The “Facilitated Transition” hypothesis as an explanation for the gemdialkyl effect

A new technique of searching the conformational space of transition states was used to explore the cause of the rate acceleration in the gem-dialkyl effect in intramolecular cyclization reactions. Several previous hypotheses were discarded and a new hypothesis was advanced based on this new data. This hypothesis, the “Facilitated Transition” hypothesis, states that increased steric hindrance reduces the overall activation energy by facilitating rotation through the transition state. The older “Reactive Rotamer” hypothesis was eliminated by generating all conformations of the starting materials using the and computer programs and demonstrating that no relationship is found between rate increase and a change in the concentration of “reactive rotamers”.     

Effectivity of solvents-A new approach in non-aqueous acid-base titrimetry

A new approach for selection of a suitable solvent system as a medium for non-aqueous acid—base titration is proposed. The essence of the approach is the development of a new criterion called “effectivity”. The latter is based on consequences of the Brønsted and Izmailov acid—base theories and represents a quantitative measure for improving or worsening the titration conditions of acids and bases in non-aqueous solvents as compared with water. The “effectivity” E is given by the relation E = ΔpK_s - ΔpK_s where ΔpK_s is the difference between the logarithmic values of the autoprotolysis constants of water and the solvent in question, and ΔpK is the so-called medium effect. The latter is a constant value which shows that acids and bases with the same charge alter their strength to the same extent when transferred from water into a non-aqueous solvent. The medium effect is calculated by statistical treatment of a great number of acid—base constants determined experimentally both in water and the non-aqueous solvent in question. The effectivity of the solvents most often used in non-aqueous acid—base titrimetry, determined by this approach, shows that in many cases these solvents offer significant advantages over water, but drawbacks are also observed. Some limitations of the approach are discussed. Special attention is paid to dimethylsulphoxide and its mixtures with water, which prove to be highly effective media for the acid—base titration of many substances.     

Zur nomenklatur der phane—II

The new term “Phanes” has been clearly defined and a nomenclature system tentatively developed. This system is comprehensible and of general application and at the same time relatively simple. The notations “nucleus”, “bridge”, “number of bridge members” and “number of ring members” are defined. In order to get a definite characterisation of the phanes which contain a carbocyclic nucleus, a carbocyclic and heterocyclic bridge the following terms: “carbophanes”, “carba-phanes” and “hetera-phanes” have been newly introduced. The prefix “hetera-” has been proposed as a general expression and as a representative term for the syllables “aza-”, “oxa-”, “thia-” and so on. The so called “a-nomenclature” is clearly called “hetera-nomenclature”. The new expressions “heteralogous” and “substitulogous” are explained. As the various examples will show, the “Phane-Nomenclature” can also be applied to the naming of complicated metallocenophanes.     

Albumin binding and insertion into PS-b-PEO monolayers at air-water interface

Interaction of human serum albumin with poly(styrene)-b-poly(ethylene oxide) (PS-b-PEO) monolayer at air/solution interface was studied by measuring surface pressure. The density of PEO chains in the monolayer was controlled using Langmuir trough barriers. The thickness of PS-b-PEO monolayer prior to and after albumin adsorption was computed from in situ surface plasmon resonance (SPR) measurements. Depending on the initial PEO surface density the surface pressure kinetics of albumin insertion displayed two different regimes: below the PEO “pancake-brush” transition albumin binding was initially very rapid and itself induced the “pancake-brush” transition in the monolayer, and above the “pancake-brush” transition where some albumin penetration into the free PS-b-PEO monolayer still occurred into the PEO “brush”. In the case of SPR-immobilized monolayer, more than 0.1 PEO chain/nm² was required to inhibit albumin or ferritin adsorption. A half-reduction of albumin adsorption required approx. three-fold higher PEO surface density than the half-reduction of ferritin adsorption.     

Intramolecular hydrogen bonding and calixarene-like structures in p-cresol/formaldehyde resins

The nature of the strong hydrogen bonds found in p-cresol/formaldehyde (PCF) resins, compared to ordinary phenolic compounds, is studied. The evidence from FTIR spectroscopy indicates that this strong interaction is due to intramolecular hydrogen bonding from “calixarene-like” structures. The formation of this structure in PCF is enabled by its “linear” (all-ortho-linkage) structure, which is not present in branched resins. Additionally, a transition is observed at around 175 to 200°C where the intramolecular hydrogen bonded structure is lost. This structure cannot be recovered upon cooling or annealing due to restrictions on conformational rotations that are coupled to a new pattern of intermolecular hydrogen bonding. However, the structure is reformed by dissolving the resin in solution and casting new films.     

Calorimetric glass transitions in the amorphous forms of water: a comparison

The calorimetric glass transition behaviour in the amorphous forms of water is reviewed: for a heating rate of 30 K min⁻¹ the onset temperature, or T_g, of the glass transition is 136±1 K for hyperquenched glassy water and annealed vapour-deposited amorphous solid water, and 129±1 K for the low-density form of pressure-amorphized hexagonal ice. The increase in heat capacity in the glass transition region is between 1.6–2 J K⁻ mol⁻ for the three amorphous forms. Annealing of the samples a few degrees below T_g or heating several degrees above the glass transition region has no influence on the onset temperatures at 136 K and 129 K respectively, which is contrary to ‘normal’ behaviour. The results are discussed with respect to the ‘structure’ of the three amorphous forms of water below the glass transition region and a “gel-like” state of water above T_g.     

Transition metal complexes with long-chain amines thermal behaviour and crystal structure of (n-CaH2a+1NH2)2ZnCl2

The thermal behaviour of (n-C_aH_2n+1NH₂)₂ZnCl₂ complexes with n = 6, 8, … 16 has been investigated by DSC and by temperature variable IR and X-ray powder diffraction techniques. Complexes with n = 12,14,16 show solid—solid phase transition which are “melting” transitions of the hydrocarbon regions of the structure. The crystal structure of both the low and the high temperature polymorphs is characterized by the piling of sandwiches, each formed by an “inorganic” layer sandwiched between two alkylammonium layers.     

Semiempirical molecular orbital studies on p-quinodimethane. Electronic structure assignments based on total energies and comparisons with photoelectron and absorption spectra

The electronic structure of p-quinodimethane has been investigated using both CNDO/S and INDO molecular orbital approximations. It is found that the energetically favorable configuration is a “quinoid” construction leading to a spin-paired singlet ground state. Comparisons of the calculated excitation energies and orbital orderings with optical and photoemission measurements are consistent with this assignment. The “quinoid” configuration is found to be energetically unfavorable toward the formation of a low-lying triplet or “biradical”-like state. Charge density distributions, however, suggest a high ground state chemical reactivity.     

Optical potential theory of diatom-diatom scattering. II. Indistinguishable molecules

The theory of vibro-rotational energy exchange in “indistinguishable” diatom-diatom scattering is formulated in terms of effective potentials. Due the necessity of considering both “symmetric” and “antisymmetric” molecular two-particle states, the formalism has a characteristic (two-by-two matrix structure. A mathematical generalization of Schwinger's theory of sources allows a compact derivation of exact and approximate expressions for t “optical” and “transition” potentials of elastic and inelastic processes, respectively. Finally, considerations based on a partial wave analysi of the working equations, suggest that the present theory should be more readily amenable to numerical implementations than the close coupling approach     

Co-aggregation of fullerene C60 and thiophene in the non-aromatic solvent cyclohexene

Co-aggregation of fullerene C₆₀ and thiophene has been studied calorimetrically in cyclohexene at 25 °C. The total aggregation heat is found to depend on initial concentration of thiophene and fall between −1.9 and −5.8 kJ mol⁻¹. The corresponding thiophene/fullerene molar ratio (“co-aggregation number”) ranges from 7 to 12. The data are rationalized by formation of heteromolecular nanoaggregates with intermolecular contacts of both fullerene–thiophene and fullerene–fullerene types. A physical model describing interaction between fullerenes and π-donors in solution is substantiated and used to explain heterogeneity of composites containing fullerenes.     

Inversion of states in two conformers of 1,8-bis(dimethylamino) naphthalene—the proton sponge

The S₁↔S₀ transitions of two conformers of 1,8-bis(dimethylamino) naphthalene, the “proton sponge”, have been studied by semiempirical AM1 calculations. They reveal that “inversion of states” occurs in the asymmetric conformer DMAN-2, which in the gas phase may be emitted from the ¹L_a state in comparison to the ¹L_b state in symmetric DMAN-1. It was also concluded that because of the mixed character of the HOMO-1 orbital in both conformers, a certain CT contribution to the S₀↔S₁ transition has to be taken into account. The calculated maxima of absorption and emission have been compared to those experimentally obtained in supersonic expansion.     

Are pore size distributions in microfiltration membranes measurable by two-phase flow porosimetry?

The issue of evaluating equivalent pore diameter distributions in membrane microfilters from gas-liquid (g-l) porosimetry data has been critically examined. Experiments performed with one isotropic and one composite anisotropic membrane in both possible orientations revealed conspicous dependence of the obtained (g-l) porosimetry peaks on imposed pressure ramp rates, p. Interference of this kinetic effect can be eliminated from the measured data by extrapolation to p = 0. The ramp rate effect is most likely caused by tortuous pore length distribution, and relatively long times required for liquid expulsion. For two experiments, the observed effects of p could be reconciled with predictions of the Schlesinger-Bechhold theory [Bechold et al., Kolloid Z., 55 (1931) 172–198]. The data obtained with the thin top layer of the composite membrane facing intruding air directly did deviate somewhat from the theory. Pores characterized by (g-l) porosimetry are likely of the “throat type”, and their size distribution is considerably more narrow than that obtained for the “node-type” pores by SEM-image analysis [Zeman and Denault, J. Membrane Sci., 71 (1992) 221–231]. A single bivariate distribution function was constructed for these two distinct pore populations. Flow-weighted or number fraction distributions can be calculated from the extrapolated porosimetry data. For narrow ranges of “throat” diameters, these distributions are fairly similar.     

In Process Citation

Analyses for the metals Cd, Pd and Cu in the estuary and bay of the Scine have been made. The site was chosen because of the very high tides, which produce considerable water displacement. For the purpose of analysis the metals have been divided into three categories: free metals, metals forming complexes that are unstable at low pH, and metals forming stable complexes that are destroyed in an acidic medium by ultraviolet radiation. The special properties of copper have been used to establish a “complexing capacity” of the different water samples studied. By making the simple hypothesis of the existence of an equilibrium of the type Cu + L CuL, where L represents the ligand, the total ligand concentration, as well as the stability constants of the CuL complexes, has been determined. These results are presented and discussed.     

“Hole-burning spectra”. A new method for obtaining fine structure in absorption spectra of organic molecules

A new method is proposed for obtaining fine structure in broad-band absorption spectra of organic molecules in solid solutions at low temperature. This method is based on the laser burning of a series of narrow gaps in inhomogeneously broadened absorption spectra. A special spectral set-up has been developed for the measurement of the differential spectrum (a “hole-burning spectrum”) of “burnt” and “unburnt” samples. As an example the hole-burning spectrum of Zn-tetraphenylporphine in ethanol has been considered.     

Nanogram detection of m-dinitroaromatics and their derivatives A re-evaluation of the spot-test based on the janovsky reaction

The spot-test for m-dinitroaromatics and their derivatives, based on the Janovsky reaction, is re-investigated. The sensitivity of the spot-test has been much enhanced by changing the solvent and using the novel “pellet” spot-test. With the “pellet” spot-test and dimethylsulphoxide, 4–50 ng of m-dinitroaromatic compounds and their derivatives can be detected. The change from acetone (used in the original spot-test) to dimethylsulphoxide changes the reaction product in most cases.     

Kosmotrope character of maltose in water mixtures

It is well known that water plays a fundamental role for living beings, because the nature of water transformations provides for the ability to preserve biostructures. Solute can be classified as “kosmotropes” or “chaotropes” depending on the interaction strength with water. In the case of solutes destroying the natural hydrogen bonded network of water, called “kosmotropes” or “structure-makers”, the denaturation processes can be inhibited.

The aim of this work is to investigate the vibrational behaviour of maltose/H₂O mixtures in order to characterise the changes induced by the sugar on the H₂O hydrogen-bonded network. The obtained findings point out that maltose has a destructuring effect on the water tetrahedral network and emphasise its kosmotrope character.     

The enthalpy of formation of IrO2 and thermodynamic functions

The Gibbs energy of formation of IrO₂(s) has been measured by means of oxygen dissociation pressure measurements, and by EMF measurements using ZrO₂ (+ CaO) as the solid electrolyte. In addition, high-temperature enthalpy increments of IrO₂ have ben measured from 416 to 940 K using a drop calorimeter. A “third law” evaluation of the experimental results and data from literature has been made. For the enthalpy of formation of IrO₂(s) the value ΔH°_f (298.15 K) - −(59.60 ± 0.03) kcal mole⁻¹ has been selected. The thermodynamic functions of IrO₂(s) have been calculated in the temperature range 298–1200 K.     

The radiation resistance of the bioburden from medical devices

An extensive study of the radiation resistance of microbial species constituting the bioburden of a number of different medical devices obtained from Japanese medical device manufacturers has been carried out. A standard protocol for determining radiation resistance was used and validated at the fourteen centres involved in the study. Individual microbial isolates from the bioburden obtained from seven different devices manufactured in these centres were studied. A total of 3742 unselected isolates were obtained, of which 197 failed to survive long enough for subsequent radiation resistance studies. The remainder were subjected to an initial screen test to identify those organisms that were sensitive to the lethal effects of radiation with a D₁₀ of < 1.5kGy. The 465 isolates that survived the screen doses were then tested for survival in an incremental series of radiation doses using methods similar to those of Whitby (1979) and Yan and Tallentire(1995). The isolates from “dry” devices were more resistant than those obtained from the one water filled (“wet”) device studied. The overall distribution of radiation resistance among the isolates was considered to be similar to that forming the “Standard Distribution of Resistance” (SDR) included in the ISO International Standard 11137 “Sterilization of Health Care Products — Requirements for validation and routine control — Radiation sterilization”.     

Can triorganoboroxins exist in a “monomeric” R---B=O form? MNDO calculations and ebulliometric molecular weight determination

MNDO calculations were made for triethylboroxin (EtBO)₃ and triphenylboroxin (PhBO)₃ using both X-ray determined and optimized geometry of these molecules. The results were compared with hypothetical “monomeric” molecules R---B=O. Calculated energies of trimerization are about −200 kJ mol⁻¹ for both compounds and confirm the much higher stability of the “trimer”. Ebulliometric determination of molecular weight of triphenylboroxin in 2-pentanone confirms its trimeric character.