Self-assembled monolayers on Pt(111): molecular packing structure and strain effects observed by scanning tunneling microscopy

Self-assembled monolayers (SAMs) of octanethiol and benzeneethanethiol were deposited on clean Pt(111) surfaces in ultrahigh vacuum (UHV). Highly resolved images of these SAMs produced by an in situ scanning tunneling microscope (STM) showed that both systems organize into a super-structure mosaic of domains of locally ordered, closely packed molecules. Analysis of the STM images indicated a (square root 3 x square root 3)R30 degrees unit cell for the octanethiol SAMs and a 4(square root 3 x square root 3)R30 degrees periodicity based on 2 x 2 basic molecular packing for the benzeneethanethiol SAMs under the coverage conditions investigated. SAMs on Pt(111) exhibited differences in molecular packing and a lower density of disordered regions than SAMs on Au(111). Electron transport measurements were performed using scanning tunneling spectroscopy. Benzeneethanethiol/Pt(111) junctions exhibited a higher conductance than octanethiol/Pt(111) junctions.     

Rapid inorganic ion analysis using quantitative microchip capillary electrophoresis

Rapid quantitative microchip capillary electrophoresis (CE) for online monitoring of drinking water enabling inorganic ion separation in less than 15 s is presented. Comparing cationic and anionic standards at different concentrations the analysis of cationic species resulted in non-linear calibration curves. We interpret this effect as a variation in the volume of the injected sample plug caused by changes of the electroosmotic flow (EOF) due to the strong interaction of bivalent cations with the glass surface. This explanation is supported by the observation of severe peak tailing. Conducting microchip CE analysis in a glass microchannel, optimized conditions are received for the cationic species K+, Na+, Ca2+, Mg2+ using a background electrolyte consisting of 30 mmol/L histidine and 2-(N-morpholino)ethanesulfonic acid, containing 0.5 mmol/L potassium chloride to reduce surface interaction and 4 mmol/L tartaric acid as a complexing agent resulting in a pH-value of 5.8. Applying reversed EOF co-migration for the anionic species Cl-, SO42- and HCO3- optimized separation occurs in a background electrolyte consisting of 10 mmol/L 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) and 10 mmol/L HEPES sodium salt, containing 0.05 mmol/L CTAB (cetyltrimethylammonium bromide) resulting in a pH-value of 7.5. The detection limits are 20 micromol/L for the monovalent cationic and anionic species and 10 micromol/L for the divalent species. These values make the method very suitable for many applications including the analysis of abundant ions in tap water as demonstrated in this paper.     

Identification of alkyl carbazoles and alkyl benzocarbazoles in Brazilian petroleum derivatives

The present work describes (a) the identification and characterization of an impurity, 2,4,5,7-tetrabromo-6-hydroxy-9-(2,3,4,5-tetrachlorophenyl)-3H-xanthen-3-one (BCPX), in the color additives D&C Red Nos. 27 and 28 (phloxine B) and (b) the determination of the extent and level of BCPX contamination in certified lots of these colors. For these purposes, BCPX (a compound not previously reported in the literature) was synthetically prepared. Test portions from 42 certified lots of D&C Red Nos. 27, 28 and 27 lakes were analyzed for BCPX using an HPLC method that included gradient elution and UV-vis photodiode array detection. Those lots were submitted for certification by both domestic (six) and foreign (six) manufacturers during the past 4 years. Of the test portions analyzed, 32 (76.2%) contained BCPX in amounts ranging from 0.01 to 3.21%. The remaining 10 test portions (23.8%) contained no detectable BCPX or less than 0.008%, which is the limit of quantification for the present method. The analyses revealed substantial differences in the level of BCPX across different manufacturers. The wide range of BCPX levels found in the analyzed lots suggests that the presence of BCPX in D&C Red Nos. 27 and 28 may be avoided or significantly reduced during the manufacturing process.     

Distonoid ions

By Yates, Bouma, and Radom's definition, distonic radical ions are those formally arising by ionization of diradicals or zwitterionic molecules (including ylides). These ions differ, therefore, from conventional radical ions by displaying the charge site and unpaired electron site (spin) localized mandatorily on separate atoms or group of atoms; that is, these sites are separated in all of their major resonance forms. Many conventional radical ions with a major resonance form in which charge and spin sites reside formally on the same atom or group of atoms display, however, high degree of discretionary (non-mandatory) charge-spin separation. By analogy with the metal/metalloid terminology, we propose that these distonic-like radical ions be classified as distonoid ions. Radical ions would, therefore, be divided into three sub-classes: conventional, distonic, and distonoid ions. B3LYP/6-311 + G(d,p) calculations for a proof-of-principle set of radical cations are used to demonstrate the existence of many types of distonoid ions with a high degree of discretionary charge-spin separation. Reliable calculations are indispensable for probing distonoid ions, since an ion that was expected to be distonoid (by the analysis of its resonance forms) is shown by the calculations to display a characteristic conventional-ion electronic distribution. Similarly to many distonic radical ions, and in sharp contrast to a conventional radical ion (ionized 1,4-dioxane), the gas-phase intrinsic bimolecular reactivity with selective neutrals of a representative distonoid ion, ionized 2-methylene 1,3-dioxolane, is found to include dual ion-radical type reactions.     

Electrophilic aromatic nitration: understanding its mechanism and substituent effects

Theoretical calculations and gas-phase mass spectrometric studies were performed for the reaction of the naked (NO2+) and monosolvated (CH3NO2.NO2+) nitronium ion with several monosubstituted aromatic compounds. From these studies, we propose a general model for regioselectivity based on the single-electron transfer (SET) mechanism and an alternative mechanistic scheme for electrophilic aromatic nitration. This scheme considers the SET and the polar (Ingold-Hughes) mechanisms as extremes in a continuum pathway, the occurrence and extents of both mechanisms being governed mainly by the ability, or lack of ability, of the aromatic compound to transfer an electron to NO2+.     

Natural compounds obtained through centrifugal molecular distillation

Soybean oil deodorized distillate (SODD) is a byproduct from refining edible soybean oil; however, the deodorization process removes unsaponifiable materials, such as sterols and tocopherols. Tocopherols are highly added value materials. Molecular distillation has large potential to be used in order to concentrate tocopherols, because it uses very low levels of temperatures because of the high vacuum and short operating time for separation and, also, it does not use solvents. However, nowadays, the conventional way to recover tocopherols is carrying out chemical reactions prior to molecular distillation, making the process not so suitable to deal with natural products. The purpose of this work is to use only molecular distillation in order to recover tocopherols from SODD. Experiments were performed in the range of 140-220 degrees C. The feed flow rate varied from 5 to 15 g/min. The objective of this study was to remove the maximum amount of free fatty acids (FFA) and, so, to increase the tocopherol concentration without add any extra component to the system. The percentage of FFA in the distillate stream of the molecular still is large at low feed flow rates and low evaporator temperatures, avoiding thermal decomposition effects.     

Highly sensitive and selective measurement of underivatized methylmalonic acid in serum and plasma by liquid chromatography-tandem mass spectrometry

Methylmalonic acid (MMA) is a functional biomarker of vitamin B12 deficiency. Measurement of plasma MMA is challenging due to its small molecular weight and hydrophilic nature. Several liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods have been developed for measuring plasma MMA. However, these methods involve lengthy sample preparation, long chromatographic run time, inadequate sensitivity, or interference from succinic acid (SA). Here we report a novel LC-MS/MS method for quantitation of underivatized MMA in serum or heparinized plasma with high sensitivity and selectivity. Sample preparation involved only strong anion exchange solid phase extraction. The extract was purified by online turbulent flow and analyzed on an Organic Acids column. MS/MS analysis was performed in negative electrospray mode, and the analytical time was 6 min. The use of ion ratio confirmation in combination with chromatographic resolution from SA greatly enhanced the selectivity. No interference was observed. This method was linear from 26.2 to 26,010.0 nM with an accuracy of 98-111 %. Total coefficient of variation was less than 4.6 % for three concentration levels tested. Comparison with a reference laboratory LC-MS/MS method using leftover patient serum specimens (n = 48) showed a mean bias of -2.3 nM (-0.61 %) with a Deming regression slope of 1.016, intercept of -6.6 nM, standard error of estimate of 25.3 nM, and a correlation coefficient of 0.9945. In conclusion, this LC-MS/MS method offers highly sensitive and selective quantitation of MMA in serum and plasma with simple sample preparation.     

Mechanism of UV-induced formation of Dewar lesions in DNA

The importance of a backbone: The mechanism of formation of Dewar lesions has been investigated by using femtosecond IR spectroscopy and ab?initio calculations of the exited state. The 4π?electrocyclization is rather slow, occurs with an unusual high quantum yield, and--surprisingly--is controlled by the phosphate backbone.     

An augmented penalty function method with penalty parameter updates for nonconvex optimization

Given an augmented Lagrangian scheme for a general optimization problem, we use an epsilon subgradient step for improving the dual function. This can be seen as an update for an augmented penalty method, which is more stable because it does not force the penalty parameter to tend to infinity. We establish for this update primal-dual convergence for our augmented penalty method. As illustration, we apply our method to the test-bed kissing number problem.