Nonlocal density functional calculation of gas phase heats of formation

A general procedure is presented for computing the gas phase heats of formation of a wide variety of organic compounds. ΔE for the formation of the molecule from its elements at 0 K is obtained from density functional calculations (Gaussian 92/DFT) for optimized geometries. This result is converted to ΔH at 298 K by assuming ideal behavior and adding the translational, rotational, and vibrational energies. Additive correction terms corresponding to the various coordination states of the carbons, nitrogens, and oxygens were developed using a database of 54 compounds. The experimental ΔH values of these compounds are then reproduced with an average absolute error of 3 kcal/mol and a standard deviation of 4 kcal/mol. For a group of 10 test cases that were not part of the database, the average absolute error is 3.5 kcal/mol and the standard deviation is 4.1 kcal/mol. © 1995 by John Wiley & Sons, Inc.     

Inclusion compounds of psychotropic agents and cyclodextrins

Literature data on inclusion compounds of psychotropic agents (hypno-sedative, anticonvulsive and antiepileptic drugs, neuroleptics and anxiolytics) with cyclodextrins and their derivatives are presented. Inclusion compounds of two novel psychotropic agents, gidazepam and cinazepam, with-cyclodextrin (1 : 1 and 1 : 2) were obtained. The structure of these complexes has been established by one- and two-dimensional¹H-NMR (for solutions in DMSO) and IR (for solid state) spectroscopy.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August, 1995.     

Assessing the balance between protein-protein interactions and enzyme-substrate interactions in the channeling of intermediates between polyketide synthase modules.

6-Deoxyerythronolide B synthase (DEBS) is the modular polyketide synthase (PKS) that catalyzes the biosynthesis of 6-deoxyerythronolide B (6-dEB), the aglycon precursor of the antibiotic erythromycin. The biosynthesis of 6-dEB exemplifies the extraordinary substrate- and stereo-selectivity of this family of multifunctional enzymes. Paradoxically, DEBS has been shown to be an attractive scaffold for combinatorial biosynthesis, indicating that its constituent modules are also very tolerant of unnatural substrates. By interrogating individual modules of DEBS with a panel of diketides activated as N-acetylcysteamine (NAC) thioesters, it was recently shown that individual modules have a marked ability to discriminate among certain diastereomeric diketides. However, since free NAC thioesters were used as substrates in these studies, the modules were primed by a diffusive process, which precluded involvement of the covalent, substrate-channeling mechanism by which enzyme-bound intermediates are directly transferred from one module to the next in a multimodular PKS. Recent evidence pointing to a pivotal role for protein-protein interactions in the substrate-channeling mechanism has prompted us to develop novel assays to reassess the steady-state kinetic parameters of individual DEBS modules when primed in a more "natural" channeling mode by the same panel of diketide substrates used earlier. Here we describe these assays and use them to quantify the kinetic benefit of linker-mediated substrate channeling in a modular PKS. This benefit can be substantial, especially for intrinsically poor substrates. Examples are presented where the k(cat) of a module for a given diketide substrate increases >100-fold when the substrate is presented to the module in a channeling mode as opposed to a diffusive mode. However, the substrate specificity profiles for individual modules are conserved regardless of the mode of presentation. By highlighting how substrate channeling can allow PKS modules to effectively accept and process intrinsically poor substrates, these studies provide a rational basis for examining the enormous untapped potential for combinatorial biosynthesis via module rearrangement.     

Coulometric determination of the flow rate of small masses of substances II. Application to on-line measurement of the EOX-parameter

 Today’s microprocessor-controlled coulometric systems can generate complex functions obtained from the response of the microcoulometric cell. A previously developed procedure for determining the flow-rate (mass per unit time of detected species) has been accomplished by integrating, in the software, the full time dependence of the reaction of the coulometric cell and their electrodes, as well as of the titration curve. This leads to a time dependence of the measured signal, which has a fast response time without any delay. The titration curves are determined by means of various procedures. The applicability of the method is demonstrated for the case of the continuous analysis of the EOX-parameter (Extractable Organic Halogens X=Cl, Br, I) by coupling the coulometric detection with a FIA system. Results of continuous measurements of aqueous samples containing different chlorinated compounds as analytes are presented by taking into account different flow velocities in the FIA system as well as different measuring times. Recovery yields of the FIA system better than 70% have been obtained. Received: 7 November 1995/Revised: 15 May 1996/Accepted: 18 May 1996     

High-resolution/higher-order derivative spectrophotometry for determination of ternary mixtures of B-complex vitamins in pharmaceuticals

A method for the determination of ternary mixtures of the B-complex vitamins B6, B1 and B12, by fifth-derivative spectrophotometry is described. The procedure works without any separation step or solving of equations. Beer's law has been valid up to 100 μg/mL for each compound. Detection limits at p=0.01 level of significance have been calculated to 0.009, 0.054 and 0.005 μg/mL and determination limits of 0.027, 0.19 and 0.017 μg/mL have been found for the respective vitamin. The procedure has been successfully applied to the simultaneous determination of vitamins B6, B1 and B12 in synthetic mixtures and in commercial injections and tablets for these drugs (without other vitamins) with a high reliability and repeatability. Received: 14 December 1994/Revised: 3 February 1995/Accepted: 9 February 1995     

A variational density matrix approach with nonlocal effective potential

We show that using the Colle–Salvetti correlation-energy functional (Colle and Salvetti in Theoret Chim Acta 37:329, 1975) in the Hartree–Fock-type procedure suggested by Kohn and Sham (Phys Rev 140:A1133PR, 1965), one can calculate quite accurately electronic properties of systems in which the “dynamical” correlation energy is dominant. We compare our results with those obtained by Grabo and Gross (Chem Phys Lett 240:141, 1995) using the optimized effective potential method, and we discuss characteristics and advantages of our procedure.     

An efficient approach to calculation of zero-flux atomic surfaces and generation of atomic integration data

A new robust method for variational determination of atomic zero-flux surfaces is presented. The zero-flux surface sheets are expressed in terms of variational trial functions in prolate spheroidal coordinates. The trial functions are optimized with a Newton procedure to satisfy the zero-flux condition on a grid. The data required for radial integrations are generated by an adaptive quadrature procedure that employs model electron densities and utilizes an original third-order algorithm for linear search. Results of test calculations involving variational determination of atomic surfaces are presented for a representative set of 20 molecules. The new approach is both less time consuming and substantially more accurate than the previously published algorithms. © 1995 John Wiley & Sons, Inc.     

Problems of validation of computerised instruments for accredited chemical laboratories

 Some problems of validation of computerised instruments are reviewed briefly, taking essential standards and guides into account. The significant role of certified standard reference materials is underlined. An attitude of suppliers towards the validation of instruments is presented, and producers' responsibilities and obligations are discussed. The "black-box" concept is recommended as a preliminary step for the validation of computerised instruments. Two examples for gel permeation chromatography are given that illustrate a bad manufacturer's practice (BMP) and good manufacturer's practice (GMP). In the case of BMP, a need is expressed for a guide and for regulations that should be implemented into the quality assurance system. It has been proposed that the EURACHEM/VAM draft of guidance for qualification/validation of instruments should be amended by incorporating the "black-box" approach as a preliminary procedure for validation of computerised instruments, a retrospective validation procedure if the need for current validation was not foreseen or not specified, and a procedure (or selection rules) for qualification of the supplier. Moreover, the mechanisms of inspection to control the observance of the standardised rules and commonly recognised recommendations should also be considered by international quality organisations. Received: 19 November 1996 · Accepted: 20 March 1997     

A MASSive Laboratory Tour. An Interactive Mass Spectrometry Outreach Activity for Children

It is imperative to fascinate young children at an early stage in their education for the analytical sciences. The exposure of the public to mass spectrometry presently increases rapidly through the common media. Outreach activities can take advantage of this exposure and employ mass spectrometry as an exquisite example of an analytical science in which children can be fascinated. The presented teaching modules introduce children to mass spectrometry and give them the opportunity to experience a modern research laboratory. The modules are highly adaptable and can be applied to young children from the age of 6 to 14 y. In an interactive tour, the students explore three major scientific concepts related to mass spectrometry; the building blocks of matter, charged particle manipulation by electrostatic fields, and analyte identification by mass analysis. Also, the students carry out a mass spectrometry experiment and learn to interpret the resulting mass spectra. The multistage, inquiry-based tour contains flexible methods, which teach the students current-day research techniques and possible applications to real research topics. Besides the scientific concepts, laboratory safety and hygiene are stressed and the students are enthused for the analytical sciences by participating in “hands-on” work. The presented modules have repeatedly been successfully employed during laboratory open days. They are also found to be extremely suitable for (early) high school science classes during laboratory visit-focused field trips.

Certification of reference materials by European Commission services

The programmes of the European Commission for the certification of reference materials are presented: the Standards, Measurement and Testing Programme (formerly BCR Programme) funds research and development aiming at improved ability to measure and at improved comparability of measurements; reference materials result from many of its projects; the Institute for Reference Materials and Measurements (formerly CBNM) prepares some of these, stores nearly all of them, will distribute them from 1995 on and certifies some additional ones. The principles adopted for the certification of BCR reference materials are explained. The procedure for a typical matrix reference material is that a group of laboratories first refines their methods (e.g. by intercomparisons and technical discussion) until they achieve agreement of results to within random experimental error and then analyses the reference material; certified value and uncertainty are calculated on the basis of their results. Recent achievements and projects in progress are listed.     

Semidirect algorithms for third-order electron propagator calculations

An efficient procedure for third-order electron propagator calculations of ionization energies and electron affinities is reported. Diagonal self-energy expressions that are suitable for large molecules are empolyed. The outer-valence Green's function method also is implemented. An integral transformation program for direct and semidirect algorithms is modified to store only nonzero integrals according to Abelian point group symmetry. Contributions to self-energy matrix elements that depend on electron repulsion integrals with four virtual orbital indices are computed in a direct way. Intermediate batches of integrals are created by sort procedures while avoiding storage of transformed integrals in the main memory. This method permits calculation of electron binding energies for C with a 231 atomic orbital basis and for Zn(C₅H₅)₂ with a 220 atomic orbital basis on an IBM RISC/6000 Model 550. During these calculations, the CPU is engaged approximately 90% of the time. © 1995 John Wiley & Sons, Inc.     

Ko-LABSUE: a quasi non-dependent data format package fork o-quantification

Thek _o-standardization method has been extensively used in the Pierre Süe laboratory during the last few years. We had developed two different new programs which both used parts of a commercial package (Canberra or Nuclear Data), for example: isotope parameter editor, automatic peak search, nuclide identification. However, further adaptation of these packages is not easy, due to the specific data handling of the two programs. In order to overcome these constraints, we decided to write a completely new program, Ko-LABSUE, which is as independent of the manufacturer specifications as possible. All of the different steps of thek _o-quantification (automatic peak search, nuclide identification, calculation of elemental concentration etc.) are processed by modules written in TURBO-PASCAL. Only two unit modules have to be re-written to process manufacturer data files of different formats: the modules used to read the spectrum data and the detector efficiency curve. This program runs on an IBM-PC and is currently on -test in our laboratory.     

Evaluating the state-of-the-practice in soil measurements in relation to environmental regulations

 The results of chemical analyses of soils show large variations due to the heterogeneity of the material, differences in pretreatment and analytical methods, and unavoidable random errors. In this article, the analytical aspects in the determination of heavy metals in soils are highlighted. Both within-laboratory and between-laboratory variation must be minimized by validation and standardization procedures, respectively, in order to keep the variations resulting from the analytical procedure at an acceptably low level. Even then, the best results will show uncertainties of 5–20%, an issue that should be recognized by governmental and legal bodies. Received: 9 November 1995 Accepted: 1 December 1995     

Density functional theory for systems of very many atoms

The standard Kohn-Sham formulation of density functional theory (DFT ) is limited, for practical reasons, to systems of less than about 50-100 atoms. The computational effort scales as N, where N_at is the number of atoms and 2 < α > 3. (By comparison, conventional configuration interaction methods are limited to 5-10 atom systems.) This article deals with the prospect of practical methods that scale linearly in N_at and may thus allow calculations for systems of 10³-104 atoms. The physical reason (“near-sightedness”) for linear scaling is presented. Implementations of linear scaling DFT by the use of generalized Wannier functions or the one-particle density matrix are discussed. © 1995 John Wiley & Sons, Inc.     

Determination of Elemental Sulfur in Bottom Sediments Using High-Performance Liquid Chromatography

A procedure for determining elemental sulfur in bottom sediments by microcolumn reversed-phase high-performance liquid chromatography was developed. The analytical range was 4–1200 g/g (in terms of the dry weight of a sediment). The procedure is based on the direct injection of acetone extracts of sediments into a chromatographic column. The detection limit was 5 ng/peak (signal-to-noise ratio of 3 : 1); the relative standard deviation was 5.6%. Errors introduced at particular stages of analysis and the total errors were evaluated for different sampling techniques. The results of determining elemental sulfur in the core samples of bottom sediments from Lake Baikal are presented.     

Electron-probe microanalysis of light elements in multiphase diffusion couples

Wavelength-dispersive electron-probe microanalysis of transition metal carbide and nitride diffusion couples was investigated as a tool to quantitatively determine portions of phase diagrams. In crosssections, diffusion couples generally show a sequence of several phases composed of crystallites with different orientations and hardness values. Thus, the polishing procedure can adversely affect the accuracy of the analysis due to creation of an uneven surface (relief effect). In addition, as a result of the polishing process the edges of the samples are rounded so that the concentration at the original sample surface is difficult to measure. The most rapid and straightforward procedure to detect influences of the relief effect is a plot of the non-normalized analytical total vs. distance, even where the concentration profile is discontinuous at the interface between phase bands. It is shown that careful experimental preparation, as well as the application of restricted diffusion geometry, has several advantages for establishing phase diagrams.Presented at the EMAS'95 (1995 Meeting of the European Microbeam Analysis Society), May 14–19, 1995, St. Malo, France. Microstructures of this work are accessible under the URL: http://info.tuwien.ac.at/physmet/images.htm     

Evaluating uncertainty in analytical measurements: the pursuit of correctness

 Simple in principle, the evaluation of uncertainty, especially in chemical analysis, is not a routine task and needs great care to be correct. This can be seen, particularly, from an examination of the EURACHEM Guide, Quantifying Uncertainty in Analytical Measurement (1995), which is the most important document on the subject. The examination reveals, in the author's opinion, a shortage of correctness in some principal details of the uncertainty estimation process as presented in worked examples in the Guide, and the author has therefore formulated some "in pursuit of correctness" rules for estimating uncertainty. The rules and respective comments are concerned with the following items: (1) choosing an appropriate distribution function in type B evaluation of uncertainty, (2) the necessity for consideration of separate contributions to the combined uncertainty, and (3) taking account of actual influence factors in the uncertainty estimation process. Furthermore, the problem of estimation of conditional versus overall uncertainty is touched upon in connection with comparative trials where only internal consistency of results is required. Received: 29 January 1998 · Accepted: 10 February 1998     

Measurements of Heat of Zeolite Dehydration by Scanning Heating

A procedure for measurement of the heat of zeolite dehydration by scanning heating has been designed. Simultaneous data on heat flow (DSC) and mass loss (TG) are required for evaluation. The heating rate depends on the experimental conditions (point-spread function, sample mass, crucible design, and calorimetric reproducibility). Dehydration measurements have three advantages as compared with the sorption procedure: i) one can investigate samples with irreversible dehydration; ii) no approximation model is needed for calculation of the partial molar heat of dehydration; and iii) the procedure is not labor-consuming.The procedure was tested on the natural zeolites heulandite, chabazite and mordenite. The results are close to those measured by the sorption procedure. The partial molar heat of dehydration was found to depend on the water content. It increases from 50 to 87 J mol^–1 K^–1 for heulandite, from 53 to 81 J mol^–1 K^–1 for chabazite, and from 51 to 71 J mol^–1 K^–1 for mordenite.The approximation of the heat of sorption by linear regression was found to be wrong. Detection of a phase transitioN after this approximation has no meaning.This revised version was published online in November 2005 with corrections to the Cover Date.     

Hybrid iodoperfluoroalkane-ferrocene supramolecular arrays: the shortest contacts iodine forms with nitrogen atoms and unsaturated moieties

α,ω-Diiodoperfluoralkanes self-assemble in solution and in the solid phase with (S)-(−)-N,N-dimethyl-1-ferrocenylethylamine. The formed adducts have been characterized by DSC, IR, Raman, and NMR techniques. Single crystal X-ray analysis of one of the obtained adducts shows that perfluorinated and organometallic modules alternate giving rise to infinite one-dimensional (1D) chains. The interacting modules are connected through strong n-type halogen bondings, involving nitrogen and iodine atoms, and weak π-type halogen bondings, involving Cp rings and iodine atoms. Both interactions are the shortest reported to now in their respective classes. Theoretical calculations have been performed to quantify the observed π-type halogen bonding. For the first time a metallocene-perfluorocarbon (PFC) hybrid material is described.     

Atomic radii scales and electron properties deduced from the charge density

A procedure to represent Hartree-Fock electron densities in atoms [L. Fernandez Pacios, J. Comp. Chem., 14 , 410 (1993)] defines ρ(r) as a reduced expansion of exponential functions. These analytically modeled densities (AMDs) are used in this article to develop a simple computational procedure for analyzing different atomic radii scales implemented in the commercial software system MATHEMATICA. The analysis is focused on the physical information associated to a given atomic radius as deduced from calculations depending on ρ(r). The amount of electron charge contained in the sphere of the given radius as well as the distinct contributions to the potential energy integrated up to that radius are obtained within the AMD formulation for main-group atoms H—Kr. The ASCII file needed to run the procedure within MATHEMATICA is also presented. © 1995 by John Wiley & Sons, Inc.