HF Reaction Research Articles

Synthesis and Reactivity of a Sterically Crowded Tris(pyrazolyl)borate Hafnium Benzyl Complex

The reaction of Hf(CH2Ph)4 and Tl[TpMs*] (TpMs* = HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)−) yields TpMs*Hf(CH2Ph)3 (1), bibenzyl, and Tl0 via ligand exchange followed by thermal decomposition of Tl[CH2Ph]. Compound 1 reacts with [H(OEt2)2][B(C6F5)4] to form TpMs*Hf(CH2Ph)2(OEt2)+ (2, 100%) and with [Ph3C][B(C6F5)4] or B(C6F5)3 to form an unstable species characterized by NMR as TpMs*Hf(CH2Ph)2+. 1/[Ph3C][B(C6F5)4] behaves similarly to TpMs*HfCl3/MAO in olefin polymerization, producing high molecular weight, linear PE, and ethylene/1-hexene copolymer with good hexene incorporation. These results are consistent with the proposal that similar cationic TpMs*Hf(X)R+ species (X = Cl or R) are active species in these catalysts. Ethylene polymerization by 1/[Ph3C][B(C6F5)4] at low temperature proceeds without significant chain transfer, enabling the synthesis of bromo benzyl double-end-capped linear PE (PhCH2(CH2CH2)nBr).

Reactions of Hf +, Ta +, and W + with O 2 and CO: Metal carbide and metal oxide cation bond energies

The reactions of Hf +, Ta +, and W + with O 2 and CO are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. All three reactions with O 2 form diatomic metal oxide cations in exothermic reactions that occur at the collision rate. In the CO systems, formation of both diatomic metal oxide and metal carbide cations is observed to be endothermic. The energy-dependent cross sections in the latter systems are interpreted to give 0 K bond energies (in eV) of D 0(HfC +) = 3.19 ± 0.03, D 0(TaC +) = 3.79 ± 0.04, D 0(WC +) = 4.76 ± 0.09, D 0(HfO +) = 6.91 ± 0.11, D 0(TaO +) = 7.10 ± 0.12, and D 0(WO +) = 6.77 ± 0.07. The present experimental values for TaO + and WC + agree well with literature thermochemistry, those for HfO + and WO + refine the available literature bond energies, and those for HfC + and TaC + are the first measurements available. The nature of the bonding in MO + and MC + is discussed and compared for these three metal ions and analyzed using theoretical calculations at a B3LYP/HW+/6-311+G(3df) level of theory. Bond energies for all MO + and MC + species are calculated using geometries calculated at this level and single point energies determined at B3LYP, CCSD, CCSD(T), QCISD, and QCISD(T) levels of theory with the same basis set. Reasonable agreement between the theoretical and experimental bond energies for the three metal oxide and three metal carbide cations is found. Potential energy surfaces for reaction of the metal cations with CO are also calculated at the B3LYP level of theory and reveal additional information about the reaction mechanisms.

2,4,6-Triamino-1,3,5-triazine-1,3-diium aquapentafluoridoaluminate

The title compound, (C3H8N6)[AlF5(H2O)], was obtained by solvothermal synthesis from the reaction of aluminium hydroxide, 1,3,5-triazine-2,4,6-triamine (melamine), aqueous HF and water at 323 K for 48 h. The structure consists of [AlF5(H2O)]2− octa­hedra and diprotonated melaminium cations. Cohesion is ensured by a three-dimensional network of hydrogen bonds.

Correlation of attack and recoil angles for the Li + HF reaction: An exact quantum mechanical study at low and high total angular momentum

The total angular momentum ( J = 1, 10) S matrix elements of the Li + HF reaction have been calculated using a time independent method, and matrix transformed to the stereodirected and Gauss–Legendre discrete variable representations. This paper extends to higher partial waves the study on the stereodynamics of the Li + HF reaction performed at a total angular momentum J = 0. The work is focused on the orbital angular momentum ( L) substates rather than on the projections K on the intermolecular axis and emphasis is given to a space-fixed rather than to a body fixed view of the collision process. A detailed comparison of stereodirected and discrete variable representations is also made.

Transition state structure, energetics, and rate constants for the F ( 2P) + C 2H 6 → C 2H 5 + HF reaction

Geometries, energies, frequencies, and rate constants of the reaction F ( 2P) + C 2H 6 → C 2H 5 + HF were computed using the MP2 and CCSD(T) approaches, correlation-consistent basis sets, and an extrapolation scheme to assess the complete basis set (CBS) limit energies. Values of the enthalpy of reaction at 0 K ( Δ H 0 0 ) calculated with the CCSD(T)/cc-pVQZ method and extrapolated to the CBS limit are equal to −35.2 and −36.7 kcal/mol, respectively (Exp. −35.3 ± 0.5 kcal/mol). Rate constants calculated with the zero-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment.

The special features of rotationally resolved differential cross sections of the F + H2 reaction at small scattering angles

We studied the nature and collision energy dependence of the maximum that appears in the angular distributions of the HF (v′ = 3) product of the F + H2 (v = 0; j = 0, 1, 2) → H + HF (v′, j′) reaction at small scattering angles θ in the center-of-mass frame. This maximum and its increase as the collision energy increased were discovered in the well-known experiment described by D.M. Neumark, A.M. Wodtke, G.N. Robinson, C.C. Hayden, and Y.T. Lee, J. Chem. Phys. 82 (7), 3045 (1985). In order to determine the nature of the maximum, we performed quantum-mechanical simulation of the reaction on the Stark-Werner ground state potential energy surface at collision energies of 1.84, 2.74, and 3.42 kcal/mol corresponding to the above-mentioned experiment and calculated the vibrationally and rotationally resolved differential cross sections dσv′j′/dΩ of the reaction. The maximum under consideration was found to be due to a superposition of two effects, namely, the absence of HF (v′ = 3; j′) products with large j′ because of energy restrictions and an increase in the relative amplitude of quantum-mechanical oscillations on dσv′j′/dΩ cross sections at small j′ and θ as v′ increased. Oscillations on dσ3j′/dΩ cross sections with small j′ are responsible for the maximum observed.

Coherent-states dynamics of the H + + HF reaction at ELab = 30 eV: A complete electron nuclear dynamics investigation

Results of a complete investigation of the H + + HF reaction at E Lab = 30 eV with the electron nuclear dynamics (END) and the coherent-states dynamics (CSD) theories are herein presented. Current END–CSD methodology employs frozen Gaussian wave packet in the semiclassical limit of ℏ → 0 for the nuclei, and a single-determinantal Thouless coherent state (CS) for the electrons. The simulated 400 CS trajectories from five independent HF target orientations provide a complete description of the reactive processes in this system, including: non-charge-transfer scattering (NCTS), charge-transfer scattering (CTS), hydrogen fluoride dissociation (H–F D), and hydrogen rearrangement (HR). Several aspects of the reactions dynamics, such as mechanistic details and rainbow angles effects, are discussed. Differential and integral cross sections are evaluated via a novel CS formulation of those properties in conjunction with semiclassical techniques. The calculated total differential cross section shows an excellent agreement with available experimental results.

Temperature-Programmed Desorption Analyses of Sol–Gel Deposited and Crystallized HfO2 Films

Characterizations of sol–gel-derived hafnium dioxide (HfO2) films on silicon (Si) substrates were carried out using X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). Significant water (H2O) desorption and accompanying structural changes in the HfO2 film were observed from the TPD curves. The HfO2 film exhibited two distinct H2O desorption states. One state was due to the desorption of physisorbed H2O and/or chemisorbed Hf–OH saturated in air at the surface area of the HfO2 film. The other state depicted by major peaks was caused by the desorption of H2O which may be produced by the reaction of Hf–OH bonds and/or tightly locked in the micropores between crystal grains in the crystallized bulk HfO2 film (monoclinic crystalline state). For amorphous HfO2 film fired at 450 °C, the major peak of the TPD curve was symmetrical, indicating that H2O was detected as a result of the reaction of Hf–OH bonds, which is reaction-controlled (the second-order reaction), whereas at 550 and 700 °C (monoclinic), the major peak of the TPD curve was nonsymmetrical (diffusion-controlled; the first-order reaction). On the basis of the capacitance–voltage (C–V) characteristics, the dielectric constant (permittivity εHfO2 ) of the sol–gel-derived HfO2 film was calculated to be 19, which compares well to those reported for HfO2 prepared by chemical vapor deposition (CVD) and physical vapor deposition (PVD).

Sodium tris(glycinium) bis(hexafluorosilicate) glycine trisolvate

The title compound, Na(+) x 3C(2)H(6)NO(2)(+) x 2SiF(6)(2-) x 3C(2)H(5)NO(2), arose from an unexpected reaction of glycine and HF with the glass container. It is an unusual hybrid organic-inorganic network built up from chains of vertex-sharing NaF(4)O(2) and SiF(6) octahedra. A pair of glycinium/glycine molecules bridges the chains into a sheet via a centrosymmetric O...H...O link. The other organic species interact with the network by an extensive N-H...F hydrogen-bond network, including bifurcated and trifurcated bonds. Finally, an extremely short C-H...O interaction (H...O = 2.25 Angstrom) is seen in the crystal structure. The Na atom has site symmetry overline1.

Reaction rate of beryllium with fluorine ion for Flibe redox control

An experimental effort to apply Flibe (a mixed molten salt of 2LiF + BeF 2) to a self-cooled liquid blanket of a fusion reactor was carried out under a Japan–US collaboration called JUPITER-II. Maintaining Flibe under a reducing atmosphere is a key issue to transform TF to T 2 with a faster reaction rate compared with the residence time in blanket. One of the tasks was to clarify whether or not the redox control of Flibe can be achieved with Be. The dissolution rate of a Be rod and the reaction rate of Be + 2HF = BeF 2 + H 2 in Flibe were experimentally determined. Sufficiently fast rates of the Be dissolution and the reduction reaction of HF to H 2 were clarified by our redox control experiment. Close agreement was obtained between experiments and our simplified complete-mixing model. In particular, the reaction between Be and a F − ion immediately after their contact was found to be limited by diffusion of the F − ion.

Quantum mechanical study of the correlation of attack and recoil angles for the Li + HF reaction: Stereodirected versus discrete variable representations

The zero total angular momentum ( J = 0) S matrix elements, calculated using a time independent method for the Li + HF reaction, have been matrix transformed to the stereodirected and Gauss–Legendre discrete variable representations. Although the results in the two representations are (as expected) quantitatively different with respect to the angular selectivity and specificity of the reactive process, the qualitative similarity has allowed us to draw for the first time conclusions with respect to some characteristics of the potential energy surface.

ALD of Hafnium Dioxide Thin Films Using the New Alkoxide Precursor Hafnium 3‐Methyl‐3‐pentoxide, Hf(mp)4.

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Quantum and quasiclassical state-selected O( 1D) + HF reaction dynamics and kinetics on a new MRCI ground singlet potential energy surface

About 20 000 MRCI single point energies for the O( 1D) + HF → F( 2P) + OH reaction have been calculated and fitted to obtain the ground 1A′ potential energy surface (PES). After successfully comparing with spectroscopic data related to the deep HOF molecular well the more demanding tests involving the whole surface with quasiclassical trajectory calculations (QCT) – first ‘validated’ by comparison to quantum (wavepacket) results – were performed. A reasonable agreement of the room temperature reaction rate coefficient with the more reliable existing kinetic experiment as well as the way the rate coefficient, k( T), changes with temperature and the shape of the products angular distribution in detailed dynamical calculations are reasonable guarantees of the PES quality.

ALD of Hafnium Dioxide Thin Films Using the New Alkoxide Precursor Hafnium 3-Methyl-3-pentoxide, Hf(mp)4

A new alkoxide precursor, Hf(mp)4 [mp=3-methyl-3-pentoxide, OC(CH3)(C2H5)2] has been employed in the atomic layer deposition (ALD) of HfO2 thin films using water (H2O) as the oxygen source. The self-limiting ALD process via alternate surface reactions of Hf(mp)4 and H2O is confirmed from thickness measurements of the HfO2 films grown with varied Hf(mp)4 supply time and number of Hf(mp)4/H2O ALD cycles. The ALD temperature window for this precursor is found to be between 250 and 350°C. Under optimal reaction conditions, the growth rate of the HfO2 films is ∼0.9 A per cycle. Amorphous films can be obtained across the entire temperature range with atomically flat and uniform surfaces. X-ray photoelectron spectroscopy (XPS) and depth-profiling Auger electron spectroscopy (AES) indicate that the films are stoichiometric with negligible amounts (less than 2%) of carbon impurities.

Coupled diabatic potential energy surfaces for studying the nonadiabatic dynamics at conical intersections in angular resolved photodetachment simulations of OHF−→OHF+e−

An energy-based method is proposed for the diabatization of the OH(2Pi)+F(2P)-->O(3P)+HF(1Sigma+) reaction. It is demonstrated that the diabatic representation obtained is regularized, i.e., the residual derivative couplings do not present singularities at the conical intersections appearing along the reaction path. This method only requires the knowledge of the 1,2 3A" and 1 3A' eigenvalues and does not require any adjustable parameter. Thus, many convergence problems arising in other derivative-based diabatization methods are avoided, and the description of the configuration space along the reaction path is enormously simplified. Three-dimensional coupled diabatic energy surfaces are obtained by an interpolation procedure using approximately 4000 accurate ab initio points. The angular resolved photodetachment cross sections are obtained in the diabatic and adiabatic representations using a wave packet method. An excellent agreement is obtained with recent experimental data [D. M. Neumark, Phys. Chem. Chem. Phys. 7, 433 (2005)] for high electron kinetic energies where only the triplet electronic states contribute.

Electret properties and chemical modification of cellular polypropylene films

The electret properties of virginal cellular polypropylene (PP) films and chemically modified cellular PP films by extraction from CH 2Cl 2 solution, oxidation in a mixture solution of H 2SO 4, CrO 3 and H 2O and fluorination in a hydrofluoric acid (HF) solution, were systematically studied by measuring the open-circuit thermally stimulated discharge (TSD) current spectra, charge TSD spectra and isothermal charge decay. The results point out that there are more deep traps than shallow traps in the surface region while the contrary case occurs in the bulk region. The thermal stability of charge storage of the chemically modified cellular PP film is significantly improved in comparison with that of the virginal one. Light irradiation or reacting at elevated temperature has remarkable promotion effect on the reaction of HF with the extracted and oxidized film. Moreover, a method for investigating the dynamic changes of mean charge depth relative to its initial value during heating was proposed.

Theoretical predictions of complex formation of group-4 elements Zr, Hf, and Rf in H2SO4 solutions

Summary A process of step-wise complexation of group-4 elements Zr, Hf, and Rf in H2SO4 solutions has been considered theoretically. Relative values of the free energy change of the M(SO4)2(H2O)4, M(SO4)3(H2O)2 2− and M(SO4)4 4− (M = Zr, Hf, and Rf) formation reactions from hydrated and partially hydrolyzed cations have been calculated using the fully relativistic density functional theory method. They proved to be very similar for reactions of Zr and Hf. At low H2SO4 concentrations, Hf should, however, have a slight preference for the complex formation, especially, when hydrolysis takes place, while with increasing H2SO4 concentration and decreasing hydrolysis, the trend should be Zr > Hf. According to the results, Rf should exhibit a weaker tendency to complex formation than Zr and Hf at the entire range of acid concentrations, i.e., for all types of complexes. Thus, the predicted trend is Zr > Hf ≫ Rf. The same trend is also found for the formation of R4M(SO4)4, where R is a monovalent cation. Accordingly, the trend in the K d (distribution coefficient) values in anion-exchange separations of group-4 elements should be: Zr > Hf ≫ Rf. The obtained trend for Zr and Hf is in agreement with literature data on extraction of these elements in macro-concentrations from sulphuric acid solutions and with recent experiments on extraction of single species by TOA in toluene using the SISAK technique.

Why is hafnium so unreactive?: Experimental and theoretical studies of the reaction of Hf + with methane

The reaction of atomic hafnium cations with CH 4 and CD 4 is studied using a guided ion beam tandem mass spectrometer. In contrast to most third-row transition metal ions, the dehydrogenation reaction to form HfCH 2 + + H 2 is endothermic. At higher energies, other products, HfCH +, HfCH 3 +, and HfH +, the latter being the predominant species, are observed. Implicit in the behavior of the cross sections for HfH +, HfCH 2 +, and HfCH 3 + is a H Hf + CH 3 intermediate. Modeling of the endothermic cross sections provides for 0 K bond dissociation energies (in eV) of D 0(Hf + CH) = 5.10 ± 0.15, D 0(Hf + CH 2) = 4.37 ± 0.07, D 0(Hf + CH 3) = 2.12 ± 0.26, and D 0(Hf + H) = 1.97 ± 0.11. These experimental bond energies are in good agreement with density functional calculations at the B3LYP/HW+/6–311 ++ G(3 df,3 p) level of theory. Theoretical calculations reveal the mechanism of the reaction and illustrate the geometric and electronic structures of the individual products and intermediates. Unlike its first and second-row congeners, which have quartet ground states and must change spin to dehydrogenate methane, Hf + retains its ground state doublet configuration throughout the dehydrogenation reaction, demonstrating that spin-restrictions are not responsible for the relatively low reactivity of Hf +. Instead, this can be attributed to the unfavorable doubly occupied 6 s orbital in the 2D ground state of Hf +.

Reactivity of Tetraneopentylhafnium, Hf(CH2tBu)4, with Silica Surfaces

The reaction of Hf(CH2tBu)4 with a silica surface treated at 800 °C affords a unique surface organometallic species in only one surface environment, (⋮SiO)Hf(CH2tBu)3 (1-SiO2-(800)). In contrast, with SiO2-(500) two surface species, (⋮SiO)Hf(CH2tBu)3 (1-SiO2-(500)) and (⋮SiO)2Hf(CH2tBu)2 (2−SiO2-(500)), in a molar ratio of 70:30 are obtained. Thermal treatment of 1-SiO2-(800) at increasing temperatures leads to the successive evolution of neopentane, isobutene, and isobutane as well as several alkanes varying from C1 to C5. Polyisobutenes are also formed on the surface. The mechanism by which such decomposition occurs suggests a succession of γ-H eliminations with formation of neopentane followed by β-methyl transfer and formation of isobutene and [Hf]−Me. This isobutene is reinserted into [Hf]−Me with formation of isopentene and [Hf]−H. A comparison of the analytical data of 1-SiO2-(800) and (⋮SiO)Zr(CH2tBu)3 indicated the hafnium complex exhibits in EXAFS shorter Hf−C and Hf−O bonds and a larger Hf−Cα−Cβ angle and in 2D J-resolved NMR spectra a lower 1J(Cα−H) value. These differences underlined a larger steric hindrance in the coordination sphere of the Hf metal. The thermal stability of 1-SiO2-(800) was monitored by infrared spectroscopy, in batch and continuous flow reactors, and proved that 1-SiO2-(800) was more stable than (⋮SiO)Zr(CH2tBu)3 and more active in alkane hydrogenolysis.

Preparation and characterization of iminopyrrolyl hafnium complexes as catalyst precursors for α-olefin polymerization

Amido, chloro, and benzyl hafnium complexes bearing 2-( N-aryliminomethyl)pyrrole [(R-pyr)H, 1a: R = DIP = 2-{ N-(2,6-diisopropylphenyl)iminomethyl}, 1b: R = XYL = 2-{ N-(2,6-dimethylphenyl)iminomethyl}, 1c: R = ANI = 2-{ N-(4-methoxyphenyl)iminomethyl}, 1d: R = TOL = 2-{ N-(4-methylphenyl)iminomethyl}] were prepared and characterized. The coordination modes of (R-pyr) 2Hf(NMe 2) 2 ( 2a– d) depend on the bulkiness of the aromatic substituent at the imine nitrogen of the ligands. In contrast, all (R-pyr) 2HfCl 2 ( 3a– d) have a C 2-symmetric octahedral structure. These diamido- and dichloro-bis(iminopyrrolyl)hafnium complexes, when combined with MMAO, become active catalysts for ethylene polymerization. Two types of hafnium benzyl complexes, (DIP-pyr)Hf(CH 2Ph) 3 ( 4a) and (DIP-pyr) 2Hf(CH 2Ph) 2 ( 5a), were prepared by the alkane elimination reaction from reactions of Hf(CH 2Ph) 4 with controlled amounts of iminopyrrole ( 1a). The mono(iminopyrrolyl)hafnium tribenzyl complex ( 4a) exhibited a high catalytic activity for 1-hexene polymerization.