Narrow Results

The clean Si(111)(7×7) surface has been exposed to ethylene (C₂H₄) doses, up to 7000 L (1 L=10^-6Torr×1 s) at most, under ultrahigh vacuum. The structural and electronic property changes of the surface have been studied by low energy electron diffraction (LEED), Auger electron spectrometry (AES) and photoemission yield spectroscopy (PYS). The interaction presents two types of kinetic adsorption, where the first is produced below 3000 L of C₂H₄. In the first step, the ethylene molecule is adsorbed molecularly and the initial sticking coefficient S₀ is very low (S₀≈2×10^-3). At the saturation (≈ 5000–6000 L), the valence band is fitted by a power law (E — 4.42)^2.5eV.^1,2 The surface is then a stronger scattering for photoemitted electrons. We attribute this result, produced at the second step of adsorption, to the C₂H₄-π orbital and hydrogen liberated by this molecule, which break the Si–Si surface bonds.

The high-resolution Fourier transform infrared absorption spectrum of the ν₂ + ν₁₂ combination band of normal ethylene (¹²C₂H₄) in the 3050–3105 cm^-1 region was recorded at a resolution of 0.0063 cm^-1 and at an ambient temperature of 296 K. Upper state rovibrational analysis was carried out using a standard Watson's Hamiltonian in asymmetric reduction in I^r representation. The band center, rotational constants and centrifugal distortion constants up to quartic terms of the upper ν₂ + ν₁₂ = 1 state were determined from the final fit that included 102 infrared transitions. The root-mean-square deviation of the fit was 0.000729 cm^-1.

A detailed computational study is performed on the radical-molecule reactions between HCO/HOC and ethylene (C₂H₄) at the Gaussian-3//B3LYP/6-31G(d) level. For the HCO + C₂H₄ reaction, the most favorable pathway is the direct C-addition forming the intermediate H₂CCH₂CHO, followed by a 1,2-H-shift leading to H₃CCHCHO. Subsequently, there are two highly competitive dissociation pathways for H₃CCHCHO: one is the formation of the direct H-extrusion product H₂CCHCHO + H, and the other is the formation of C₂H₅ + CO via the intermediate H₃CCH₂CO. The overall reaction barrier is 14.1 and 14.6 kcal/mol respectively, at the G3B3 level. The quasi-direct H-donation process to produce C₂H₅ + CO with the barrier 16.5 kcal/mol is less competitive. Thus, only at higher temperatures, the HCO + C₂H₄ reaction could play a role. In contrast, the HOC + C₂H₄ reaction just need to overcome a small barrier 2.0 kcal/mol to generate C₂H₅ + CO via the quasi-direct H-donation mechanism. This is suggestive of the potential importance of the HOC + C₂H₄ reaction in combustion processes. However, the direct C-addition channel is much less competitive. The present kinetic data and orbital analysis show that the HCO radical has much higher reactivity than HOC, although the latter is more energetic. Till now, no kinetic study on the HOC radical has been reported, the present study can provide useful information on understanding the reactivity and depletion mechanism of the energetic HOC radical.

The mechanistic study on the insertion of ethylene into the Zr–H bond of Cp₂ZrH₂ to give Cp₂Zr(H)(CH₂CH₃) is performed with the aid of density functional theory calculations. Two possible insertion pathways are proposed. One is the side-insertion and the other is the central-insertion. The rate-determining step is the ethylene insertion in the former path and formation of the adduct in the latter path. Our results of calculations predicted that the central-insertion path is preferred kinetically over the side-insertion path. Two types of Zr⋯H–C agostic interactions having different stabilities are described.

The addition reaction of germylenoid H₂GeFMgF with ethylene (C₂H₄) was first investigated in this work. All of the stationary points were optimized using the M062X method in conjunction with the 6-311$+$G (d, p) basis set and the corresponding energies were calculated using the QCISD method with the 6-311$++$G (d, p) basis set. The calculated results demonstrated that there are two different channels in the addition reaction of H₂GeFMgF with C₂H₄, while only the channel I is feasible once the C₂H₄ approaches H₂GeFMgF in the proper conditions. The solvent effect upon the addition reaction was also investigated using PCM model. The results demonstrated that the tetrahydrofuran (THF) solvent could accelerate the addition reaction.

Three ansa-metallocenes (Me₂C)(Me₂Si)Cp₂TiCl₂(1), [(CH₂)₅C](Me₂Si)Cp₂TiCl₂(2) and (Me₂C)(Me₂Si)Cp₂ZrCl₂(3) with larger dihedral angles and longer distance from metal to the center of Cp planes were synthesized and used as catalysts for ethylene polymerization in the presence of methylaluminoxane (MAO). In the case of ethylene polymerization, compared the feature structures of unbridged metallocenes, singly bridged metallocenes and doubly bridged metallocenes 1, 2, 3, there exhibit the relationship between structural characteristics and the catalytic properties with behavior of polymers. And the view that the dimensions of the short dimethylsilylene and isopropylidene bridges of metallocene 3 do not match the co-ordination requirement of the larger zirconium atom can also explain the result for ethylene polymerization. Catalyzed by complexes 1 and 2 respectively, the copolymerizations of ethylene with 1-hexene, 1-octene, especially the long chain α-olefins (1-tetradecene, 1-hexadecene) were investigated, and showed high incorporations and comonomer effects in almost all the examples. The melting points of the copolymers are approximately 4%–31% below those of the homopolymers, and the enthalpies of fusion decrease greatly due to the lower crystallinity. The microstructures of the copolymer were analyzed with ¹³C-NMR to determine the comonomer sequence distributions and number–average sequence lengths. Comparison between results for the complexes 1 and 2 with different bridging groups respectively showed that the nature of the bridge has a significant effect on both the incorporation and sequence distribution of the α-olefin. Because of the bridges in ansa-metallocene complexes, the copolymerization behavior was different in using catalyst 1 and catalyst 2.

Precise, efficient copolymerizations of ethylene with cyclic olefins [norbornene (NBE), cyclopentene (CPE)] using nonbridged half-titanocenes of type, Cp'TiCl₂(L) (Cp' = cyclopentadienyl group, L = aryloxo, ketimide)-MAO catalyst systems have been summarized. CpTiCl₂(N = C^tBu₂) exhibited both remarkable catalytic activity and efficient NBE incorporation for ethylene/NBE copolymerization: The NBE incorporation by Cp'TiCl₂(X)X = N = C^tBu₂(X)(X = N = C^tBu₂, O-2,6-ⁱPr₂C₆H₃; Cp'=Cp, C₅Me₅, indenyl was related to the calculated coordination energy after ethylene insertion. (^tBuC₅H₄)TiCl₂(N=C^tBu) exhibited significant catalytic activities with efficient CPE incorporations in ethylene/CPE copolymerization in the presence of MAO. The polymerization proceeded with exclusive 1,2-CPE incorporation, affording high molecular weight (alternating) copolymers with uniform distributions. Therefore, the cyclopentadienyl fragment (Cp') plays an essential role in terms of both the activity and the efficient cyclic olefin incorporation.

Emulsion copolymerization of styrene and ethylene catalyzed by a series of neutral nickel(II) complexes was carried out in an aqueous system to give high-molecular-weight copolymers. The copolymers and emulsions were characterized by an array of techniques including NMR, GPC, TEM, WAXD and DSC. The results indicate that the copolymers obtained are mostly block copolymers of polyethylene with random insertion of styrene units, and their M_w is in the range of 10⁵–10⁶. By enhancing the electron withdrawing of the substituents on the phenoxy ring, the ethylene contents in the copolymers varied from 0 to 52%. The molecular weight distribution of the copolymers was bimodal. With reducing the steric effect of substituents on the aniline ring, the solid contents of the emulsions decreased. The polydispersity of the copolymer varied from 11.1 to 2.8, and the ethylene content in the copolymer was below 10%. The homopolymer of styrene was obtained when the substitutent on the aniline ring was hydrogen.

Bis(imino)pyridyl Fe(II) complexes are important catalysts in ethylene oligomerization for preparing α-olefins. The metal net charge-activity relationship of bis(imino)pyridyl Fe(II) complexes was investigated by molecular mechanics (MM) and net charge equilibration (QEq) method with modified Dreiding force field. It was found that metal net charge was in reverse ratio to ethylene oligomerization activity. Electron-donor substituents with less steric hindrance to the central metal were favorable to Fe complex activity. Metal net charge-activity relationship could be used to assist the design of new Fe oligomerization catalysts with higher activity.

Various catalysts have been used for achieving the CO₂ reduction reaction (CO₂RR), among which Cu₂O has received widespread attention due to its high activity and selectivity. In this work, we focus on the morphology control of Cu₂O nanoboxes (NBs) regulated by poly (vinyl pyrrolidone) and graphene oxide and report the synthesis of a hollow structure by facet-controlled etching. Cu₂O NBs etched for 8 days exhibited enhanced CO₂RR selectivity and activity, reaching a superior C₂H₄ Faraday efficiency of $\sim$35.4% at −1.2 V (versus Ag/AgCl), which was much higher than that of unetched catalysts. The improved performance was attributed to the synergistic effect of various crystal facets exposed on the NB surface. The hollow NBs showed relatively high structural stability, demonstrating the potential for practical applications. This study introduces an efficient approach to boost the catalyst properties by specifically constructing the structure and controlling the morphology.

Absolute cross-sections for dehydrogenation of an ethylene molecule onto , Ti_nO⁺ (n = 2−25), , and were measured as a function of the cluster size, n, in a gas-beam geometry at a collision energy of 0.4 eV in an apparatus equipped with a tandem-type mass spectrometer. It is found that (i) the di-dehydrogenation proceeds on and Ti_nO⁺, and the mono-dehydrogenation on , and , (ii) the di- dehydrogenation cross-section increases gradually with the cluster size of and Ti_nO⁺, (iii)the mono-dehydrogenation cross-section increases rapidly above a cluster size of ∼ 18 for , ∼13 and ∼ 18 for , and ∼ 10 for , and (iv) therapid increase of the cross-section for (M = Fe, Co and Ni) occurs ata cluster size where the 3d-electrons start to contribute to the highest occupied levels of . These findings lead us to conclude that the 3d-electrons of lay a central role in the dehydrogenation on .