Contrast mechanism in non-contact SFM imaging of ionic surfaces

We analyse the mechanisms of contrast formation in NC-SFM imaging of ionic surfaces and calculate constant frequency shift scanlines of the perfect surfaces of NaCl and MgO. Non-contact SFM operation is modelled in a perturbed oscillator model using an atomistic simulation technique for force–field calculations. We demonstrate that the contrast in NC-SFM imaging of ionic surfaces is based on the interplay between the van der Waals interaction and the electrostatic interaction of the tip with the surface potential and the local surface polarisation induced by the tip. The results emphasise the importance of the tip-induced relaxation of the surface ions in the tip–surface interaction and image contrast.     

Calculation of the frequency shift in dynamic force microscopy

A theoretical study of the quality and the range of validity of different numerical and analytical methods to calculate the frequency shift in dynamic force microscopy is presented. By comparison with exact results obtained by the numerical solution of the equation of motion, it is demonstrated that the commonly used interpretation of the frequency shift as a measure for the force gradient of the tip–sample interaction force is only valid for very small oscillation amplitudes and leads to misinterpretations in most practical cases. Perturbation theory, however, allows the derivation of useful analytic approximations.     

Schrödinger operators with singular potentials from the space of multiplicators

We study Schrödinger operatorsT+Q, whereT=?Δ is the Laplace operator andQ is the multiplication operator by a generalized function (distribution). We also consider generalizations for the case of the polyharmonic operatorT = (-δ) ⁿ      

Accurate electrical and spectroscopic properties ofX 1Σ+ BeO from coupled-cluster methods

Summary This paper reports a series of coupled-cluster (CC) calculations through CCSDT on the theoretically challenging ground state of the BeO molecule. Along with CC methods, quadratic configuration interaction (QCI) approximations to CC theory have been used (QCISD and QCISD(T)), which show several dramatic failings. Equilibrium electrical properties (, _xx , and _zz ) and basic spectroscopic properties (r _e, _e,D _e, and infrared intensity (I)) have been computed. Basis set and electron correlation effects are analyzed in order to arrive at accurate values of the dipole moment and polarizability, which are not known experimentally. For the dipole moment, we obtain a value of 6.25 D, with an uncertainty of about 0.1 D. For _xx and _zz , we suggest respective values of 32 and 36 atomic units (a.u.) and error bars of about 1 and 2 a.u. With extended basis sets, the spectroscopic propertiesr _e, _e, andD _e are reproduced to high accuracy, which is the first time this has been achieved for this species byab initio methods. At the highest calculation levels,I is predicted to be very small. AlthoughI has not been measured, some support for this prediction comes from a recent infrared study of BeO-rare gas complexes. The QCI methods are shown to be much more sensitive to basis set, and even with large basis sets yield values of _zz andI which differ from CC results by an order of magnitude and three orders of magnitude, respectively. These differences doubtless arise from the importance of single excitations (T ₁) for this molecule, as several terms involvingT ₁ are neglected in the QCISD approximation compared with CCSD. We also report CC calculations with Brueckner orbitals, which yield results similar to those obtained with restricted Hartree-Fock orbitals.     

Thermodynamic properties of organic compounds in aqueous solution. II. Apparent molal heat capacities of piperidines,morpholines, and piperazines

Apparent molal heat capacities of some piperidine, morpholine, and piperazine derivatives in aqueous solution have been determined by adiabatic calorimetry in the temperature range 20–55°C and in the molality range 0.2–1m. Comparison of experimental values with those calculated through group contributions, found for monofunctional compounds, indicates strong interactions between the hydrophilic centers. An interpretation is given of the possible mechanism of this interaction. Also, values of ΔC _p for the addition reaction of proton to nitrogen centers of mono- and bifunctional organic compounds are examined.     

Retention and selectivity effects caused by bonding of a polar urea-type ligand to silica: a study on mixed-mode retention mechanisms and the pivotal role of solute-silanol interactions in the hydrophilic interaction chromatography elution mode

The separation properties of five silica packings bonded with 1-[3-(trimethoxysilyl)propyl]urea in the range of 0–3.67 μmol m⁻² were investigated in the hydrophilic interaction chromatography (HILIC) elution mode. An increase of the ligand surface density promoted retention of non-charged polar compounds and even more so for acids. An opposite trend was observed for bases, while the amphoteric compound tyrosine exhibited a U-shaped response profile. An overall partitioning retention mechanism was incompatible with these observations; rather, the substantial involvement of adsorptive interactions was implicated. Support for the latter was provided by column-specific changes in analyte retention and concomitant selectivity effects due to variations of salt concentration, type of salt, pH value, organic modifier content, and column temperature. Silica was more selective for separating compounds differing in charge state (e.g. tyramine vs. 4-hydroxybenzoic acid), while in cases where structural differences of solutes resided in non-charged polar groups (e.g. tyramine vs. 5-hydroxydopamine, nucleoside vs. nucleobase) more selective separations were obtained on bonded phases. Hierarchical cluster analysis of the home-made urea-type and three commercial amide-type bonded packings evinced considerable differences in separation properties. The present data emphasise that the role of the packing material under HILIC elution conditions is hardly just the polar support for a dynamic coating with a water-enriched layer. Three major retention mechanisms are claimed to be relevant on bare silica and the urea-type bonded packings: (i) HILIC-type partitioning, (ii) HILIC-type weak adsorption such as hydrogen bonding between solutes and ligands or solutes and silanols (potentially influenced by individual degrees of solvation, salt bridging, etc.), (iii) strong electrostatic (ionic) solute–silanol interactions (attractive/repulsive). Even when non-charged polar bonded phases are used, solute–silanol interactions should not be discounted, which makes them a prime parameter to be characterised by HILIC column tests. Multi/mixed-mode type separations seem to be common under HILIC elution conditions, associated with a great deal of selectivity increments. They are accessible and controllable by a careful choice of the type of packing, the mobile phase composition, and the temperature.     

Contributions to reversed-phase column selectivity. I. Steric interaction

In reversed-phase chromatography (RPC), the restricted retention of "bulky" solutes can occur in one of two ways, giving rise to either "shape selectivity" or "steric interaction." Starting with data for 150 solutes and 167 monomeric type-B alkylsilica columns, the present study examines the steric interaction process further and compares it with shape selectivity. The dependence of column hydrophobicity and steric interaction on column properties (ligand length and concentration, pore diameter, end-capping) was determined and compared. The role of the solute in steric interaction was found to be primarily a function of solute molecular length, with longer solutes giving increased steric interaction. We find that there are several distinct differences in the way shape selectivity and steric interaction are affected by separation conditions and the nature of the sample. Of particular interest, steric interaction exhibits a maximum effect for monomeric C(18) columns, and becomes less important for either a C(1) or C(30) column; shape selectivity appears unimportant for monomeric C(1)-C(18) columns at ambient and higher temperatures, but becomes pronounced for C(30) - as well as polymeric columns with ligands ≥C(8). One hypothesis is that shape selectivity involves the presence or creation of cavities within the stationary phase that can accommodate a retained solute (a primarily enthalpic process), while steric interaction mainly makes greater use of spaces that pre-exist the retention of the solute (a primarily entropic process). The related dependence of hydrophobic interaction on column properties was also examined.     

The Liquid Junction Potential in Potentiometric Titrations. X. The Calculation of Potentials Across Liquid Junctions of the Type AY|AY+HY+BYz(B)+AyL for Emf Cells Where Strong or Weak Complexes are Formed at Constant Ionic Strength and −log10[H+]≤7

Equations are derived for the calculation of the total potential anomalies in Emf cells with liquid junctions (ΔE _J ) in mV and at 25.0 °C, defined as

Improved detection of multi-phosphorylated peptides in the presence of phosphoric acid in liquid chromatography/mass spectrometry

In contrast to lower phosphorylation states (e.g. the tryptic monophosphopeptide FQpSEEQQQTEDELQDK from bovine beta-casein), the specific detection of multi-phosphorylated peptides (e.g. the tetraphosphopeptide RELEELNVPGEIVEpSLpSpSpSEESITR from tryptic digestion of bovine beta-casein) has often been problematic for liquid chromatographic mass spectrometric (LC/MS) analysis owing to their high affinity for adsorption to exposed surfaces. We observed an enhancement in the overall detection of phosphopeptides on addition of phosphoric acid (0.1-1.0%) to the sample solution; a 10-fold increase in sensitivity was determined for the detection of two tryptic phosphopeptides and also a significant improvement in the detection of the tetraphosphopeptide. Using capillary LC with ion trap tandem MS for detection and identification, the achievable detection limits were 50 fmol and 50 pmol for the monophosphopeptide and the tetraphosphopeptide, respectively. Phosphoric acid is believed to act as a blocking agent to available silanol groups on both the silica capillary surface and the C(18)-bonded stationary phase silica surface.