Kinetics of nonequilibrium methylsiloxane polymerization and rearrangement

Nonrandom reactions were demonstrated experimentally during the polymerization of (CH₃)₃SiOSi(CH₃)₃ plus [(CH₃)₂SiO]₄ and the rearrangement of (CH₃)₃SiO[(CH₃)₂SiO]₅Si(CH₃)₃ or (CH₃)₃SiO[(CH₃)₂SiO]₈Si(CH₃)₃, using sulfuric acid-treated Fuller's earth as catalyst. Cyclic and linear reaction products were analyzed by gas–liquid chromatography. A four-step polymerization mechanism was proposed to account for the approach to equilibrium from either the forward or reverse direction. Reaction rate expressions for this mechanism were reduced to a finite set of nonlinear ordinary differential equations. These were solved by using a fourth-order Runge-Kutta numerical integration on a Burroughs B5500 computer. The calculated molecular weight distributions thus obtained were found to agree at all times with the distributions from polymerization and rearrangement experiments.     

Interpretation der Schwingungsspektren von (CH3)2SO2, (CH3)2SO(NH) und (CH3)2S(NH)2 unter Verwendung eines Kopplungsstufenverfahrens

Normal Coordinate Analysis of (CH₃)₂SO₂, (CH₃)₂SO(NH), and (CH₃)₂S(NH)₂ using the Method of Stepwise Coupling The qualitative assignment of the vibrational spectra of (CH₃)₂SO₂ ( 1 ), (CH₃)₂SO(NH) ( 2 a ), and (CH₃)₂S(NH)₂ ( 3 a ) and of the C and N deuterated derivatives of 2 a and 3 a is used in a normal coordinate analysis by the method of stepwise coupling. The force constants and the energy distributions are calculated in symmetry coordinates using a generalized valence force field.     

Transition metal promoted alkylations of unsaturated alcohols: the methylation and ethylation of 3-buten-1-ol using titanium tetrachloride-organoaluminum systems

The ethylation and methylation of the olefinic linkage in 3-buten 1-ol by incorporating the alkenol into a titanium-organoaluminum system was studied under a variety of conditions. Systems were derived from titanium tetrachloride and the organoaluminum compounds Al(C₂H₅)₃, Al(C₂H₅)₂Cl, Al(CH₃)₃, and Al(CH₃)₂Cl. With diethylaluminum chloride the major products obtained were 1-hexanol, 3-methyl-1-pentanol, trans-3-hexen-1-ol, and 1-butan I. Triethylaluminum gave no alkylation products. Dimethylaluminum chloride and trimethylaluminum gave product distributions similar to the analogous diethylaluminum chloride system.     

Preparation and characterization of acrylates and polyacrylates having variable fluorine contents and distributions

Five acrylic esters having different fluorine contents and distributions in their side-groups (i.e., CH₂=CHC(O)OR, where R = ? C(CH₃)₂C₆F₄H, ? C(CH₃)₂C₆F₅, ? C(CF₃)₂C₆F₅, ? C(CF₃)₂C₆H₅, and ? C(CH₃)₂C₆H₅) have been prepared from the reactions of the lithium salts of their corresponding alcohols with acryloyl chloride. These monomers are polymerized under identical conditions by the radical initiator AIBN and five polyacrylates were prepared having the structure of ? [ ? CH₂CHC(O)OR? ]_n? . These addition polymers were compared and fully characterized by GPC, VPO, DSC, TGA, NMR, IR, and UV-visible spectroscopies, and they showed potential for practical applications. Significant differences in their thermal stabilities were found with respect to fluorine contents and distributions in these polyacrylates, and the highest stability arises from CF₃ substitutions in the side-chains of the polymers. © 1994 John Wiley & Sons, Inc.     

Force field studies of some trifluoromethyl- and trichloromethyl-mercury-II) compounds

Force constants of [Hg(CF₃)₂], [Hg(CCl₃)₂], [Hg(CF₃)X] (X = Cl, Br, or I) and [Hg(CCl₃)X] (X = Cl or Br) have been calculated using a valence force field and wavenumber data from solutions. The potential energy distributions show substantial mixing between the symmetrical stretching and umbrella deformation coordinates of the trihalomethyl groups. The high degree of mixing of HgC and HgX stretching coordinates in [Hg(CF₃)Br] and [Hg(CF₃)I] accounts for the discontinuous frequency and intensity trends in the [Hg(CF₃)X] series.The results are discussed in comparison with methylmercury and other trifluoromethyl systems.     

High polymerization of methyl methacrylate with organonickel/MMAO systems

A series of nickel complexes, including Ni(acac)₂, (C₅H₅)Ni(η³‐allyl), and [NiMe₄Li₂(THF)₂]₂, that were activated with modified methylaluminoxane (MMAO) exhibited high catalytic activity for the polymerization of methyl methacrylate (MMA) but showed no catalytic activity for the polymerization of ethylene and 1‐olefins. The resulting polymers exhibited rather broad molecular weight distributions and low syndiotacticities. In contrast to these initiators, the metallocene complexes (C₅H₅)₂Ni, (C₅Me₅)₂Ni, (Ind)₂Ni, and (Me₃SiC₅H₄)₂Ni provided narrower molecular weight distributions at 60 °C when these initiator were activated with MMAO. Half‐metallocene complexes such as (C₅H₅)NiCl(PPh₃), (C₅Me₅)NiCl(PPh₃), and (Ind)NiCl(PPh₃) produced poly(methyl methacrylate) (PMMA) with much narrower molecular weight distributions when the polymerization was carried out at 0 °C. Ni[1,3‐(CF₃)₂‐acac]₂ generated PMMA with high syndiotacticity. The NiR(acac)(PPh₃) complexes (R = Me or Et) revealed high selectivity in the polymerization of isoprene that produced 1,2‐/3,4‐polymer at 0 °C exclusively, whereas the polymerization at 60 °C resulted in the formation of cis‐1,4‐rich polymers. The polymerization of ethylene with Ni(1,3‐tBu₂‐acac)₂ and Ni[1,3‐(CF₃)₂‐acac]₂ generated oligo‐ethylene with moderate catalytic activity, whereas the reaction of ethylene with Ni(acac)₂/MMAO produced high molecular weight polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4764–4775, 2000     

Geometries and properties of bimetallic phosphido-bridged complex Cp(CO)2W(μ-PPh2)W(CO)5 and Cp(CO)3W(μ-PPh2)W(CO)5

Complete geometry optimizations were carried out by HF and DFT methods to study the molecular structure of binuclear transition-metal compounds (Cp(CO)₃W(μ-PPh₂)W(CO)₅) (I) and (Cp(CO)₂W(μ-PPh₂)W(CO)₅) (II). A comparison of the experimental data and calculated structural parameters demonstrates that the most accurate geometry parameters are predicted by the MPW1PW91/LANL2DZ among the three DFT methods. Topological properties of molecular charge distributions were analyzed with the theory of atoms in molecules. (3, −1) critical points, namely bond critical point, were found between the two tungsten atoms, and between W1 and C10 in complex II, which confirms the existence of the metal–metal bond and a semi-bridging CO between the two tungsten atoms. The result provided a theoretical guidance of detailed study on the binuclear phosphido-bridged complex containing transition metal–metal bond, which could be useful in the further study of the heterobimetallic phosphido-bridged complexes.     

An alternative to pseudopotentials: a core—valence approximation scheme for molecular calculations

A core—valence approximation scheme is described in which the integral-evaluation time varies as (N_valence)⁴ rather than (N_total)⁴. The scheme is based on expanding core—valence overlap distributions in terms of a mono-centric basis centred at the core. The method is illustrated with comparative calculations on NF₃ and PCl₃.     

Bis(phosphinimino)methanide Borohydride Complexes of the Rare‐Earth Elements as Initiators for the Ring‐Opening Polymerization of ε‐Caprolactone: Combined Experimental and Computational Investigations

Rare‐earth‐metal borohydrides are known to be efficient catalysts for the polymerization of apolar and polar monomers. The bis‐borohydrides [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln=Y, Lu) have been synthesized by two different synthetic routes. The lanthanum and the lutetium complexes were prepared from [Ln(BH₄)₃(THF)₃] and K{CH(PPh₂NSiMe₃)₂}, whereas the yttrium analogue was obtained from in situ prepared [{CH(PPh₂NSiMe₃)₂}YCl₂]₂ and NaBH₄. All new compounds were characterized by standard analytical/spectroscopic techniques, and the solid‐state structures were established by single‐crystal X‐ray diffraction. The ring‐opening polymerization (ROP) of ε‐caprolactone initiated by [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln=Y, Lu) was studied. At 0 °C the molar mass distributions determined were the narrowest values (M?_w/M?_n=1.06–1.11) ever obtained for the ROP of ε‐caprolactone initiated by rare‐earth‐metal borohydride species. DFT investigations of the reaction mechanism indicate that this type of complex reacts in an unprecedented manner with the first B? H activation being achieved within two steps. This particularity has been attributed to the metallic fragment based on the natural bond order analysis.     

Vibrational energy disposal in reactions of fluorine atoms with hydrides of groups III,IV and V

The HF infrared chemiluminescence from the reactions of F atoms with B₂H₆, CH₄, CH₃F, CH₂F₂, CH₂Cl₂, CH₃ONO. CH₃NO₂, NH₃ (and ND₃). PH₃ and HNCO has been observed from a 300 K flowing-afterglow reactor. Experiments were done for a range of CH₄ and F atom concentrations to identify conditions which were free of vibrational relaxation and secondary reactions, and these conditions were used to assign initial HF(v) vibrational distributions for each reaction. The emission intensity from each reaction also was compared to that from CH₄ in order to obtain the relative HF formation rate constants at 300 K. Since the absolute rate constant for F + CH₄ is well established, the combination of all of these data provides absolute rate constants for HF(v) formation at 300 K. The ND₃ reaction was studied to obtain information on more vibrational levels in order to better estimate the HF(v = 0) and DF(v = 0) components of the ammonia distributions. With NH₃ and ND₃ there is no significant isotope effect on the energy disposal. Except for NHCO, for which an addition-elimination channel is possible, the HF(v) distributions are inverted and <f_v > = 0.60. Differences between the HF(v) distributions reported here and some other reports in the literature are noted: the present data are discussed as representative of direct H atom abstraction for 300 K Boltzmann conditions. The HCl infrared chemiluminescence from the F + CHCl₂ secondary reaction also was observed; the HCl(v) distribution was v₁: v₂: v₃: v₄: v₅ - 0.47: 0.23: 0.18: 0.08: 0.04.     

Infrared chemiluminescence in the reactions of atomic fluorine with ethanol,propanol-(2) and some deuterated analogs

In the reactive systems F+C₂H₅OH, F+C₂D₅OD, F+C₂H₅OD, F+(CH₃)₂CHOH, F+(CD₃)₂CHOH, and F+(CD₃)₂CDOH the infrared emission spectra were recorded from HF and/or DF in the fundamental region. Hydrogen abstraction takes place from CH and OH bonds. Vibrational relaxation was suppressed and rotational relaxation took place only to a minor extent. HF(DF) excitation reaches the thermodynamic limit within error limits in all cases. The vibrational distributions of HF for the systems F+(CD₃)₂CHOH, F+(CD₃)₂CDOH show no populati inversion. The vibrational distribution of HF for all other systems and all the DF vibrational distributions obtained show population inversion. Inform theory was used to describe the results of those reaction channels that could be studied separately because of isotopic substitution. The results are c to the systems F + methanol and deuterated analogs investigated before in our laboratory, and to the F+CH₄, F+CD₄, and F+H₂O₂ reactio     

Cluster analysis of periodic distributions; application to conformational analysis

A method is described to analyze observed conformations for molecular fragments with more than three torsional degrees of freedom. The method is an adaption of statistical cluster analysis to multidimensional, symmetric, periodic distributions of data points. Application to the molecular fragment M(PPh₃)₂ with eight torsional degrees of freedom reveals a model of conformational interconversion. The model implies gearing motion of the two PC₃ fragments alternating with stepwise inversions of the helicities of the PPh₃-propellers.     

Characterization of Molecular Weight Distributions by the Standard Deviation

The standard deviation [sgrave] is known to be an absolute measure of Gaussian (G) distributions, because it (or multiples of it) always determines a constant fraction of the material. However, this is not true for distribution functions other than Gaussian. The weight fractions corresponding to weight standard deviations (±[sgrave]_w) of Schulz-Flory distributions depend on the polymolecularity index xw/xn, approaching a limiting value of W(x_w ± [sgrave]_w) = 86.5 for x_w/x_n →∞. The use of weight standard deviations is meaningless for generalized logarithmic normal (LN) distributions if x_w/x_n 52. The weight fractions W(X_n ± [sgrave]_n) around the number-average degree of polymerization of G and LN distributions first go through a maximum before decreasing with increasing x_w/x_n. The weight fractions W(x_n ± [sgrave]_n) decrease steadily with higher x_w/x_n.     

Electronic structure and spectra of [Ru(NH3)5pyz]2+ and [(NH3)5Ru-pyz-Ru(NH3)5]4+

The electronic structure and spectra of [Ru(NH₃)₅pyz]²⁺ and [(NH₃)₅Ru-pyz-Ru(NH₃)₅]⁴⁺ are calculated by the INDO (CINDO-E/S) method. Changes in molecular orbitals, charge distributions, and bond order indices of the pyrazine molecule and [Ru(NH₃)₅pyz]²⁺ complex in the [(NH₃)₅Ru-pyz-Ru(NH₃)₅]⁴⁺ binuclear complex are analyzed. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 4, pp. 12–23, July–August, 1994. Translated by. O. Kharlamova     

A Triatomic Silicon(0) Cluster Stabilized by a Cyclic Alkyl(amino) Carbene

Reduction of the neutral carbene tetrachlorosilane adduct (cAAC)SiCl₄ (cAAC=cyclic alkyl(amino) carbene :C(CMe₂)₂(CH₂)N(2,6‐iPr₂C₆H₃) with potassium graphite produces stable (cAAC)₃Si₃, a carbene‐stabilized triatomic silicon(0) molecule. The Si?Si bond lengths in (cAAC)₃Si₃ are 2.399(8), 2.369(8) and 2.398(8) Å, which are in the range of Si?Si single bonds. Each trigonal pyramidal silicon atom of the triangular molecule (cAAC)₃Si₃ possesses a lone pair of electrons. Its bonding, stability, and electron density distributions were studied by quantum chemical calculations.     

Anisotropic interactions from double rainbows in Na atom—molecule collisions

Crossed molecular beam experiments have been performed measuring differential cross sections for Na scattered from CH₃I, (CH₃)₃CCl, (CH₃)₃CBr and (CH₃)₃CI at collision energies of 0.170 to 0.220 eV. The angular distributions show pronounced double rainbow structure which is attributed to the anisotropy of the interaction potential. The origin and the conditions for this structure are investigated by calculations in the infinite order sudden approximation. The effect is used to determine the anisotropic interaction potentials for the measured systems which all exhibit minima in the linear configuration of the collision complex ranging from 30 to 60 meV.     

Cation distribution and interatomic interactions in oxides with heterovalent isomorphism: XI. (La1−xHox)2SrAl2O7 solid solutions

Interatomic distances and distributions of rare-earth and strontium cations over two sites in the structure of (La_1?x Ho_x)₂SrAl₂O₇ solid solutions were determined by means of full-profile X-ray structural analysis. Introduction of holmium cations into the La₂SrAl₂O₇ oxide results in ordered distribution of Ln³⁺ cations and stabilization of the perovskite-like structure.     

Bis(β‐diketiminato) lanthanide amides: synthesis,structure and catalysis for the polymerization of l‐lactide and ε‐caprolactone

Treatment of the chlorides (L^2,6‐iPr2_Ph)₂LnCl (L^2,6‐iPr2_Ph = [(2,6‐ⁱPr₂C₆H₃)NC(Me)CHC(Me)N(C₆H₅)]^?) with 1 equiv. of NaNH(2,6‐ⁱPr₂C₆H₃) afforded the monoamides (L^2,6‐iPr2_Ph)₂LnNH(2,6‐ⁱPr₂C₆H₃) (Ln = Y ( 1 ), Yb ( 2 )) in good yields. Anhydrous LnCl₃ reacted with 2 equiv. of NaL^2,6‐iPr2_Ph in THF, followed by treatment with 1 equiv. of NaNH(2,6‐ⁱPr₂C₆H₃), giving the analogues (L^2,6‐iPr2_Ph)₂LnNH(2,6‐ⁱPr₂C₆H₃) (Ln = Sm ( 3 ), Nd ( 4 )). Two monoamido complexes stabilized by two L^2‐Me ligands, (L^2‐Me)₂LnNH(2,6‐ⁱPr₂C₆H₃) (L^2‐Me = [N(2‐MeC₆H₄)C(Me)]₂CH)^?; Ln = Y ( 5 ), Yb ( 6 )), were also synthesized by the latter route. Complexes 1 , 2 , 3 , 4 , 5 , 6 were fully characterized, including X‐ray crystal structure analyses. Complexes 1 , 2 , 3 , 4 , 5 , 6 are isostructural. The central metal in each complex is ligated by two β‐diketiminato ligands and one amido group in a distorted trigonal bipyramid. All the complexes were found to be highly active in the ring‐opening polymerization of L‐lactide (L‐LA) and ε‐caprolactone (ε‐CL) to give polymers with relatively narrow molar mass distributions. The activity depends on both the central metal and the ligand (Yb < Y < Sm ≈ Nd and L^2‐Me < L^2,6‐iPr2_Ph). Remarkably, the binary 3/benzyl alcohol (BnOH) system exhibited a striking ‘immortal’ nature and proved able to quantitatively convert 5000 equiv. of L‐LA with up to 100 equiv. of BnOH per metal initiator. All the resulting PLAs showed monomodal, narrow distributions (M_w/M_n = 1.06 ? 1.08), with molar mass (M_n) decreasing proportionally with an increasing amount of BnOH. The binary 4/BnOH system also exhibited an ‘immortal’ nature in the polymerization of ε‐CL in toluene. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling the electronic structure of graphite intercalation compounds with lithium by metal complexes with polybenzene systems

The electronic structure of intercalation compounds obtained by inclusion of an alkali metal in graphite is considered using molecular complexes C₆H₆Li₂, (C₆H₆)₂Li, and (C₆H₆)₃Li₂ in the lowest energy states as examples. Modeling the electron distributions of the intercalates requires inclusion of metal d-orbitals in the basis set of AOs and rejection of purely ionic models. It was found that exclusion of the lithium d-AO from the basis set significantly increases the ionic character of the distributions. Moscow State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 3, pp. 450–457, May–June, 1996.     

Direct MP2 molecular dynamics studies of H atom reaction with CD4 and CH4

The mechanism and dynamics of the H + CD₄ → CD₃ + HD (I) and H + CH₄ → CH₃ + H₂ (II) reactions have been investigated by electronic structure methods. The minimum‐energy path and vibrational frequencies along the intrinsic reaction coordinate are calculated at MP2/cc‐pVDZ level. Energy distributions of the products are also obtained by the direct classical trajectory calculations at the MP2/ cc‐pVDZ level. It is found that most of the available energy appears as product translational energy, and very little of the available energy is partitioned into internal excitation of the HD (H₂) product for reaction I (II), which is in agreement with the experimental evidence. The results indicate that the experimental results could be reproduced by the direct MP2 molecular dynamics calculations. The rotational state distributions of the products show the HD (H₂) products are formed with lower rotational quantum numbers than the CD₃ (CH₃) products. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011