Toward spectroscopic accuracy of ab initio calculations of vibrational frequencies and related quantities: a case study of the HF molecule

Calculations at various coupled-cluster (CC) levels with and without the inclusion of linear r _{i j} -dependent terms are performed for the HF molecule in its ground state with a systematic variation of basis sets. The main emphasis is on spectroscopic properties such as the equilibrium distance r _e and the harmonic vibration frequency ω_e. Especially with the R12 methods (including linear r _{i j} -dependent terms), convergence to the basis set limit is reached. However, the results (at the basis set limit) are rather sensitive to the level of the treatment of electron correlation. The best results are found for the CCSDT1-R12 and CCSD[T]-R12 methods (CCSD[T] was previously called CCSD+T(CCSD)), while CCSD(T) overestimates ω_e by ≈6 cm⁻¹. The good agreement of conventional CCSD(T) with experiment for basis sets far from saturation (e.g. truncated at g-functions) is probably the result of a compensation of errors. The contribution of core-correlation is non-negligible and must be included (effect on ω_e≈5 cm⁻¹). Relativistic effects are also important (23 cm⁻¹), while adiabatic effects are much smaller (<1cm⁻¹) and non-adiabatic effects on ω_e can be simulated in replacing nuclear by atomic masses; for rotation nuclear masses appear to be the better choice, at least for hydrides. From a potential curve based on calculations with the CCSDT1-R12 method with relativistic corrections, the IR spectrum is computed quantum-mechanically. Both the band heads and the rotational structures of the observed spectra are reproduced with a relative error of ≈10⁻⁴ for the three isotopomers HF, DF, and TF. Received: 3 July 1998 / Accepted: 4 August 1998 / Published online: 28 October 1998     

Conjugate polymer-based membranes for gas separation applications: current status and future prospects

Conjugate polymers provide the possibility of exploiting both the chemical and physical attributes of the polymers for membrane-based gas separation. The presence of delocalized π electrons provides high chain stiffness with low packing density, thus making the membrane a rigid structure that favors facilitated transport. Historically, the polymeric membranes were constrained by the tradeoff relationship between gas permeability and gas selectivity. So, different methods were investigated to prepare the membranes that can overcome the limitation. In recent years, electroconductive polymeric membranes have gained attention with their enhanced transportation properties combining the separation behavior depending on both molecular size discrimination as well as the facilitated transport. They offer better selectivity toward polar gases such as CO₂ because of the increased solubility. This review is aimed to provide a literature survey on gas separation using conjugate polymers such as polyaniline, polypyrrole, and some derivatives of polythiophenes. It contains various methods used by different researchers to enhance the gas separation properties of the membranes with improved mechanical and thermal stability such as changing the morphology and membrane preparation methods. In addition, it provides the pros and cons of various factors affecting the conjugate polymer membrane performance. The major challenges and future work that can be done in improving the transportation properties through the membrane to achieve viable membranes are also discussed so that they can be used for commercial and practical applications in the future.     

Cubic structure II double clathrate hydrates with tetra(n-propyl)ammonium fluoride

A double clathrate hydrate with the composition THF·0.5(n-Pr)₄NF·16H₂O and cubic structure II (CS-II,a=17.67 Å) has been obtained. Its experimental density is 1.053±0.001 g/cm³; its melting point is 8.1°C, i.e. 3.1°C higher than that of the THF·17H₂O hydrate. The double hydrates of acetone, 1,4-dioxan, trimethyleneoxide and 1,3-dioxolane with (n-Pr)₄NF have melting points of –14.8, –5.5, –2.6 and –9.6°C, respectively. With pressure increase up to 6 kbar the melting points of the double hydrates increase monotonously in contrast to common CS-II hydrates. The friability of the structure of the hydrates (the packing coefficient) and their sensitivity to pressure (dT/dP) are compared.The results of this work have been reported at the International Seminar on Inclusion Compounds, Jaszowiec (Poland), 24–26th September 1987.     

Ligand-Promoted Transformation of the μ3-η2-Vinylidene Ligand in the Reaction between RuCo2(CO)9(μ3-η2-C=CHPh) and 4,5-Bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd). X-Ray Structure of RuCo2(CO)7(bpcd)(μ3-η2-C=CHPh)

The reaction of the heterometallic vinylidene cluster RuCo₂(CO)₉(₃-²-C=CHPh) with the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) proceeds readily in the presence of Me₃NO to furnish the new cluster RuCo₂(CO)₇(bpcd)(₃-²-C=CHPh) as the sole product. This cluster has been isolated by preparative chromatography and characterized in solution by IR spectroscopy. The molecular structure was determined by X-ray diffraction analysis, which has confirmed the chelation of the bpcd ligand to the ruthenium center and the change in the coordination mode exhibited by the vinylidene ligand. RuCo₂(CO)₇(bpcd)(₃-²-C=CHPh) crystallizes in the triclinic space group P , a = 10.5788(9), b = 11.909(1), c = 19.526(2) Å, = 84.491(9)°, = 78.068(8)°, = 63.760(7)°, V = 2158.7(4) ų, Z = 2, and d _calc = 1.581.     

The formation of solid solutions in the tetrahydrofuran-tetra(n-propyl)ammonium fluoride-water system

The phase diagram of the tetrahydrofuran (THF)-tetra(n-propyl)ammonium fluoride (Prp₄ NF)-water system has been studied by differential thermal analysis (DTA). Solid solutions have been detected of the general formula THF·x Prp₄NF·(17–2x) H₂O, wherex changes from 0.06 to 0.5. The formation of solid solutions is due to the variable degree of the filling of the fragment consisting of four D-cavities in CS-II clathrate hydrates by the tetra(n-propyl)ammonium cation. The solid solution obtained is peculiar in that its vacancies are occupied by the blocks of four D-cavities each. Besides, this kind of displacement of two water molecules by an ionic pair Prp₄N⁺+F^–, where only the charges of the substituting and substituted groups are equal, takes place here. It appears that this type of solid solution has been discovered for the first time.     

N-Polyfluoroethyl and N-2-chlorodifluorovinyl derivatives of azoles

Reactions of tetrafluoroethylene, chlorotrifluoroethylene and 1,2-dichlorodifluoroethylene with N-potassium salts of imidazole, 2-methylbenzimidazole, 3,5-dimethylpyrazole, 1,2,4-triazole, and benzotriazole lead to the formation of the corresponding N-(1,1,2,2-tetrafluoroethyl), N-(2-chloro-1,1,2-trifluoroethyl), and N-(2-chloro-1,2-difluorovinyl) azoles. Treatment of N-(2-chloro-1,2-difluorovinyl) and N-(2-chloro-1,1,2-trifluoroethyl) derivatives of azoles with tetramethylammonium fluoride is a useful synthetic method for the preparation of heterocycles with 1,2,2,2-tetrafluoroethyl group attached to nitrogen.     

121,123Sb NQR study of antimony(III) fluoride complexes

Antimony(III) complexes with nitrogen-containing ligands: 2SbF₃·Gly, SbF₃·Gly, SbF₃·2NA, SbFO·Gly, MSb₂F₇ (M=Et₂NH₂, Bu₄N, HNA⁺), MSbF₄ (M=Et₂NH₂, Pr₂NH₂, Bu₄N, HNA⁺, HGly⁺), M₂SbF₅ (M=Et₂NH₂ and Pr₂NH₂), where Gly is glycine (⁺NH₃CH₂COO⁻) and NA is nicotinamide (β-C₅H₄NCONH₂), were studied by^121,123Sb NQR spectroscopy at 77 K. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2232–2236, November, 1998.     

阴离子交换树脂支载氟阴离子试剂催化Knoevenagel反应

在无水乙醇中，阴离子交换树脂载支载氟阴离子试剂能顺利地催化芳醛与丙二 氰和氰基乙酸乙酯进行Knoevenagel缩合反应，以良好产率生成相应E式烯烃。     

乙烯基混金属羰基簇取代衍生物的合成和分子结构

The mono- and di- substituted derivatives of the parent cluster {μ₃-H(Ph)C=C}FeCo₂(CO)₉ (1) have been synthesized by facile substitution reactions. The compound {μ₃-H(Ph)C=C}FeCo₂(CO)₇(dppe) (2)， {μ₃-H(Ph)C=C}FeCo₂(CO)₈(PPh₃) (3) and {μ₃-H(Ph)C=C}FeCo₂(CO)₈(AsPh₃) (4) were characterized by elemental analysis， IR， ¹H ，³¹P NMR， MS and X-ray crystallography. The new cluster 2 crystallizes in monoclinic space group P2₁/n， with a=0. 8614(5)nm， b=1.6404(8)nm， c=2.6574(14)nm， β=96.3222(12)°， V=3.732(3)nm³， Z=4， S=0.886， Δρ_max=0.335e·nm^-3， Δρ_min=-0.379e·nm^-3. Final R indices [I >2σ(I)]： R₁=0.0432， wR₂=0.0695. CCDC： 164600.     

Rhenium(I)‐Catalyzed Generation of α,β‐Unsaturated Carbene Complex Intermediates from Propargyl Ethers for the Preparation of Cycloheptadiene Derivatives

The rhenium(I)‐catalyzed generation of α,β‐unsaturated carbene complex intermediates from easily available propargyl ethers was achieved for the concise construction of cycloheptadiene derivatives through the formal [4+3] cycloaddition reaction with siloxydienes.