SOME PITFALLS IN THE USE OF DERIVATIVE SPECTRA

Abstract— Several difficulties are described in the use of derivative spectra: increase in the number of apparent bands, disproportionate band heights and difficulties associated with numerical differentiation.     

Differential signal detection system for improved analysis of overlapping signals in liquid chromatography

We have developed a differential signal detection system for the analysis of liquid chromatographic signals using two or more detectors and a differential amplifier circuit. The proposed detection system is an improvement on conventional chromatography in which signals are detected by means of a single detector. The differential signal detection system eliminates difficulties of isolating minor components of the signal which are masked by the major component, as well as difficulties of separating two components of the signal having similar retention times.     

Uncertainty of measurement. Twenty years afterwards

The application of the concept "uncertainty" causes considerable difficulties. In this paper an analysis of the intrinsic contradictions of the concept itself and its relationship with the statements of metrology is given with the aim of establishing possible reasons for these difficulties. As a result of this analysis several examples are presented to demonstrate the conflicts of the concept and its several statements in the general fundamental notions of metrology.     

分子模拟在超临界流体领域中的应用

"分子模拟作为一种全新的研究手段已经在化学、化工、材料、生物等领域受到了广泛的关注。特别是在超临界流体技术方面,其应用不但可以获得许多结构、传递以及热力学信息,而且可以部分取代昂贵的超临界实验。本文从超临界流体在主体相和受限空间中的特性以及超临界萃取、反应等几个方面对现有的分子模拟方法和成果进行总结与评述,并对进一步发展的方向和面临的困难进行了讨论。     

Remarks on the molecular mechanical calculations of functionalized hydrocarbons

Molecular mechanical calculations for hetero-substituted hydrocarbons present certain difficulties that are not encountered in dealing with hydrocarbons. Ways are proposed to overcome such difficulties, sometimes by two-step or iterative computation. The following topics are considered: fitting the force field by using data from studies in solution; ways to estimate atomic charges or, alternatively, bond moments; ways to account for field effects upon atomic charges and bond moments.     

Critical analysis and extension of the Hirshfeld atoms in molecules

The computational approach to the Hirshfeld [Theor. Chim. Acta 44, 129 (1977)] atom in a molecule is critically investigated, and several difficulties are highlighted. It is shown that these difficulties are mitigated by an alternative, iterative version, of the Hirshfeld partitioning procedure. The iterative scheme ensures that the Hirshfeld definition represents a mathematically proper information entropy, allows the Hirshfeld approach to be used for charged molecules, eliminates arbitrariness in the choice of the promolecule, and increases the magnitudes of the charges. The resulting "Hirshfeld-I charges" correlate well with electrostatic potential derived atomic charges.     

A New Synthetic Route to Selected Indenones

Indenones are useful intermediates in the synthesis of several natural products, e.g. gibberellins (1) and steroid hormones with a C-nor-D-home system (2). Despite the nernerous methods for indenone synthesized, or the yields was much lower as mentioned in earlier reports (1)(4). One of the difficulties of the usual methods is the required electrophilic cyclization meta to a methoxyl substiruent. House (5) deals with these difficulties in more detail. In our method the synthesis of 6-methoxyinde-nones is effected by substitution dara to methoxyl function.     

Problems of the calculation of intermolecular interactions on complexes with strong delocalized π-bonds within the PCILO framework

Strong delocalized π-bonds must be approximated by localized π-bonds within the PCILO framework. From this approximation some difficulties result in the calculation of intermolecular interactions. If only one subsystem involves delocalized π-bonds the difficulties seem to be not very important. But if both subsystems involve such bonds, the PCILO results are not reasonable in some cases.     

Uncertainty of measurement. Twenty years afterwards

The application of the concept “uncertainty” causes considerable difficulties. In this paper an analysis of the intrinsic contradictions of the concept itself and its relationship with the statements of metrology is given with the aim of establishing possible reasons for these difficulties. As a result of this analysis several examples are presented to demonstrate the conflicts of the concept and its several statements in the general fundamental notions of metrology. Received: 16 February 2001 / Revised: 2 April 2001 / Accepted: 9 April 2001     

Comment nonexistence of difficulties with the graph theory of aromaticity

It is stressed that, at present, there are no difficulties with the graph theory of aromaticity as long as a careful definition of resonance energy is used as a measure of aromatic stabilization.     

Controlled pore formation in organotrialkoxysilane-derived hybrids: from aerogels to hierarchically porous monoliths

Porous polysilsesquioxane gels derived from sol-gel systems based on trifunctional silanes are reviewed. Although it is well known that trifunctional silanes possess inherent difficulties in forming homogeneous gels, increasing attention is being paid on these precursors and resultant porous polysilsesquioxanes because of hydrophobicity, functionality, and versatile mechanical properties. Much effort has been made to overcome the difficulties for homogeneous gelation, and a number of excellent porous materials with various pore properties have been explored. In this critical review, we put special emphasis on the formation of a well-defined macroporous structure by making use of phase separation, which in turn is a serious problem in obtaining homogeneous gels though. Porous polysilsesquioxane monoliths with the hierarchical structure and transparent aerogels with high mechanical durability are particularly highlighted (169 references).     

Difficulties with topological resonance energy

Some difficulties with topological resonance energy (TRE) are indicated. A strict application of the TRE concept pre-diets cyclobutasdiene to be significantly more aromatic than benzene. The reasons for such an obscure result are analysed. Some algebraic properties of a new graphic polynomial are derived.     

A novel approach to the retrieval of structural and dynamic information from residual dipolar couplings using several oriented media in biomolecular NMR spectroscopy

The interpretation of residual dipolar couplings in terms of molecular properties of interest is complicated because of difficulties in separating structural and dynamic effects as well as the need to estimate alignment tensor parameters a priori. An approach is introduced here that allows many of these difficulties to be circumvented when data are acquired in multiple alignment media. The method allows the simultaneous extraction of both structural and dynamic information directly from the residual dipolar coupling data, in favorable cases even in the complete absence of prior structural knowledge. Application to the protein ubiquitin indicates greater amplitudes of internal motion than expected from traditional (15)N spin relaxation analysis.     

Estimation of rate constants from growth and decay data

Many experiments in chemical kinetics are initiated by a fast pulse, such as electric discharge, shock wave, flash lamp, or laser. After this pulse one observes the production and subsequent decay of a reactive intermediate. One then postulates a mechanism and adjusts the associated rate constants so as to minimize the difference between the results of the experiment and the prediction of the mechanism. The parameters to be estimated are usually strongly correlated, so that it is not possible to determine them separately. These estimated parameters are of little value unless we can also estimate statistically valid confidence limits for them. The difficulties are discussed which frequently arise in estimating parameters and confidence limits for a kinetic mechanism which is widely used in interpreting laser excitation and fluorescence measurements, that is, first-order production and decay. These difficulties, and methods for dealing with them, are illustrated with realistic data. The estimation problem is particularly ill conditioned when the production and loss rates are nearly equal. In some experimental systems this can be avoided, but in others it is inevitable.     

On the gauge invariance of the schrodinger equation

The problem of gauge invariance in relation to (approximate) calculations of molecular properties is considered. We show that these difficulties are not avoided by making a unitary transformation to a formalism which is independent of the electromagnetic field potentials.     

A critical evaluation of the current state-of-the-art in quantitative imaging mass spectrometry

Mass spectrometry imaging (MSI) has evolved into a valuable tool across many fields of chemistry, biology, and medicine. However, arguably its greatest disadvantage is the difficulty in acquiring quantitative data regarding the surface concentration of the analyte(s) of interest. These difficulties largely arise from the high dependence of the ion signal on the localized chemical and morphological environment and the difficulties associated with calibrating such signals. The development of quantitative MSI approaches would correspond to a giant leap forward for the field, particularly for the biomedical and pharmaceutical fields, and is thus a highly active area of current research. In this review, we outline the current progress being made in the development and application of quantitative MSI workflows with a focus on biomedical applications. Particular emphasis is placed on the various strategies used for both signal calibration and correcting for various ion suppression effects that are invariably present in any MSI study. In addition, the difficulties in validating quantitative-MSI data on a pixel-by-pixel basis are highlighted.

Exploratory applications of diffusion ordered spectroscopy to liquid foods: an aid towards spectral assignment

NMR spectroscopy has, in many instances, proved to be an important tool for the rapid, non-invasive analysis of liquid foods. However, achieving improved and more complete spectral assignment poses several difficulties due to strong signal overlap and low abundance of many components of interest. This paper presents preliminary applications of diffusion ordered spectroscopy (DOSY) to complex liquid foods such as fruit juices and beer, as a complementary aid to spectral assignment based on the hydrodynamic volume, and hence diffusivity of different components. The usefulness of this technique is shown mainly for aliphatic and aromatic compounds found in juices and beer and the difficulties posing challenges for future work are discussed.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 25: Mechanisms of aldehyde and carboxylic acid formation

Aldehydes and acids can be formed in numerous reactions in oxidizing polyethylene melts. Significant amounts of aldehydes result from β-scission of alkoxy radicals that are formed on bimolecular hydroperoxide decomposition. There are also large amounts of aldehydes expected from acid-catalyzed decomposition of allylic hydroperoxides as soon as enough acids have accumulated for efficient catalysis. There are difficulties in explaining the formation of aldehydes at a constant rate in sufficient amount for explaining the experimental data. There are much less difficulties with the constant rate of carboxylic acid formation. The α,γ-keto-hydroperoxides that are formed on chain propagation might account for the bulk of the acids formed at a constant rate.The foremost problems with the acids pertain to their formation at increasing rates in the initial as well as in the advanced stages. Formation and decomposition of α,β-di-hydroperoxides and α,γ-di-hydroperoxides is a possibility in this respect. Similarly, α,β-keto-hydroperoxides might be formed on peroxidation in the α-position to ketone groups in the advanced stages. There are considerable difficulties in elucidating the exact role of the aldehydes that are usually seen as the main precursors of the acids. Although there are many possibilities for transformation of aldehydes into acids, the free radical mechanisms envisaged usually have considerable disadvantages. These disadvantages result essentially from fast decarbonylation of acyl radicals and even faster decarboxylation of acyl-oxy radicals. Direct transformation of peracids into acids on reaction with double bonds is always a possibility. Moreover, in the low temperature range (150-160 °C) where hydroperoxides are accumulating, direct reaction of aldehydes with primary and/or secondary hydroperoxides will also yield acids.     

Theoretical interpretation of the valence X-ray photoelectron spectrum of TiC

The valence X-ray photoelectron spectrum of TiC is interpreted in terms of the density of states and in terms of a simple model for transition matrix elements. The conceptual difficulties arising with both the initial and the final state are discussed.     

Liquid crystalline polymers: molecular simulation of some polyethers containing oxetanic rings in the main chain

This paper presents a conformational analysis of some polyethers containing the oxetanic ring in the main chain. The studied polymers were synthesized by phase transfer catalysis, using 3,3-bis(chloromethyl)-oxetane and various bisphenols. The analysis was performed using the Cerius² program (version 2.0) molecular simulation software for material science, designed by Molecular Simulations Incorporated (MSI). This study tries to elucidate some aspects concerning the difficulties appearing in the liquid crystalline property's characterization. These difficulties are induced by the cross-linking processes that take place in certain situations due to the opening of the oxetanic rings at high temperature.