Electronic rearrangements during chemical reactions. II. Planar dissociation of ethylene

The direct dissociation of ethylene into two methylenes is studied along the least motion reaction path by means of an ab initio multiconfiguration self-consistent-field (MCSCF ) calculation. All eight configurations arising from those valence orbitals that form the CC bonds, seven of them singlet coupled and one triplet coupled, are taken into account. The HCH bond angle is optimized along the entire reaction path. Separate MCSCF optimizations are carried through for the lowest two states of ¹A_g symmetry. The (¹A_gσ²π²) ethylene ground state dissociates into two (³B₁σπ) ground-state methylenes. The (¹A_gσ²π*²) excited state of ethylene dissociates into two (¹A₁σ²) excited methylenes. It is established that both these dissociations proceed without any barrier in the energy curve. In the ground state, where orbital symmetry is conserved, the π-bond breaks before the σ-bond, and the calculated heat of reaction agrees within 6 kcal/mol with the experimental value. In the excited state, where orbital symmetry is not conserved, the nonbonded repulsion between methylene σ₂ lone pairs is found to blend into the antibonding character of the excited ethylene, yielding an energy curve that is everywhere repulsive. However, the variation of the HCH angle during the dissociation process is not simple, initially it expands and subsequently it contracts. Quantitative analytical approaches are developed which furnish conceptual interpretations of the orbital changes and configurational changes along the reaction path.     

Variegated micelle surfaces: correlating the microstructure of mixed surfactant micelles with bulk solution properties

Electron paramagnetic resonance, viscosity, and small-angle neutron scattering (SANS) measurements have been used to study the interaction of mixed anionic/nonionic surfactant micelles with the polyampholytic protein gelatin. Sodium dodecyl sulfate (SDS) and the nonionic surfactant dodecylmalono-bis-N-methylglucamide (C12BNMG) were chosen as "interacting" and "noninteracting" surfactants, respectively; SDS micelles bind strongly to gelatin but C12BNMG micelles do not. Further, the two surfactants interact synergistically in the absence of the gelatin. The effects of total surfactant concentration and surfactant mole fraction have been investigated. Previous work (Griffiths et al. Langmuir 2000, 16 (26), 9983-9990) has shown that above a critical solution mole fraction, mixed micelles bind to gelatin. This critical mole fraction corresponds to a micelle surface that has no displaceable water (Griffiths et al. J. Phys. Chem. B 2001, 105 (31), 7465). On binding of the mixed micelle, the bulk solution viscosity increases, with the viscosity-surfactant concentration behavior being strongly dependent on the solution surfactant mole fraction. The viscosity at a stoichiometry of approximately one micelle per gelatin molecule observed in SDS-rich mixtures scales with the surface area of the micelle occupied by the interacting surfactant, SDS. Below the critical solution mole fraction, there is no significant increase in viscosity with increasing surfactant concentration. Further, the SANS behavior of the gelatin/mixed surfactant systems below the critical micelle mole fraction can be described as a simple summation of those arising from the separate gelatin and binary mixed surfactant micelles. By contrast, for systems above the critical micelle mole fraction, the SANS data cannot be described by such a simple approach. No signature from any unperturbed gelatin could be detected in the gelatin/mixed surfactant system. The gelatin scattering is very similar in form to the surfactant scattering, confirming the widely accepted picture that the polymer "wraps" around the micelle surface. The gelatin scattering in the presence of deuterated surfactants is insensitive to the micelle composition provided the composition is above the critical value, suggesting that the viscosity enhancement observed arises from the number and strength of the micelle-polymer contact points rather than the gelatin conformation per se.     

Novel intramolecular cyclopropanation reaction of unsaturated beta-keto esters

[reaction: see text] Fused cyclopropane beta-keto esters are versatile intermediates for the synthesis of many biologically active natural products. Here we report a new intramolecular cyclopropanation reaction of unsaturated beta-keto esters. In the presence of I(2), Et(3)N, and Lewis acids such as Mg(ClO(4))(2) and Yb(OTf)(3), beta-keto esters 1 bearing various olefin substituents were transformed to fused cyclopropanes 2 in a highly stereospecific manner with moderate to good yields. The mechanism of the reaction was also investigated.     

The determination of sulphur and phosphorus by cool-flame chemiluminescence emission spectrometry with an electrothermal atomiser for sample introduction

The technique described allows simple and rapid determination of sulphur and phosphorus in solutions of samples and sample digests. A graphite-tube atomisation device is employed for the introduction of microlitre volumes of liquid samples, after desolvation, into a cool argon—hydrogen entrained air flame in which the chemiluminescent emission from S₂ and HPO species is monitored by using a wide band-pass monochromator system.     

Film and intraparticle mass transfer during the adsorption of metal ions onto bone char

The sorption of three metal ions, namely, copper, cadmium, and zinc, onto bone char has been studied in terms of equilibrium and rate studies. Equilibrium studies have been analyzed using the Langmuir isotherm equation and the maximum sorption capacities for the metals were 0.477, 0.709, and 0.505 mmolg(-1) bone char for cadmium, copper, and zinc ions, respectively. The kinetic experimental data were used to analyze the effect of external film boundary layer and intraparticle mass transfer resistance on the sorption process and its significance. Four methods of determining the external film transport coefficient were developed and tested; three utilized experimental data to obtain the coefficient and the fourth method was completely empirical. The three experimentally based models give very similar results and consequently similar values of the deviation error values, whereas the error values for the empirical correlation were greater than these three values. The results also demonstrated that the methods for determining the film coefficient could be integrated into more complex diffusion-transport models such as film-intraparticle diffusion processes.     

Synthetic studies on potential metabolites of carcinogenic aza aromatic hydrocarbons. Part II. Non-k-region oxidised and reduced derivatives of 7-methylbenz[c]acridine

Two phenolic derivatives of the weakly carcinogenic aza aromatic hydrocarbon 7-methylbenz[c]acridine were prepared as reference compounds for studies of the mammalian metabolism of the carcinogen. Also, 1, 2, 3, 4- and 8, 9, 10, 11-tetrahydro-7-methylbenz[c]acridine were made as intermediates for synthetic studies directed toward preparation of dihydrodiols of 7-methylbenz[c]acridine.