). Compounds reported herein have been characterized by elemental analyses, IR, NMR, EI-MS, and single-crystal X-ray diffraction techniques.     

Protonated benzofuran,anthracene, naphthalene,benzene, ethene,and ethyne: measurements and estimates of pK(a) and pK(R)

Aqueous solvolyses of acyl derivatives of hydrates (water adducts) of anthracene and benzofuran yield carbocations which undergo competitive deprotonation to form the aromatic molecules and nucleophilic reaction with water to give the aromatic hydrates. Trapping experiments with azide ions yield rate constants k(p) for the deprotonation and k(H2O) for the nucleophilic reaction based on the "azide clock". Combining these with rate constants for (a) the H(+)-catalyzed reaction of the hydrate to form the carbocation and (b) hydrogen isotope exchange of the aromatic molecule (from the literature) yields pK(R) = -6.0 and -9.4 and pK(a) = -13.5 and -16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pK(R) = -6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile-water mixtures, and pK(a) = -20.4 for the 2-protonated naphthalene from combining k(p) with an exchange rate constant. The differences between pK(R) and pK(a) correspond to pK(H2O), the equilibrium constant for hydration of the aromatic molecule (pK(H2O) = pK(R) - pK(a)). For naphthalene and anthracene values of pK(H2O) = +13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pK(a) = -24.3 is derived from an exchange rate constant and an assigned value for the reverse rate constant close to the limit for solvent relaxation. Combining this pK(a) with calculated values of pK(H2O) gives pK(R) = -2.4 and -2.1 for protonated benzenes forming 1,2- and 1,4-hydrates, respectively. Coincidentally, the rate constant for protonation of benzene is similar to those for protonation of ethylene and acetylene (Lucchini, V.; Modena, G. J. Am. Chem Soc. 1990, 112, 6291). Values of pK(a) for the ethyl and vinyl cations (-24.8) may thus be derived in the same way as that for the benzenonium ion. Combining these with appropriate values of pK(H2O) then yields pK(R) = -39.8 and -29.6 for the vinyl and ethyl cations, respectively.