Extrapolation of real-space quantum chemical calculations from finite-size super cells to the ideal infinite system. I. Theory

A method is proposed how to calculate the correct density matrix of an infinite polymeric chain from that of a standard finite supercell calculation. The density matrix of the finite supercell is transformed into k-space for all k-values allowed by the periodic boundary conditions. The k-dependent matrices are then unitarily transformed, with each unitary matrix being represented by a set of complex rotation matrices. It is shown that the corresponding angles can be interpolated and extrapolated toward the zone boundaries in a straghtforward manner and that this extrapolation can be done from any finite supercell with reasonable accuracy. This gives rise to an infinite system density matrix for which all fundamental properties are guaranteed by construction. This infinite system density matrix may be used to construct a corrected density matrix for the finite supercell calculation. © 1994 John Wiley & Sons, Inc.     

Continuum wavefunctions for morse molecules

The properties of continuum wavefunctions for the regions of Morse potentials above the dissociation limit are discussed in the context of photodissociation processes of diatomic molecules.     

Chemical ionization mass spectrometry of nitrosamines

The mass spectra of eight nitrosamines have been recorded, with excitation by chemical ionization (CI) and electron impact (EI). Comparison of the intensities of the base peaks under CI and El conditions gives intensity ratios in the range 1.4-1.9 for low resolution measurements and up to 10 for high resolution measurements, confirming the enhanced sensitivity available in the CI mode.     

Modification chimique des polyalcadiènes par les composés du type CX3Z—I Addition catalysée des composés CCl3Z sur les polybutadiènes-1,2

The influence of catalysis on chemical modification of 1,2-polybutadienes (M_n < 10,000) by CX₃Z reagents (X = halogen; Z = functional group or halogen), in order to obtain polymers soluble in common solvents, has been studied. The work has been carried out with carbon tetrachloride, trichloromethylphosphonyldichloride (CCl₃POCl₂) and ethyl trichloroacetate (CCl₃CO₂Et) in the presence of solvents such as C₆H₆, CH₂Cl₂. Among the compounds tested as catalysts for the addition of CX₃Z to 1,2-polybutadiene (transition metal salts and complexes; radical initiators) only the systems formed with metal salts such as CuCl₂, FeCl₃, RuCl₃ 3H₂O, Fe₂(CO)₉ and Cl₂Ru(PPh₃)₃ were found to show high catalytic activity. The most active is Cl₂Ru(PPh₃)₃: used without or with cocatalysts (Et₃NHCl/benzôin, alcohols), it leads always, to the best yields whatever the nature of CX₃Z. The activity of the other catalysts is generally dependent on CX₃Z structure. Some secondary reactions (cross-linking, cyclization, transesterification) have been found.     

über Katalysatoren- und Fermentmodelle (1)

Ohne Zusammenfassung Die Arbeit wurde aus Mitteln des Marshall-Planes (ERP-Mittel) und mit Spenden aus dem „Fonds Chemie“ ausgeführt. Wir sprechen den zust?ndigen Stellen für die Gew?hrung der Beihilfe unseren verbindlichsten Dank aus. Der Deutschen Forschungsgemeinschaft danken wir für die überlassung einer Ultrazentrifuge, den Behring-Werken für die uns zur Verfügung gestellten Serumalbumin- und -globulin-Pr?parate.     

Use of reference pools to compare the qualitative and quantitative determination of polychlorinated biphenyls by packed and capillary gas chromatography with electron capture detection. Part 1. Serum

Serum for reference pools of in vivo polychlorinated biphenyls (PCBs) was obtained from four goats that had received one dose (100 mg kg-1) of a selected technical Aroclor (AR) (1016, 1242, 1254 or 1260) and were allowed to recover for 30 d. These pools were used to assess the differences in an analytical method that determines and quantifies PCBs using packed-column gas chromatography (PCGC) (quantified on the basis of mean mass percent. data for grouped PCB peaks) and capillary-column gas chromatography (CCGC) (quantified on the basis of percent. composition data for specific congeners). With CCGC, results were statistically significantly different (p less than or equal to 0.0002) from results with PCGC for ARs 1016, 1242 and 1254 but not for AR 1260 (p = 0.23). When comparing these gas chromatographic methods using bovine serum spiked in vitro with the same ARs at 17-25 p.p.b., it was found that the methods were not statistically significantly different for any of the ARs (p = 0.30-0.92). Levels of serum PCB determined by the two methods for 12 persons, divided into two groups according to exposure, were compared using the paired t-test. Group 1 consisted of three persons with dietary and/or environmental exposure; one with dietary and/or environmental exposure in addition to occupational exposure dating back 20 years. Group 2 consisted of eight persons with recent occupational exposure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Building mass spectrometers and a philosophy of research

Development of the techniques of ion kinetic energy spectrometry and mass-analyzed ion kinetic energy spectrometry is described. The extension of these concepts to the method of tandem mass spectrometry for direct mixture analysis is traced, and a rationale for the construction of hybrid mass spectrometers is presented. Collisions of polyatomic ions with surfaces are discussed as an outgrowth of gaseous collisions. An attempt is made to describe a philosophy of research that guided the construction of a dozen mass spectrometers and the exploration of organic ion chemistry in as many contexts.     

Variable step molecular dynamics: An exploratory technique for peptides with fixed geometry

A new algorithm is presented for performing molecular dynamics simulations of peptides with fixed geometry, with the aim of simulating conformational changes and of exploring conformational space. The principle of the method is to expand the potential energy as a Taylor's series in the coordinates around the current point, retaining the force and its first two derivatives, and obtain a series solution of the resulting differential equations using a method due to Lyapunov. By choosing the time step so that the second term in the series is small compared to the first, the true solution can in principle be approximated to any desired degree of accuracy. The algorithm has been used to solve numerically Lagrange's equations of motion for N-acetyl alanine amide and N-acetyl methionide amide, regarded as fixed at their C-termini, under the influence of the ECEPP/2 potential energy function, and time steps of 15–30 fsec have been achieved with little variation in the total energy. Possible directions for future development are discussed.     

Circular dichroism—LXVIII compounds from valerian—XII: On the chiroptical properties of valepotriates and related compounds

The CD of saturated iridoids containing a 2.9-dioxatricyclo[4.3.1.03,7] decan skeleton derived from valepotriates is determined by the acetal chromophore. For the less symmetric ring system of 1,6-anhydro hexapyranoses the sign of this CD is predictable. The vicinal effect for enones and diketones in this series is appreciable. A new rule for the correlation between the CD around 190–205 nm of enol ethers and their absolute conformation is given; the same reasoning is applicable to explain the known rules for cisoid and transoid vinyl cyclo-propanes.     

Dispersion interaction between helium atoms

Accurate values for the coefficients of the R^?6, R^?8 and R^?10 in the series representation of the dispersion interaction between two helium atoms at distance R are obtained by a simple variation method.