A configuration interaction study of phosphine using bonded functions

A computational study is made of the effect of basis set upon the energy, properties and inversion barrier of the phosphine molecule. The calculations are performed at both the SCF and CI level. The flexibility of the double zeta basis is discussed in the light of the results.     

Assembly of colloidal particles into microwires using an alternating electric field

We have investigated the dielectrophoretic assembly of colloidal gold, carbon black, and carbon nanotubes into electrical wires. The resulting microwires have diameters less than 1 microm, with lengths ranging from 5 microm to 3 mm. Current-voltage curves for these wires indicate an ohmic response, where the resistance is determined by the type of colloid and by the frequency of the alternating field used to grow the wires. The predicted frequency dependence of dielectrophoresis is confirmed by experiment. Measurements of the threshold voltage for initial wire growth are also presented. These experiments demonstrate that a variety of nanoparticles can be assembled into microwires for sensor applications.     

The effect of thermal processing on PVC—III. Photo-oxidation of unstabilized PVC

The rate of photo-oxidation of unstabilized PVC is markedly dependent on the severity of the processing operation. In mildly processed PVC, peroxy gel appears to be a very effective photo-initiator whereas in more severely processed polymer, solvent soluble polymer containing polyconjugation, hydroperoxides and carbonyl groups is primarily responsible for photo-initiation. The auto-retardation of photo-oxidation which occurs in all samples is believed to be due to the build-up of a highly absorbing surface layer of degraded polymer.     

Reductive spiroannulation of nitriles with secondary electrophiles

The scope of reductive decyanation and spiroannulation reactions has been expanded to include secondary electrophiles for potentially useful transformations. Secondary phosphates and chlorides, as well as terminal epoxides, cyclize in a stereospecific fashion. Both endo and exo modes of cyclization were observed with terminal epoxides.     

Classification of binary mass spectra of toxic compounds with an inductive expert system and comparison with SIMCA class modeling

The performance of an inexpensive, inductive rule-building expert shell system, based on the ID3 algorithm, was compared to that of SIMCA class modeling in classifying the binary mass spectra of 78 toxic and related compounds. The compressed mass spectra consisted of 17 masses chosen by using information theory. The expert rules verified the six main classes and two subclasses found with SIMCA class modeling. These classes were: all benzenes and all alkanes/ alkenes (alka(e)nes); nonhalobenzenes, chlorobenzenes, bromoalka(e)nes, and chloroalka(e)nes; and mono-, dichloroalka(e)nes and polychloroalka(e)nes. Training set classification accuracies obtained with the expert system were 93–100% as opposed to 62–98% for SIMCA. For 73 compounds, the expert rules gave a classification accuracy of 97–100% vs. 79–96% for SIMCA. Predictive accuracy for the four main classes was 78%. In general, fewer masses were involved with the rules than with the SIMCA models, and the rules are normally optimized with regard to minimum number of steps in the rule, not minimum number of variables. The expert rules work best with closed sets of objects where all possibilities can be included in the training sets. The expert rules represent planes partitioning the multidimensional measurement space (hypercube) into a subvolume nearest the SIMCA cylinders for an appropriate class. Overall, the performance of the expert system was very good.     

A comparison of neutron activation and alpha spectroscopy analyses of thorium in crystalline rocks

The concentrations of Th in samples of crystalline rock from three drillcore sections were analysed independently by instrumental neutron activation analysis and by chemical separation and alpha spectroscopy. The two methods show good general agreement over an approximate concentration range of 1 to 100 ppm Th. Variations in results between the two methods are not of a systematic nature and probably arise from sample heterogeneity. The results confirm the reliability of both methods and provide a useful comparison of the standards and reference materials used. The study indicates that, in cases where Th isotopic information is not required, the simpler and more rapid neutron activation analysis provides a satisfactory method.     

Probabilities of peak occurrence and information content of the NBS/EPA/MSDC mass-spectral data base

The probabilities of peak occurrence and contents of binary information were calculated for the 43 990 mass spectra in the 1987 NBS/EPA/MSDC data base. The median molecular weight of compounds in the data base was 230. Compounds composed of combinations of C, H, N, and O comprised 64% of the data base. The numbers of base peaks per mass channel are tabulated. A subset of compounds (30480) with low molecular weights was selected as a volatile-compound data base; the median molecular weight of this group was 189. The probabilities and information contents for the whole set of spectra and the volatile hydrocarbons, oxygenated hydrocarbons, chlorocarbons and chlorohydrocarbons and bromohydrocarbons were calculated. The most common peak in the entire data base and in the volatile set occurred at mass 41. All peaks in both of these sets of spectra with probabilities greater than 0.50 occurred below mass 78. The probabilities over the total and volatile-compound data base showed a general decrease with increasing mass channel with a division into odd- and even-mass curves which converged at high masses. Mass channels with 0.90–1.0 bit information content occurred below ca. mass 100. Information contents decreased with increasing mass and the two odd- and even-mass curves were superimposed on the general trend.     

Decarboxylative aldol reactions of allyl beta-keto esters via heterobimetallic catalysis

Mild and selective heterobimetallic-catalyzed decarboxylative aldol reactions involving allyl beta-keto esters have been developed. The reaction is promoted by Pd(0)- and Yb(III)-DIOP complexes at room temperature and involves the in situ formation of a ketone enolate from allyl beta-keto esters followed by addition of the enolate to aldehydes. The reaction is a new example of heterobimetallic catalysis in which the optimized reaction conditions require the addition of both metals.     

Ab initio stochastic optimization of conformational and many-body degrees of freedom

Moderate to large size molecules in solution have complex energy surfaces due to intramolecular (conformational) and intermolecular (many-body) interactions. The first principles Monte Carlo (FPMC) method, previously shown to effectively locate minimum-energy structures for systems with only many-body complexity, has been extended to address conformational flexibility by adding three new Monte Carlo move types. The primary advantage of the FPMC method is the ability to efficiently locate minimum energy structures of molecules with conformational flexibility in the presence of explicit solvent molecules using highly accurate quantum chemical calculations. The additions to FPMC were validated by studying conformers of glycerol, glyceraldehyde, and a large humic acid monomer unit. The structure of glyceraldehyde in the presence of one and two water molecules was also explored to demonstrate the power of FPMC to study systems with both conformational and many-body degrees of freedom.