Model studies of the chemisorption of hydrogen and oxygen on the Au (1 0 0) surface

 The convergence of chemisorption energy for hydrogen and oxygen on gold clusters is studied. Two theoretical approaches have been employed; wavefunction methods at the self-consistent-field second–order M?ller–Plesset level and density functional theory and the two methods are compared. Relativistic effective core potentials exploited in the former approach were developed in this work. Received: 25 October 1999 / Accepted: 21 February 2001 / Published online: 11 October 2001     

Fully relativistic correlated benchmark results for uranyl and a critical look at relativistic effective core potentials for uranium

 A complete set of fully-relativistic benchmark results for the bond lengths and vibrational frequencies of uranyl at various levels of correlation treatment are presented. It is shown that the relativistic and correlation effects are of the same magnitude and should be treated on an equal footing. Results of uranyl calculations using various relativistic effective core potentials (RECP) are presented. Appropriate choices for RECPs for use in density functional theory (DFT) calculations in the local density approximation (LDA) and with the gradient-corrected B3LYP exchange-correlation functional are discussed. The conclusion is reached that small-core RECPs need to be used and that the best results as compared to the benchmark values are obtained by using a DFT functional that includes some fraction of Hartree-Fock Exchange. Received: 18 May 2001 / Accepted: 25 July 2001 / Published online: 30 October 2001     

Extracting parameters for base-pair level models of DNA from molecular dynamics simulations

 A method is described to extract a complete set of sequence-dependent energy parameters for a rigid base-pair model of DNA from molecular dynamics (MD) simulations. The method is properly consistent with equilibrium statistical mechanics and leads to effective inertia parameters for the base-pair units as well as stacking and stiffness parameters for the base-pair junctions. We give explicit formulas that yield a complete set of base-pair model parameters in terms of equilibrium averages that can be estimated from a time series generated in an MD simulation. The expressions to be averaged depend strongly both on the choice of coordinates used to describe rigid-body orientations and on the choice of strain measures at each junction. Received: 12 July 2000 / Accepted: 5 January 2001 / Published online: 3 May 2001     

Study of association of 2-methoxyethanol in the aqueous phase

 Monte Carlo simulations have been carried out for 2-methoxyethanol in an isothermal–isobaric ensemble (NPT) at 298.15 K and 1 atm pressure. The optimized potential for liquid simulation force field parameters has been used for modeling 2-methoxyethanol and the TIP4P model for water. Intramolecular rotations are described by an analytical potential function fitted to ab initio energies. It has been shown that the water molecules can form hydrogen bonds between adjacent O atoms of CH₃OCH₂CH₂OH in aqueous media. The self-association of 2-methoxyethanol in aqueous media has been studied by statistical perturbation theory. Received: 9 October 2000 / Accepted: 5 January 2001 / Published online: 3 May 2001     

Molecular Schrödinger–Riccati calculations. Test for the hydrogen-ion molecule

 The hydrogen-ion molecule has been used as a test system for the application of the Schr?dinger–Riccati formulation to molecular calculations. Some of the points discussed are the characteristics (quasiconstancy of the local energies, size, number of points) of the sampling region to be chosen, the dependence on the starting function, the precision of the calculations, and the excellent behaviour of the predicted function (by comparison to an accurate function). Received: 1 May 2001 / Accepted: 25 July 2001 / Published online: 11 October 2001     

Multi-Component Reactions of Aldehydes,Amines, and Ketones I. Synthesis and Crystal Structure Elucidation of a Novel Product of Condensation of Salicylaldehyde, 1-Methyl-4-piperidone,and Methylamine

Summary.  The condensation of salicylaldehyde, 1-methyl-4-piperidone, and methylamine has been studied as a model reaction. It was found that at a molar ratio of salicylaldehyde: 1-methyl-4-piperidone = 2:1 and an excess of methylamine the previously unknown heterocyclic compound 2-(3,6,13-trimethyl-1,2,3,4,4a,5,6,7-octahydro-7,12a-epiminopyrido[4,3-b][1,5]benzoxazocin-5-yl)-phenol was formed; its structure was elucidated by spectroscopic methods and X-ray analysis. A reaction pathway for this new multi-component condensation is suggested and discussed. Received August 20, 2001. Accepted (revised) November 15, 2001     

Release of cationic drugs from loaded clay minerals

The adsorption of promethazine chloride [10-(2-dimethylammonium propyl) fenothiazine chloride] and buformin hydrochloride (1-butylbiguanidine chloride) on montmorillonite was studied in previous work. The present article focuses on the desorption of these molecules from their organocomplexes in a medium of artificial intestinal juice (pH 7.0 ± 0.1) at the temperature of the human body (37 ± 0.5 °C). The desorption was investigated by kinetic studies, basal spacing measurements and Fourier transform IR studies. Important quantitative differences were observed: buformin, which adsorbed in a monolayer coverage, exhibited a very high desorption rate, whereas promethazine formed a pseudotrilayer arrangement and showed a lower dissolution rate. Received: 20 January 2001 Accepted: 8 March 2001     

Bader's interatomic surfaces are unique

 In this work we comment on the statement about the nonuniqueness of the solution of Bader's equation for defining atoms in molecules reported in the article of P. Cassam-Chena? and D. Jayatilaka in Theoretical Chemistry Accounts (2001) 105: 213–218 Received: 10 May 2001 / Accepted: 7 September 2001 / Published online: 3 June 2002     

NMR chemical shifts in the low–pH form of a-chymotrypsin. A QM/MM and ONIOM–NMR study

 The relationship between hydrogen bonding and NMR chemical shifts in the catalytic triad of low-pH α-chymotrypsin is investigated by combined use of the effective fragment potential [(2001) J Phys Chem A 105:293] and ONIOM–NMR [(2000) Chem Phys Lett 317:589] methods. Our study shows that while the His57 N^δ1−H bond is stretched by a relatively modest amount (to about 1.060 ?) this lengthening, combined with the polarization due to the molecular environment, is sufficient to explain the experimentally observed chemical shifts of 18.2 ppm. Furthermore, the unusual down-field shift of H^ɛ1 (9.2 ppm) observed experimentally is reproduced and shown to be induced by interactions with the C=O group of Ser214 as previously postulated. The free-energy cost of moving H^δ1 from His57 to Asp102 is predicted to be 5.5 kcal/mol. Received: 26 September 2001 / Accepted: 6 September 2002 / Published online: 21 January 2003 Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 222nd National Meeting of the American Chemical Society, 2001 Correspondence to: J. H. Jensen e-mail: jan-jensen@uiowa.edu Acknowledgements. This work was supported by a Research Innovation Award from the Research Corporation and a type G starter grant from the Petroleum Research Fund. The calculations were performed on IBM RS/6000 workstations obtained through a CRIF grant from the NSF (CHE-9974502) and on supercomputers at the National Center for Supercomputer Applications at Urbana-Champaign. The authors are indebted to Visvaldas Kairys for help with the CHARMM program, and to Daniel Quinn for many helpful discussions.     

Carbon blacks as dissipative colloid systems with universal properties

As a new approach the stationary size distribution of particles and aggregates of particles of carbon blacks is described with an extended version of the model of reversible aggregation. Computer simulations of aggregation in dissipative systems lead to ensembles the reduced size distributions of which are at not too short times identical with the universal stationary distribution of the aggregation model. A crucial general point seems to be that structural fluctuations activated by continuously running chemical reactions including viscous dissipation exhibits the same statistics as it is typical for thermally activated aggregation. This leads to the suggestion that the production of carbon blacks at very high temperatures or the reorganization during the so-called CAB-procedure should show a statistically analogous fluctuation-dissipation behaviour as thermally activated aggregation. The ensemble entropy is, in any case, rapidly maximized. In the reactor, for example, a sequence of generations with quasi-stationary ensemble structures is finally built up. Dissipation-fluctuation, including highly cooperative phenomena, guarantees a unique and optimized stationary, thermodynamically “essentially entropy controlled” colloid-structure. Universality is impressively verified by the finding that the particle and aggregate size distributions of different carbon blacks belong to the same “class” so that their reduced representations fall altogether on a master curve. Consequences of our interpretation are discussed. Received: 17 April 2001 Accepted: 1 August 2001     

A quartz crystal microbalance sensor for the detection of formic acid vapors

A new quartz crystal microbalance sensor is developed to determine formic acid at low concentrations. Four previously selected polymers with acid–base characteristics were tested as possible coatings. Polyoxyethylene bis [amine] presented the best results. The sensor is rapid, sensitive [0.67 Hz/(mg/m³)], and reversible at low concentrations. The detection limit for formic acid (7.2 mg/m³) is comparable with the short term exposure limit and the threshold limit values. It presents a fast mechanical response to pressure changes, so that it can be quickly used in different environments and situations. The sensor also shows a good stability in a temperature range typical of work atmospheres (16–36 °C). It has a wide linear range (7.2–911.2 mg/m³) and a long useful time. It is also applicable to other low molecular mass carboxylic acids such as acetic acid. Received: 26 September 2000 / Revised: 8 February 2001 / Accepted: 12 February 2001     

Monte Carlo simulation of 2-ethoxyethanol in continuum configurational biased procedure: conformational analysis and association in aqueous and non-aqueous media

 Monte Carlo simulations have been carried out for 2-ethoxyethanol (C₂E₁) in isothermal-isobaric ensemble (NPT) at different temperatures and 1 atm pressure with a continuum configurational biased procedure in water and chloroform media. Hydrogen bond bridges were formed between adjacent oxygen atoms in C₂E₁ (CH₃CH₂OCH₂CH₂OH) through water molecules. We also found that the stable conformers of C₂E₁ in water and CHCl₃ are different and the effect of temperature on solute-solvent interaction energies is considerable. The self-association of C₂E₁ in aqueous and nonaqueous media has been studied by statistical perturbation theory, and the relative free energy has been obtained at different reaction coordinates by double-wide sampling method. Two minima were found in water solvent in the potential of mean force (PMF), corresponding to the contact and solvent-separated pairs, but only one minimum was found in CHCl₃ solvent corresponding to a contact pair complex. Received: 18 January 2001 / Accepted: 22 October 2001 / Published online: 21 January 2002     

Role of electronegativity in the qualitative inference of the TOF–SIMS fragment pattern of inorganic compounds

The role of the electronegativity of atoms in inorganic compounds in TOF–SIMS fragmentation is discussed. From a study of approximately 30 inorganic compounds – chlorides, oxides, nitrates, and sulfates – a simple rule has been proposed for the dependence of fragment pattern appearance on the electronegativity (electron affinity), which can be easily obtained from handbooks, and the valence of positive and negative ions in these compounds. TOF–SIMS measurements of metal and alloy surfaces, should be corrected for the ionization potentials and/or electronegativities of atoms present in surface contaminants. Received: 9 October 2000 / Revised: 26 February 2001 / Accepted: 1 March 2001     

Modeling the H5+ potential-energy surface: a first attempt

 Ab initio molecular electronic structure calculations are performed for H₅ ⁺ at the QCISD(T) level of theory, using a correlation-consistent quadruple-zeta basis set. Structures, vibrational frequencies and thermochemical properties are evaluated for ten stationary points of the H₅ ⁺ hypersurface and are compared with previous calculations. The features of the H₃ ⁺…H₂ interaction at intermediate and large intermolecular distances are also investigated. Furthermore, an analytical functional form for the potential-energy surface of H₅ ⁺ is derived using a first-order diatomics-in-molecule perturbation theory approach. Its topology is found to be qualitatively correct for the short-range interaction region. Received: 15 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001     

Interlaboratory comparisons and improvements of methods for acid number determination in used motor oils

The results of two interlaboratory comparisons of acid number determinations in used motor oils are discussed. It is shown that the comparability of the measurement results is not as good as that required by known standards for petroleum products. The problem is motor oil contaminants which accumulated during use, and which are the source of a matrix effect in the acid number determination. The standard methods’ drawbacks are analyzed and some improvements are proposed. Repeatability and accuracy of the improved methods are evaluated. Received: 11 December 2001 Accepted: 15 February 2002     

Comparison of various analytical techniques for homogeneity test of candidate standard reference materials

Three different analytical techniques: INAA, X-ray fluorescence and atomic absorption spectrometry were used at the JRC Ispra to test the total homogeneity of four candidate botanic reference materials supplied by NBS. Without adopting complex statistical formalities a “degree of inhomogeneity” was estimated in each sample for the elements considered in the test. The operating procedures and the results obtained are presented and discussed.     

LabRespond: a tool for autoverification

Autoverification is defined as the process by which a computer performs the initial verification of test results. Any data that fall outside set parameters are then reviewed by the technologist [1]. LabRespond is an open system with an integral approach to control which monitors all the phases of the laboratory process. In particular, the statistical-based multi-two-dimensional test ”frequency of occurrence” has a high detection rate for suspicious test results. The content of LabRespond, the validation rules and, in particular, the test ”frequency of occurrence” is mainly the work of members of the Dutch Working Group of Clinical Chemometrics¹(¹Members of the Dutch Working Group of Clinical Chemometrics are: Marcel Volmer, Martin van der Horst, Remi Wulkan, Kees van Dongen, Wytze Oosterhuis, Herman Ulenkate and Henk Goldschmidt.) and the staff of the laboratory of the Saint Elisabeth Hospital in Tilburg. Received: 30 July 2002 Accepted: 7 August 2002 Presented at the European Conference on Quality in the Spotlight in Medical Laboratories, 7–9 October 2001, Antwerp, Belgium     

Function and performance of silicone copolymer (VII). Dispersing ability of hydrophile-grafted acrylic copolymers and the corresponding siloxane copolymers to fumed silica. The effect of pH

Acrylic copolymers and the corresponding siloxane copolymers grafted with cationic and nonionic hydrophiles were used as dispersants to disperse fumed silica in an aqueous solution at different pH values. The dispersing ability was evaluated by viscosity and scanning electron microscopy (SEM) methods. The results showed that the dispersing abilities are functions of the dispersant concentrations and of the pH of the system. By comparing the results of SEM and viscosity methods, it can be concluded that, under comparable conditions, a suspension with a lower viscosity is more homogenously dispersed. It was also found that at pH 2 only CHE and SHE and that at pH 7 only CHE, SHE, and SQHE showed better dispersing ability. At pH 10.5, all the dispersants in this study except CQDEA resulted in a slurry of very low viscosity of 10–20 cP with 15 wt% of silica. Basically, nonionic dispersants CHE and SHE exhibit excellent dispersing ability at different pH values. Received: 16 March 2001 Accepted: 28 June 2001     

Endohedral chemical shifts in higher fullerenes with 72–86 carbon atoms

For all isolated pentagon isomers of the fullerenes C₆₀–C₈₆ with nonzero HOMO–LUMO gap and for one nonclassical C₇₂ isomer (C_{2 v} ), endohedral chemical shifts have been computed at the GIAO-SCF/3-21G level using B3LYP/6-31G* optimized structures. The experimental ³He NMR signals are reproduced reasonably well in cases where assignments are unambiguous (e.g. C₆₀, C₇₀ and C₇₆). On the basis of the calculated thermodynamic stability order and the comparison between the computed and experimental ³He chemical shifts, the assignments of the observed ³He NMR spectra are discussed for all higher fullerenes, and new assignments are proposed for one C₈₂ and one C₈₆ isomer (C₈₂:3 and C₈₆:17). The calculated helium chemical shifts also suggest the reassignment of the δ(³He) resonances of two C₇₈ isomers. Received: 26 March 2001 / Accepted: 10 May 2001 / Published online: 11 October 2001