Parametrization and comparative analysis of the BFGS optimization algorithm for the determination of optimum linear coefficients

This paper examines the numerical performance of the BFGS variable-metric algorithm and compares this performance with that of the DFP , SR1 , and OC algorithms. Numerical results indicate that the BFGS algorithm is far superior to the DFP and SR1 algorithms and comparable to the OC algorithm. Although the BFGS algorithm offers a viable method for the direct determination of localized molecular orbital coefficients, the method is not yet competitive with the more traditional methods.     

Detection of benzaldehyde and formaldehyde in the UV photolysis of gas-phase methyl benzoate

The excitation of gas-phase methyl benzoate at 240 nm leads to the observation of phosphorescence. The dispersed phosphorescence spectrum has an assigned origin of 25 270 cm⁻¹ and a prominent C=O progression of 1720 cm⁻¹, consistent with literature reports of gas-phase benzaldehyde spectroscopy. Weaker bands, which correspond to formaldehyde ν₁₇ and ν₂₅, are also evident. Time-resolved IR diode laser absorption spectroscopy has been used to probe formaldehyde. Excitation of methyl benzoate at 222 nm clearly indicates the generation of formaldehyde as a photoproduct. The temporal profile of the formaldehyde signal is consistent with significant nascent vibrational excitation in this product. The ratio of formaldehyde initially in the ground vibrational state to that in the excited vibrational states is estimated to be 0.6 ± 0.1. The proposed elimination mechanisms are analogous to those postulated for the formation of CO₂ and acetaldehyde from pyruvic acid.     

Pyrene-derived novel one- and two-component organogelators

A new class of alkyl-chain-appended pyrene derivatives 4-14 were synthesized and evaluated for their gelation abilities. Depending on the nature of the linking group, these compounds gelated a number of organic solvents, either in the presence or in the absence of the acceptor molecule 2,4,7-trinitrofluorenone (TNF). Compounds with ester, ether, or alkyl linkages gelated a number of hydroxylic and hydrocarbon solvents by means of a charge-transfer interaction with TNF, while compounds with amide, urethane and urea linkers formed gels on their own in a variety of solvents by means of pi-pi stacking and hydrogen-bonding interactions. The Xray crystal structure of urethane (S)-12 showed hydrogen-bonding and stacking features, as suggested by the model. The gels obtained were investigated by spectroscopic and electron microscopic techniques which provided structural insights.     

Comparison of gas chromatography-mass spectrometry and capillary electrophoresis in analysis of phenolic compounds extracted from solid matrices with pressurized hot water

Self-constructed pressurized hot water extraction (PHWE) equipment was used in dynamic mode to extract spiked phenolic compounds (phenol, 3-methylphenol, 4-chloro-3-methylphenol and 3,4-dichlorophenol) from sea sand and soil. Phenols were analyzed by both gas chromatography-mass spectrometry (GC-MS) and capillary zone electrophoresis (CZE) to compare the techniques and to find out if CZE is a suitable tool for analysis of phenols extracted from environmental matrix. Good recoveries of phenols spiked in sea sand were achieved at all PHWE temperatures (50, 100, 200, 300 C). GC-MS studies showed that phenols were selectively extracted from soil at 50 C but various other compounds (e.g. polyaromatic hydrocarbons) were extracted along with the phenols at 300 degrees C. In the case of CZE, phenols extracted from the soil, at 300 C were separated with good resolution at pH 9.7, and co-extracted compounds did not interfere with the analysis. The analytical values obtained by GC-MS and CZE were generally of similar magnitude.     

Electron-withdrawing substituents decrease the electrophilicity of the carbonyl carbon. An investigation with the aid of (13)C NMR chemical shifts,nu(C[double bond]O) frequency values,charge densities,and isodesmic reactions to interpret substituent effects on reactivity

(13)C NMR chemical shifts and nu(C[double bond]O) frequencies have been measured for several series of phenyl- or acyl-substituted phenyl acetates and for acyl-substituted methyl acetates to investigate the substituent-induced changes in the electrophilic character of the carbonyl carbon. Charge density, bond order, and energy calculations have also been performed. The spectroscopic and charge density results indicate that opposite to the conventional thinking, electron-withdrawing substituents do not increase the electrophilicity of the carbonyl carbon but instead decrease it. On the other hand, reaction energies of the isodesmic reactions designed show that electron-withdrawing substituents destabilize the carbonyl derivatives investigated. So, a significant ground-state destabilization of carboxylic acid esters, and carbonyl compounds in general, due to the decreased resonance stabilization, is proposed as a novel concept to explain both the increase in their reactivity and the changes in the chemical shifts and carbonyl frequencies induced by electron-withdrawing substituents.     

Alkali metal mediated resorcarene capsules: an ESI-FTICRMS study on gas-phase structure and cation binding of tetraethyl resorcarene and its per-methylated derivative

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) with additional ab initio calculations were used to examine the alkali metal cation binding selectivity (i.e., molecular recognition) and host properties of tetraethyl resorcarene (1) and its per-methylated derivative (2). The significance of intramolecular hydrogen bonding for the crown conformation was demonstrated. The presence of intramolecular flip-flop hydrogen bonding in 1 was confirmed both with calculations and in ND3-exchange experiments. All the alkali metal cations formed host-guest complexes by docking inside the cavity of the host. Complexation with the larger cations, especially Cs+, was favored. All the alkali metal cations also formed dimeric resorcarene capsules with 1. The capsules were directly H-bonded species, with no linking solvent molecules. ND3-exchange experiments and molecular modeling revealed the significance of direct intermolecular H-bonding for the crown conformation of 1 and stability of the capsule structure.     

Propagation of polar substituent effects in 1-(substituted phenyl)-6,7-dimethoxy-3,4-dihydro- and -1,2,3,4-tetrahydroisoquinolines as explained by resonance polarization concept

[structures: see text] Propagation of inductive and resonance effects of phenyl substituents within 1-(substituted phenyl)-6,7-dimethoxy-3,4-dihydro- and -1,2,3,4-tetrahydroisoquinolines were studied with the aid of 13C and 15N NMR chemical shifts and ab initio calculations. The substituent-induced changes in the chemical shift (SCS) were correlated with a dual substituent parameter equation. The contributions of conjugative (rhoR) and nonconjugative effects (rhoF) were analyzed, and mapping of the substituent-induced changes is given over the entire isoquinoline moiety for both series. The experimental results can be rationalized with the aid of the resonance polarization concept. This means the consideration of the substituent-sensitive balance of different resonance structures, i.e., electron delocalization, and the effect of the aromatic ring substituents on their relative contributions. With tetrahydroisoquinolines, the delocalization of the nitrogen lone pair (stereoelectronic effect) particularly contributes. Correlation analysis of the Mulliken atomic charges for the dihydroisoquinoline derivatives was also performed. The results support the concept of the substituent-sensitive polarization of the isoquinoline moiety even if the polarization pattern achieved via the NMR approach is not quite the same as that predicted by the computational charges. Previously the concepts of localized pi-polarization and extended polarization have been used to explain polar substituent effects within aromatic side-chain derivatives. We consider that the resonance polarization model effectively contributes to the understanding of the polar substituent effects.     

Determination of dynamic and sliding friction, and observation of stick-slip phenomenon on compacted polymer powders during high-velocity compaction

Dynamic friction, sliding friction, and the stick-slip phenomenon have been studied on compacted polymer powders during high-velocity compaction. It is particularly important from a practical point of view to distinguish the stick-slip mechanism and the sliding mechanism which occur concurrently. A practical experimental system has been successfully developed to study the dry frictional force and to measure the sliding coefficient between the polymer powder particles and the die wall during high-velocity compaction. Two new components have been introduced as relaxation assists to improve the compaction process by reducing the frictional forces. It was found that the relaxation assist device leads to an improvement in the polymer powder compaction process by giving a more homogeneous opposite velocity and a better locking of the powder bed in the compacted form with less change in dimensions. The subsequent movement of the particles can be reduced and the powder bed attains a higher density with a minimum total elastic spring-back. The relative time of the stick-slip phenomenon during the compacting stage is also reduced so that the time needed to transfer from an intermittent stick-slip state to a smooth sliding state is reduced and the powder bed slides smoothly. It was found that the dynamic, dry frictional force is intermittent (stick-slip mechanism) at low compaction rates but that at high compaction rates is becomes more smooth (sliding mechanism). Both mechanisms depend on the nature of the powder and on the compaction conditions. At the beginning of the compaction stage, the sliding coefficient decreases due to an increase in the radial to axial stress ratio until the maximum pressure has been reached. During the reorganization stage, more time is needed for large particles to move, rotate and slide due to their relatively large diameter and mass. As a result, the reorganization stage is extended and the stick-slip phenomenon is observed more with increasing particle size. Much better transfer of the pressure throughout the powder bed and less loss of pressure lead to a higher sliding coefficient due to the overall friction during the compaction process. It was found that the sliding coefficient is proportional to the density.

A more homogeneous density distribution in the compacted powder and a smaller pressure loss during compaction has a major influence on the sliding coefficient and on the quality of the compacted material.     

AB Initio molecular orbital calculations on the si2h4molecule

Ab initio molecular orbital calculations using an extended Gaussian basis set have been performed on C₂H₄, CH₂SiH₂ and Si₂H₄. The species CH₂ and SiH₂ have also been examined. Geometries were partially optimized and the energy difference between the planar singlet and orthogonal or twist triplet geometries of Si₂H₄ was studied in order to provide a measure of the strength of the Si-Si bond in this molecule. Mulliken population analyses were carried out on CH₂CH₂ and SiH₂SiH₂, to further study the nature of the Si-Si double bond in comparison with the C—C double bond.