Theoretical studies of [n]paracyclophanes and their valence isomers

Summary The valence isomerisations of benzene, [6]- and [7]paracyclophane to their Dewar benzene and prismane isomers are studied with the MNDO method using the unrestricted Hartree-Fock (UHF) and the configuration interaction (C.I.) approximations. The enthalpy of the reaction Dewar benzene benzene is H° _r =–68.9 kcal/mol and the activation enthalpy is H°=27.9 kcal/mol (with C.I.). The reaction path hasC _2v symmetry.The determination of several points of the lowest potential energy surface of [6]- and [7]paracyclophanes leads to a minimum reaction path having the same topology as for the potential energy surface of the nonbridged benzene. The only difference is a quantitative change in the energy values of the aromatic isomers due to the deformation introduced by the alkyl chain. For [6]paracyclophane, the activation enthalpy is H°=24.6 kcal/mol and the activation entropy is S ⁰=0.6 cal K^–1 mol^–1 calculated with C.I.The enthalpy of the reaction prismane Dewar benzene is H° _r –32 kcal/mol and the activation enthalpy is H°19 kcal/mol. The highest molecular symmetry group common to both molecules isC _2v , whereas the symmetry group of the reaction path is lowered toC _s . Along this reaction path is located a biradicaloid intermediate, separated by low activation barriers from the products. No significant changes of the potential energy surfaces are found for the bridged [n]prismanes and the [n]Dewar benzenes.All the calculated values, reaction enthalpies, activation enthalpies and entropies, are in a good agreement with literature experimental data.This article is dedicated to Professor A. Pullman     

Thermodynamic and magnetic resonance studies on the hydration of polymers: I. Interactions of water in a synthetic cation-exchange resin

Measurements of NMR spin-lattice relaxation times T ₁ were performed for sorbed H₂O and D₂O in a sulfonated ion-exchange resin at varying degrees of hydration with alkaline cations as counter ions. From the sorption isotherms at three different temperatures the partial molar enthalpies and entropies of sorption show a minimum for all alkaline cations at water concentrations of n 0.8, i.e., there are 0.8 water molecules per –SO ₃ ^– group. The first water molecules sorbed in the ion-exchange resin matrix are characterized by anisotrtopic rotational diffusion processes with correlation times of the order of ₁ 50 ns and ₂ 30 ps, respectively. This indicates that they are located in the electrostatic field between the corresponding ion pair. Although these two correlation times are very similar at a given temperature for all alkaline cations studied in the present investigation, the existence of a second spin-lattice relaxation time for sorbed H₂O at n=0.8 indicates that for Cs about 50% of the sorbed water diffuses between locations in the resin. This fraction decreases with ionic radii and with falling temperatures. For Li the amount is less than 20%.     

Viscosities of solutions of electrolytes and nonelectrolytes inN-methylacetamide at 35° and 55°C

The viscosities of dilute solutions of a number of tetraalkylammonium and alkali metal halides, tetraphenylarsonium chloride, sodium tetraphenylborate, and tetrabutylammonium tetrabutylborate, as well as several nonelectrolytes have been measured in the high dielectric constant solvent N-methylacetamide (NMA) at 35 and 55°C. The relative viscosities were fitted to the extended Jones-Dole equation, = 1 + AC^1/2 + BC + DC². The pattern of behavior of the B coefficients is roughly similar to that observed in H₂O. However, the small ions have exceptionally large B values in this solvent due to strong solvation effects, while the large organic ions do not display the sharp crossing of the Einstein law, B=2.5 _V, characteristic in H₂O of hydrophobic interaction. The D coefficients roughly parallel the B behavior and display remarkably regular ionic trends. This suggests that they arise largely from hydrodynamic origins. Nonelectrolytes have small or negative B coefficients showing that the Einstein law is not applicable at the molecular level and that nonelectrolytes are poor models for structurally similar ions. A simple mixture law is presented as an alternative to the Einstein law to explain the B coefficients.     

The Coordination Number of Lu(III) in a Mixed System of Methanol and Water

The stability constants (_1(F)) of the monofluoro complex of Lu(III) and those (_1(Cl)) of the monochloride solvent-shared ion-pair of Lu(III) have been determined in mixed solvents of methanol and water at 0.10 and 1.00 mol·dm^–3 ionic strengths, respectively. The variation in ln_1(F) with an increase in the mole fraction of methanol (X _s) in the mixed solvent system showed an acute-angled convex inflection point at X _s 0.12, an acute-angled concave inflection point at X _s 0.22, and another acute-angled convex inflection point at X _s 0.27. It was concluded that the first and the second convex inflection points denoted the CN of Lu³⁺ from CN = 8 to a mixture of CN = 8 and 7 and from CN = 8 and 7 to a mixture containing CN = 6, respectively. The concave point is the starting point of a change in the CN of Lu(III) in LuF²⁺ from CN = 8 to a mixture of CN = 8 and 7. The values of two inflection points of the CN around Lu³⁺ are consistent with the inflection points of the variation in the values of ln_1(Cl) versus the dielectric constant of the mixed solvent.     

Conformation analysis of diphosphine

Extended basis set ab initio computations are performed on HF, PNO-CI and CEPA level to determine the structure of P₂H₄ and the potential curve E() for rotation around the P-P axis. The structure parameters are optimized for dihedral angles of 0 ° (cis), 50 °, 80 ° (gauche or semi-eclipsed), 130 °, and 180 ° (trans). It turns out that P₂H₄ has a gauche equilibrium structure, a local minimum for trans which is 2.5 kJ/mol above gauche, a rather large cis barrier of 20 kJ/mol and a gauche trans barrier of 3.5 kJ/mol. The potential E() is extremely flat in the region 50 ° < < 310 °, where E() varies by less than 5 kJ/mol. Electron correlation tends to reduce the barriers but has no drastic effect on E().     

Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: II. Methylation of >Si=O Groups

The methods of optical and IR spectroscopy and quantum chemistry were used to obtain data on the direction and kinetics of the reaction of a silanone (SiO)₂Si=O with a CH₄ molecule and a methyl radical. Two mechanisms of methylation of silanone groups, molecular and free-radical, are studied. Both processes are accompanied by the formation of (SiO)₂Si(OH)(CH₃) groups. The rate constant of the molecular process is determined and its activation energy is estimated (17 kcal/mol). A methyl radical adds to the silicon atom in a silanone group to form the oxy radical (Si–O)₂Si(O)(CH₃). This radical carries a free-radical process of silanone group methylation. The main channel for the pyrolysis of (Si–O)₂Si(OH)(CH₃) groups is their decomposition with the abstraction of a methane molecule. The activation energy of this process is 70 kcal/mol. Quantum chemical methods were employed to obtain data on possible intermediates in the processes studied and these results are used to interpret spectral and kinetic data.     

Disilene,silylsilylene and their cations

An accurate examination of features of the ground state surfaces of Si₂H₄ and Si₂H ₄ ⁺ is reported; they are compared to C₂H₄ and C₂H ₄ ⁺ . For the neutral species, accurate SCF calculations show disilene to be planar, but silylsilylene has the lower energy, whereas at the correlated (CI, MP2, MP3, MP4(SD)) levels disilene becomes trans bent and has the lower energy by 6 kcal/mol. In view of a recent theoretical suggestion that this value should be 23 kcal/mol, we have used large basis sets in these investigations. Our calculations cannot support this large value. Similar investigations are reported for the cation, where the planar disilene structure is predicted to be the most stable. It may be very slightly twisted at high accuracy CI, but it is much lower in energy than the silylsilylene structure. Vibrational frequencies and infra-red intensities are also reported. Theoretical photoelectron spectra of C and Si systems are presented and compared with experiment.     

Spin conversion of positronium in paramagnetic solutions studied by age-momentum-correlation method

Correlated measurement of the lifetime and of the lineshape of the 511 keV annihilation radiation of positrons (age-momentum correlation, AMOC) has become a powerful tool for investigating reactions of positron or positronium in condensed matter as a function of time. The beam-based ⁺ AMOC method installed at the Stuttgart relativistic positron beam facility (E _kin ^e+ 4MeV) offers substantial advantages over the conventional E coincidence technique resulting in a fast data taking due to the unity detection efficiency of the ⁺ scintillator which provides the start signal for the lifetime measurement. In this paper the application of AMOC to positronium chemistry is illustrated by the study of spin conversion of positronium in the system methanol/HTEMPO as a function of HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, a paramagnetic solute) concentration. The same technique can be applied to other systems and other reactions in positronium chemistry, e. g., oxidation or complex formation.     

Isomerisierung und Ringinversion von Tribenzaspiro[5.5]undecaphanen

For the two possible ring inversions in tribenzaspiro[5.5]undecaphanes—corresponding to the isomerizations79 and97 resp.—no mutual influence is found. Thus the degenerate isomerizations (topomerizations)77 and99 can proceedvia two consecutive (non-concerted) reversals. A ring inversion path similar to that for [2.2]metacyclophane (1) is postulated. The considerable ease of the isomerization of9 as compared to1 can be understood on the basis of thermodynamic arguments.Moreover it is confirmed that the free enthalpy difference G ₃₆₀=3.84 kcal/mol of7 and9 at equilibrium originates almost entirely from the different stabilities of boat and chair shaped benzene rings.These conclusions are drawn from a kinetic study of the thermal isomerization of9 and equilibration experiments in comparison with the corresponding data of1. The activation parameters for the process97 are: G _360isom ^A =26.52 kcal/mole, H _isom ^A =25.35 kcal/mole, S _isom ^A =–3.28 cal/deg·mole (isomerization) and G _360inv ^A =27.03 kcal/mole, H _inv ^A =25.35 kcal/mole, S _inv ^A =–4.67 cal/deg·mol (ring reversal); the corresponding data for the degenerate isomerizations of7 and9—identical with those for the ring reversal in7—were deduced to be G _360inv ^B =30.87 kcal/mole, H _inv ^B =27.50 kcal/mole and S _inv ^B =–9.5 cal/deg·mole.

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Mit 7 Abbildungen

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Herrn Prof.H. Nowotny mit besten Wünschen zum 65. Geburtstag gewidmet.

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7. Mitt.:H. Keller, E. Langer undH. Lehner, Mh. Chem.107, 949 (1976).     

Adsorption and mobility of water and organic molecules in carbonaceous adsorbents. 4. Self-diffusion coefficients of water molecules and organic substances in active carbon having relatively large micropores

The self-diffusion coefficients of water in active carbon (AC) having relatively wide micropores increase in direct proportion to the degree of filling. The self-diffusion coefficients D of adsorbed C₆H₆, C₆F₆, C₆H₁₂, and H₂O molecules at 298 K do not depend on the size of the molecules and are larger, the greater the mobility of the molecules in the free liquids. The self-diffusion activation energy of the adsorbed molecules varies in the order: H₂O>C₆F₆> C₆H₁₂ C₆H₁₄. C₆H₁₄. For H₂O and C₆H₆ the activation energy increases as the characteristic size of the micropores of the AC increases.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1949–1951, September, 1989.     

Kinetics of the Thermal Desorption of Atomic Oxygen during Transformations BiO2 – x→ β-Bi2O3→ α-Bi2O3

The thermal desorption of atomic oxygen during the transformations BiO_{2 – x} -Bi₂O₃ -Bi₂O₃is shown to be due to the removal of overstoichiometric oxygen from the bulk of -Bi₂O₃. Oxygen formed at the first stage is desorbed in a molecular form. The maximum desorption rate of atomic oxygen is found before the phase transformation -Bi₂O₃ -Bi₂O₃. The activation energy of the diffusion of excess oxygen in the -Bi₂O₃lattice is 30 kcal/mol.     

Einfluß der Anionensolvatation auf die Komplexbildung in Donorlösungsmitteln

Zusammenfassung Der Einfluß der Anionensolvatation auf die Komplexbildung in Donorlösungsmitteln wird folgendermaßen beschrieben: Der thermodynamische Solvatationseffekt hängt vom Verhältnis der freien Solvatationsenthalpien zur freien Enthalpie des Gesamtvorganges der Komplexbildung ab; d. h., je geringer die Akzeptorstärke des Akzeptors, um so mehr wird durch die Solvatation die Donorstärke eines Anions im Vergleich zu derjenigen des Neutraldonors erniedrigt. Der spezifische Solvatationseffekt bringt das spezifische Solvatisierungsvermögen eines Lösungsmittels gegenüber einem Anion (bei gegebenem Akzeptor) zum Ausdruck. Protonenhaltige Lösungsmittel, z. B. Wasser, Methanol, Essigsäure, Ameisensäure, Formamid, haben eine hohe Tendenz zur Wasserstoffbrückenwechselwirkung; sie solvatisieren Halogenid- und Pseudohalogenidionen wesentlich stärker als aprotische Donorlösungsmittel. Die Donorstärke eines anionischen Liganden wird in einem solchen Lösungsmittel entsprechend erniedrigt. Zum Vergleich der Stabilität eines Komplexes in verschiedenen Lösungsmitteln können ihre Donizitäten allein nicht herangezogen werden. Die solvatisierenden Eigenschaften von Donorlösungsmitteln nehmen ab: H₂O>ROHDMSOES>ANTMSNMDMF>DMA>HMPT.

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The influence of anion-solvation on complex formation in donor solvents

The said influence is described as follows: the thermodynamic solvation effect depends on the ratio of the free enthalpy of solvation to the free enthalpy of the complex forming reaction. It is increased by decreasing acceptor strength of the acceptor. The specific solvation effect is due to the specific solvating power of a solvent towards an anion (with given acceptor). Protonic solvents are known to have higher tendencies for hydrogen bridge interactions. They are stronger solvating agents for halide and pseudohalide ions than aprotic donor solvents. The donor properties of a donor anion is decreased in such solvents and the donicity alone is no longer a useful guide to estimate the relative stabilities of a complex in such media. The solvating properties of donor solvents decrease in the following order: H₂O>ROHDMSOES>ANTMSNMDMF>DMA>HMPT.

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Herrn Professor Dr.Richard Kieffer zum 65. Geburtstag gewidmet.     

The quest for a stable silyne, RSi ≡ CR′. The effect of bulky substituents [1]

The two major fundamental obstacles which so far have prevented theisolation of stable silynes, RSiCR (1), are: (a)the existence of more stable isomers, e.g., RRC=Si: (2) and(b) their extremely facile (exothermic) dimerication. The steric andelectronic effects of various substituents R and R (R = alkoxy,alkyl, aryl and silyl; R = alkyl and aryl groups) on the stability ofRSiCR relative to the isomeric RRC=Si:(E(1-2)), and on the energy of dimerization tothe corresponding 1,3-disilacyclobutadienes (E(D)), werestudied computationally using density functional theory (DFT) and theONIOM method. The goal was to find a combination of substituents thatwill make RSiCR more stable than RRC=Si: and whichwill also prevent its dimerization. For R = R = H,E(1-2)) = 40.7 kcal/mol (i.e., 2 islower in energy than 1), and E(D) = –104.0kcal/mol. 1, R = OH, R = m-Tbt 2,6-bis[bis(silyl)methyl]phenyl, is by 11.1 kcal/mol morestable than the isomeric silylidene 2. However, thedimerization of 1, R = OH, R = m-Tbt remains highlyexothermic (by 101 kcal/mol). 1, R = R = m-Tbt and1, R = (t-Bu)₃Si, R = m-Tbt, are by 5.8 and 2.0kcal/mol, respectively, less stable than the corresponding 2.However, the dimerization of 1, R = (t-Bu)₃Si, = m-Tbt is exothermic by only 12 kcal/mol. For1, R = (t-Bu)₃Si, and R = Tbt 2,6-bis[bis(trimethylsilyl)methyl]phenyl, the corresponding1,3-disilacyclobutadiene dimer 3, dissociates spontaneously.Thus, (t-Bu₃Si)SiCTbt is predicted to be kineticallystable towards both, isomerization to (t-Bu₃Si)TbtC=Si: anddimerization to 3, making it a viable synthetic target. Thereported energies were calculated atB3LYP/6-31G^**//B3LYP/3-21G^*; good agreement is found betweenthe DFT and the ONIOM results.     

Calculation of spin densities in odd alternant hydrocarbon radicals

Spin densities in benzyl, allyl, pentadienyl and perinaphthenyl radicals have been calculated with some variants of the simplified McLachlan unrestricted SCF method based on Hückel-type and the SC Hückel-type molecular orbitals. The various estimations of the spin densities were compared one with each other and with the results of more sophisticated methods. It was shown that the SC Hückel basis and Zhidomirov and Schastnev's modification of the McLachlan approach give a best least square fit of the relation a ^exp Q ^calc .

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Zusammenfassung Die Spindichten von Benzyl-, Allyl-, Pentadienyl- und Naphtenyl-Radikalen werden mit einigen Varianten der uneingeschränkten SCF-Methode in der vereinfachten Form nach McLachlan, die auf den Hückel- und den SC-Hückel-Molekülorbitalen basiert, berechnet. Die verschiedenen Schätzungen der Spindichten werden miteinander sowie mit den Resultaten komplizierterer Methoden verglichen. Es wird gezeigt, daß die SC Hückel Basis und die von Zhidomirov und Schastnev eingeführte Modifikation der Näherung von McLachlan eine beste Näherung nach der Methode der kleinsten Quadrate an die Beziehung a ^exp Q ^calc ergeben.

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Résumé Les densités de spin ont été calculées dans les radicaux benzyle, allyle, pentadiényle et perinaphthényle à l'aide de variantes de la méthode SCF sans restrictions de spin simplifiée de Mac Lachlan, en utilisant des orbitales de type Hückel et SC Hückel. Les différentes évaluations de densité de spin sont comparées entre elles et avec celles obtenues par des méthodes plus élaborées. On montre que la base SC Hückel et la modification de Zhidomirov et Schastnev à la théorie de Mac Lachlan donnent le meilleur accord au sens des moindres carrés avec la relation a ^exp Q ^cal .

     

Aerogelation Process Simulation by a Cluster-Cluster Aggregation Algorithm

A ballistically-limited cluster-cluster aggregation (BLCA) model was developed to simulate aerogelation processes. In the model, the clusters move along linear paths, in random directions, in a finite box. When two aggregates contact each other, they are combined irreversibly to form a larger aggregate. As expected, the simulations show that the aggregation time is much shorter than that obtained with diffusion-limited cluster-cluster aggregation (DLCA) models. The minimum concentration, c_g, required for gel formation scales as L^D–3, where L is the length of the sides of the box and D is the fractal dimension of the aggregates (D 1.95). For a concentration c larger than c_g, the mean free path of the aggregating clusters, , scales as c^–1.1. The pair correlation function g(r) and its Fourier transform S(q) were determined for the single large aggregates formed at the end of the simulations. These functions indicate that there is a characteristic length which scales as c^1/(D–3). As observed previously for the DLCA model, there is a discrepancy between the fractal dimensions obtained from g(r) and S(q).     

Zwitterionic [(H3NCH2C≡CC≡CCH2NH3)]Cu2Cl4 π-Complex: Template Synthesis and Crystal Structure

Crystals of the [(H₃NCH₂CCCCCH₂NH₃)]Cu₂Cl₄ -complex, formed in the oxidative dimerization of propargylammonium cations, were obtained by ac electrochemical synthesis from CuCl₂ · 2H₂O and propargylammonium chloride ([CCCH₂NH₃]Cl) and studied by X-ray diffraction analysis (DARCh automated diffractometer, MoK radiation, /2 scan mode; 2275 reflections with F 4(F), R = 0.048). Crystals are monoclinic: space group B2/b, a = 19.591(6) Å, b = 7.299(3) Å, c = 8.636(3) Å, = 106.83(3)°, Z = 4. The structure consists of individual [(H₃NCH₂CCCCCH₂NH₃)]Cu₂Cl₄ moietes united through the elongated Cu···Cl contacts (2.827(5) Å) into chains oriented along the [010] direction. The bond of the centrosymmetric propargylammonium dimer is -coordintated by copper(I) atom and is elongated to 1.227(6) Å.     

Electrical and Optical Diagnostics of Dielectric Barrier Discharges (DBD) in He and N2 for Polymer Treatment

Synchronous, real-time optical and electrical diagnostics have been carried out on dielectric barrier discharges in flowing gases (air, He, N₂) at atmospheric pressure. A true Atmospheric Pressure Glow Discharge (APGD) is observed in N₂ when O₂ and H₂ concentrations are below 500 ppm and 2500 ppm,respectively, and the APGD regime can be beneficially modified by suitably chosen dielectric coatings. X-ray photoelectron spectroscopy (XPS) analyses of some APGD-treated polymer surfaces are presented.     

Thermal decomposition of Cu(I) phosphine complexes

The thermal decomposition of Cu(I) phosphine complexes of the general types (CuXPPh₃)₄, [CuX(PPh₃)₂] and [CuX(PPh₃)₃] was investigated.The thermal decomposition of (CuXPPh₃)₄, where X denotes Cl^–, Br^–, I^–, NO ₃ ^– and PPh₃=P(C₆H₅)₃, occurs with formation of a phosphine oxide intermediate. For the remaining complexes this intermediate was not proved in the thermal decomposition.

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Zusammenfassung Die thermische Zersetzung der Cu(I)-Phosphinkomplexe vom allgemeinen Typ (CuXPPh₃)₄ und [CuX(PPh₃)₂], wie auch [CuX(PPh₃)₃] wurde untersucht. Die thermische Zersetzung von (CuXPPh₃)₄, wobei X=Cl^–, Br^–, I^– und NO ₃ ^– bedeutet und PPh₃=P(C₆H₅)₃, verläuft unter Bildung eines Phosphinoxid Zwischenproduktes, bei den übrigen Komplexen konnte dieses im Laufe der thermischen Zersetzung nicht nachgewiesen werden.

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Résumé On a étudié la décomposition thermique des complexes phosphine-Cu(I) de formule générale (CuXPPh₃)₄, [CuX(PPh₃)₂], [CuX(PPh₃)₃]. La décomposition thermique de (CuXPPh₃)₄, où X désigne Cl^–, Br^–, I^– et NO ₃ ^– , et PPh₃=P(C₆H₅)₃, s'effectue avec formation d'un oxyde de phosphine intermédiaire; avec les autres complexes, cet intermédiaire n'a pas été mis en évidence au cours de la décomposition thermique.

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(CuXPPb₃)₄ [CuX(PPh₃)₂] [CuX(PPh₃)₃]. (CuXPPh₃)₄, X=Cl^–, Br^–, I^–, NO ₃ ^– , PPh₃=(₆₅)₃ . .

     

Field and field-gradient polarizabilities of H2O

Using a charge-perturbation approach, all components of the polarizability tensors , , A, B, and C for H₂O have been evaluated at the SCF level and, with the exception of and B, at the MC SCF level of approximation. This is the first such comprehensive evaluation of the electrical properties of water.Member of the Ottawa-Carleton Chemistry Institute     

Free Radicals in the Photolysis and Radiolysis of Polymers: IV. Radicals in γ- and UV-Irradiated Wood and Lignin

Free radicals formed in the radiolysis and photolysis of wood and lignin were studied using X-band and D-band EPR measurements. It was found that singlet spectra at g 2, which appeared upon the low-temperature (77 K) - and UV irradiation of wood and lignin, or singlets detected in a posteffect on heating the irradiated samples belong to radicals having conjugated carbon–carbon bonds. Formyl radicals in -irradiated wood and peroxide radicals in - and UV-irradiated wood were detected for the first time using EPR spectra. The radiation-chemical and quantum yields of radical formation reactions were determined. In wood at 77 K, G _R 3.2 1/100 eV and _R 2 × 10^–3.