Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

Determination of aliphatic anhydrides and acids by reversed-phase liquid chromatography

Few chromatography methods have been reported for the determination of anhydrides in mixtures or as mixed anhydrides. The potential reactivity of anhydrides with water and other common eluent components complicates possible schemes for separation and analysis. By optimizing variables that affect hydrolysis, including the stationary phase, conditions can be found to successfully analyze anhydrides as reactive as acetic anhydride. The corresponding acids can be determined at the same time. The effect of the stationary phase on anhydride hydrolysis rates may prove to be a sensitive means of probing stationary phase chemistry.     

Recent developments in semiconductor gamma-ray detectors

Summary The successful development of lithium-drifted Ge detectors in the 1960s marked the beginning of the significant use of semiconductor crystals for direct detection and spectroscopy of gamma-rays. In the 1970s, high-purity Ge became available, which enabled the production of complex detectors and multi-detector systems. In the following decades, the technology of semiconductor gamma-ray detectors continued to advance, with significant developments not only in Ge detectors but also in Si detectors and room-temperature compound-semiconductor detectors. In recent years, our group at Lawrence Berkeley National Laboratory has developed a variety of gamma-ray detectors based on these semiconductor materials. Examples include Ge strip detectors, lithium-drifted Si strip detectors, and coplanar-grid CdZnTe detectors. These advances provide new capabilities in the measurement of gamma-rays, such as the ability to perform imaging and the realization of highly compact spectroscopy systems.     

The Relationship Between Dysphonia and Sinusitis: A Pilot Study

Though some clinicians believe chronic sinusitis is associated with dysphonia, studies confirming such a relationship are not available. This pilot study investigated vocal characteristics of chronic sinusitis patients compared to those of nonsinusitis patients. Nineteen male patients were evaluated in a single-blind, nonrandomized, prospective study at a Department of Veterans Affairs Medical Center. After candidates for the experimental and control groups were thoroughly screened [nasal and laryngology examinations, sinus computed tomography (CT) scan, history], 10 chronic sinusitis patients were assigned to the experimental group, nine patients without chronic sinusitis were assigned to the control group. Visi-Pitch analysis of their speech evaluated fundamental frequency, intensity, and perturbation. The evaluator described voice quality and vowel prolongation. Videostroboscopic examination evaluated glottic closure, supraglottic activity, vocal fold edge, amplitude, mucosal wave, and nonvibrating portion. Visi-Pitch analysis and vowel prolongation for each group was compared using an independent t-test. The average fundamental frequency mean was 113.43+/-32.75 Hz for the control group and 112.29+/-19.71 for the experimental group. This study established methods and parameters to determine if a relationship between chronic sinusitis and dysphonia actually exists. It did not establish a relationship between these entities, possibly due to the small sample size enrolled in the study, but determined that 126 subjects would be necessary to achieve significant results.     

Concentration of water-soluble compounds by distillation at parts per billion concentrations

The objective of this work was to develop a procedure for concentrating water-soluble compounds for analysis by gas chromatography using direct aqueous injection. The procedure developed involves concentrating the compound by distillation using an efficient condenser. Recoveries were measured for a variety of compounds. Recoveries of about 90% were obtained for most compounds after a 10- to 20-fold concentration. We conclude that most water-soluble compounds boiling at 100 °C or less can be concentrated 10- to 20-fold using this technique. This concentration will enable these compounds to be determined in aqueous samples at the 10- to 100-ppb level by gas chromatography with direct aqueous injection of the concentrate, and even lower levels using mass spectrometry.     

The first preparation of beta-lactones by radical cyclization

beta-Lactones have, for the first time, been prepared by 4-exo-trig radical cyclization. Thus, alpha-ethenoyloxy radicals react in the presence of tributylstannane in a photothermal process to give beta-lactones. Highest yields were obtained when groups capable of stabilizing a carbon-centered radical were present at the 3-position of the alkenoate acceptor.     

Determination of acid substituents and unesterified hydroxyl groups in cellulose esters

The properties of mixed cellulose esters are strongly affected by the nature and ratio of acid substituents. Fast reliable methods are needed to determine the acid substituents of these products. For some applications it is necessary to also determine the unesterified hydroxyl content. A generally applicable method has been developed to determine acid substituents of mixed cellulose esters. The cellulose ester sample is rapidly hydrolyzed with a reagent consisting of a polar, aprotic solvent, for example dimethyl sulfoxide, with sodium hydroxide and methanol. The hydrolyzed acids are determined by reversed-phase liquid chromatography with UV detection. The results are sufficiently precise so that unesterified hydroxyl content can be calculated by difference.     

A Convenient Approach to Stereoisomeric Iminocyclitols: Generation of Potent Brain‐Permeable OGA Inhibitors

Pyrrolidine‐based iminocyclitols are a promising class of glycosidase inhibitors. Reported herein is a convenient epimerization strategy that provides direct access to a range of stereoisomeric iminocyclitol inhibitors of O‐GlcNAcase (OGA), the enzyme responsible for catalyzing removal of O‐GlcNAc from nucleocytoplasmic proteins. Structural details regarding the binding of these inhibitors to a bacterial homologue of OGA reveal the basis for potency. These compounds are orally available and permeate into rodent brain to increase O‐GlcNAc, and should prove useful tools for studying the role of OGA in health and disease.