The prediction of nucleophilic attacking sites via determinants of ‘active’ frontier and near-frontier orbitals

A method for computing the most favourable initial attacking site for a nucleophile on a transition metal complex is described, and applied to the nucleophilic substitution and addition reactions of fluoromethane, and the cationic complexes [BFe(CO)₃]⁺ (B = C₆H₇ and C₇H₉). The reactions considered are classified according to whether they are frontier or non-frontier orbitally or charge controlled. It is found that initial attack on the (polyenyl)M(CO)₃ cations is always predicted to occur at the M(CO)₃ moiety, in agreement with the experimental observation of intermediates in several such reactions and suggesting that the existence of these species is a general phenomenon.     

Routine analysis by high precision gas chromatography/mass selective detector/isotope ratio mass spectrometry to 0.1 parts per mil

Stable isotope methods are potentially quite useful for validating natural or enhanced mineral degradation of contaminants. For this reason, a continuous flow gas chromatograph (GC), isotope ratio mass spectrometer (IRMS) has been coupled with a quadrupole mass selective detector (MSD) to allow simultaneous mass spectral and stable carbon isotope ratio data to be obtained from a single chromatographic analysis. This allows the target contaminant and any extra-cellular degradation intermediates to be both qualified and quantified. Previously acceptable limits of precision (0.3 parts per mil) are undesirable given the small fractionation observed during aerobic degradation. To further understand the fate of organic contaminants and to gain information about the metabolic degradative pathway employed by a microorganism, routine isotopic analyses on a range of analytes have been performed. Quantities of sample producing mass-44 ion beam signal (I(44)) of 2 x 10(-10) to 1 x 10(-8) A were analysed. When the IRMS was tuned for high sensitivity, ion source nonlinearities were overcome by peak height correction from an algorithm that was produced using known isotopic standards of varying concentrations. This led to sample accuracy of <0.01 per thousand and sample precision of 0.1 per thousand. Copyright 1999 John Wiley & Sons, Ltd.     

The solubility of H2S in liquid sulfur

The need for fundamental data describing the dissolution of hydrogen sulfide, H₂S, in Claus recovered liquid sulfur prompted an examination of equilibrium H₂S solubility at typical industrial condensation temperatures and partial pressures. An FT/IR absorption technique has been described and new H₂S solubility measurements have been reported for partial H₂S pressures from 0.4 to 56 kPa and temperatures from 120 to 155 °C. The measurements were combined with previously reported values for atmospheric pressures of H₂S and used to calibrate a semi-empirical equation for the Henry's law solubility in liquid sulfur as a function of temperature. Estimations were compared to Claus plant field data and appear to underestimate the H₂S solubility for the first and second catalyst condenser stages. This underestimation was attributed to rapid condensation resulting in super-saturation of the recovered sulfur liquid, or an inaccurate measurement of the condensation temperatures.     

A Comparison of the Host–Guest Inclusion Complexes of 1,8-ANS and 2,6-ANS in Parent and Modified Cyclodextrins

The fluorescent molecules1-anilinonaphthalene-8-sulfonate (1,8-ANS) and2-anilinonaphthalene-6-sulfonate (2,6-ANS) areextremely sensitive to the polarity of their localenvironment, making them excellent probes for thestudy of heterogeneous systems, including cyclodextrin(CD) solutions. Both are only weakly fluorescent in ahighly polar medium, such as water, but are extremelyfluorescent in a relatively nonpolar medium, such aswithin a CD cavity. These two probes are isomers, withmajor structural differences: 1,8-ANS is much bulkierand more spherical, whereas 2,6-ANS is much morestreamlined and rod-shaped. Thus, they show majordifferences in their formation of CD inclusioncomplexes. This is reflected both in the magnitude ofthe observed fluorescence enhancement upon CDinclusion, as well as in the value of the associationconstant for complex formation. The creation of ascale for each probe for their fluorescence in CDsrelative to that in ethanol allows for directcomparisons to be made between the two probes. Theseresults are obtained and compared for the host-guestinclusion complexes of 1,8-ANS and 2,6-ANS with sixCDs: , , , and theirhydroxypropylated analogs.     

Calculations on substituted cyclopropenones and diphenylcyclopropenethione

Comparative CNDO/2 molecular orbital calculations are reported for cyclopropenone, dialkylcydopropenones, diphenylcyclopropenone and diphenylcyclopropenethione. The results are compared with ab initio calculations and with experimental studies. The effects of varying the alkyl groups (R = methyl, ethyl, n-propyl and n-butyl) are slight. The alkyl groups increase the absolute charge on the oxygen, decrease the absolute charges on the ring carbons and increase conjugation. In the diphenyl compounds, the oxygen and sulphur atoms are extremely negatively charged.     

Percolation theory and the conductivity of random close packed mixtures of hard spheres

The conductivity of random close packed mixtures of conducting and insulating spheres has been measured. The composition dependence and the critical percolation density are similar to what has been observed in crystalline systems.