Tentative Structural Assignments Research Articles

Kinetics of stepwise nitrogen adsorption by size-selected iron cluster cations: Evidence for size-dependent nitrogen phobia.

We present a study of stepwise cryogenic N2 adsorption on size-selected Fen + (n = 8-20) clusters within a hexapole collision cell held at T = 21-28K. The stoichiometries of the observed adsorption limits and the kinetic fits of stepwise N2 uptake reveal cluster size-dependent variations that characterize four structural regions. Exploratory density functional theory studies support tentative structural assignment in terms of icosahedral, hexagonal antiprismatic, and closely packed structural motifs. There are three particularly noteworthy cases, Fe13 + with a peculiar metastable adsorption limit, Fe17 + with unprecedented nitrogen phobia (inefficient N2 adsorption), and Fe18 + with an isomeric mixture that undergoes relaxation upon considerable N2 uptake.

Electrochemical Azidooxygenation of Alkenes Mediated by a TEMPO-N3 Charge-Transfer Complex.

We report a mild and efficient electrochemical protocol to access a variety of vicinally C-O and C-N difunctionalized compounds from simple alkenes. Detailed mechanistic studies revealed a distinct reaction pathway from those previously reported for TEMPO-mediated reactions. In this mechanism, electrochemically generated oxoammonium ion facilitates the formation of azidyl radical via a charge-transfer complex with azide, TEMPO-N3. DFT calculations together with spectroscopic characterization provided a tentative structural assignment of this charge-transfer complex. Kinetic and kinetic isotopic effect studies revealed that reversible dissociation of TEMPO-N3 into TEMPO• and azidyl precedes the addition of these radicals across the alkene in the rate-determining step. The resulting azidooxygenated product could then be easily manipulated for further synthetic elaborations. The discovery of this new reaction pathway mediated by the TEMPO+/TEMPO• redox couple may expand the scope of aminoxyl radical chemistry in synthetic contexts.

Tentative Structural Assignment of a Glucuronide Metabolite of Methyltestosterone in Tilapia Bile by Liquid Chromatography–Quadrupole-Time-of-Flight Mass Spectrometry

Methyltestosterone (MT), a strong androgenic steroid, is not approved for use in fish aquaculture in the United States. It is used in the U.S. under an investigational new animal drug exemption (INAD) only during the early life stages of fish. There is a possibility that farmers feed fish with MT to enhance production for economic gains. Therefore, there is a need to develop methods for the detection of MT and its metabolite residues in fish tissue for monitoring purposes. Previously, our laboratory developed a liquid chromatography-quadrupole time-of-flight (LC-QTOF) method for characterization of 17-O-glucuronide metabolite (MT-glu) in bile of tilapia dosed with MT. The system used was an Agilent 6530 Q-TOF equipped with electrospray jet stream technology, operating in positive ion mode. Retrospective analysis of the data generated in that experiment by a feature-finding algorithm, combined with a search against an in-house library of possible MT-metabolites, resulted in the discovery of a major glucuronide metabolite of MT in the bile extracts. Preliminary data indicate it to be a glucuronide of a hydroxylated MT (OHMT-glu) which persists in tilapia bile for at least 2 weeks after dosing. We present the tentative structural assignment of the OHMT-glu in tilapia bile and time course of development. This glucuronide can serve as a marker to monitor illegal use of MT in tilapia culture.

Structure of rotational bands in 253No

In-beam gamma-ray and conversion electron spectroscopic studies have been performed on the 253 No nucleus. A strongly coupled rotational band has been identified and the improved statistics allows an assignment of the band structure as built on the $\ensuremath 9/2^-[734]_{\nu}$ ground state. The results agree with previously known transition energies but disagree with the tentative structural assignments made in earlier work.

Total Synthesis of Jatrophane Diterpenes from Euphorbia characias

The enantioselective total synthesis of the jatrophane diterpene (-)-15-O-acetyl-3-O-propionylcharaciol is described. Starting from an advanced cyclopentane building block, a B-alkyl Suzuki-Miyaura cross-coupling and carbonyl addition were utilized to assemble a fully functionalized triene, and a ring-closing metathesis was then employed to construct the rigid 12-membered ring. Twenty-five years after the original report on the isolation of the natural product, our total synthesis unambiguously corroborates the original tentative structural assignment.

A Gas Chromatography/Electron Ionization−Mass Spectrometry−Selected Ion Monitoring Method for Determining the Fatty Acid Pattern in Food after Formation of Fatty Acid Methyl Esters

A method using gas chromatography/electron ionization-mass spectrometry (GC/EI-MS) in the selected ion monitoring (SIM) mode was developed for the analysis of fatty acids as methyl esters (FAMEs) in order to determine their percentage contribution to the fatty acid profile in food. In the GC/EI-MS-SIM mode, saturated fatty acids were determined with m/z 87, monoenoic fatty acids were determined with m/z 74, and polyenoic fatty acids were determined via the sum of m/z 79 and m/z 81. The ratios of these fragment ions and the GC retention data provided additional information for tentative structural assignments. The 28 FAME standards tested provided similar results for the novel GC/EI-MS-SIM method and GC/EI-MS in the full scan mode, both of which were slightly worse than GC/flame ionization detection (FID). Analysis of sunflower oil, suet, and cod liver oil verified that both major and minor fatty acids (20-60% and down to 0.001% contribution to the fatty acid pattern) were determined with sufficient quality that justifies application of the GC/EI-MS-SIM method for the analysis of food samples. Furthermore, the method was approximately 20- or approximately 10-fold more sensitive than GC/EI-MS in the full scan mode or GC/FID, respectively. The method is suited for both quantitative purposes and fatty acid identification in samples where only low amounts of lipids are available.

Tebuconazole dissipation and metabolism in Tifton loamy sand during laboratory incubationt.

The fungicide tebuconazole is widely used to control soil-borne and foliar diseases in peanuts and other crops. No published data are currently available on the extent and rate at which this compound degrades in soil. Unpublished data summarized in registration documents suggest that the compound is persistent, with 300-600 days half-life. We conducted a 63-day laboratory incubation to evaluate tebuconazole's dissipation kinetics and impact on soil microbial activity in Tifton loamy sand. Tifton soils support extensive peanut production in the Atlantic Coastal Plain region of Georgia and Alabama. Products containing tebuconazole are applied to an estimated 50% of the peanut acreage in the region. At the end of the incubation, 43 (+/-42)% of the parent compound was recovered in soil extracts. The first-order kinetic model, which gave a good fit to the dissipation data (r2 = 0.857), yielded a soil half-life (t1/2) of 49 days. This is 6-12 times more rapid than t1/2 values described in unpublished tebuconazole registration documents. Four degradates were identified. Tentative structural assignments indicated that degradates were derived from hydroxylation of the parent compound and/or chlorophenyl ring cleavage. Cleavage products showed a steady increase during the incubation, and on a molar basis were equal to 63% of the time zero tebuconazole concentration. No significant effect on soil microbial biomass was observed, indicating that when the compound is applied at normal agronomic rate it does not impact soil metabolic activity. Use of the soil-half life data derived in this study should improve the accuracy oftebuconazole fate assessments for Coastal Plain peanut production. The study also indicated that environmental assessment of selected degradates may be needed to fully evaluate risks of tebuconazole use.

Hydrated Sugars in the Gas Phase: Spectroscopy and Conformation of Singly Hydrated Phenyl β-d-Glucopyranoside

The structural investigation of a hydrated monosaccharide, phenyl β-d-glucopyranoside (PheβGlc), in the gas phase is presented. It is based upon ab initio computation coupled with the analysis of the resonant 2-photon ionization and IR ion-dip spectra of the singly hydrated clusters stabilized in a free jet expansion and follows an earlier investigation of the structure and conformations of the unhydrated sugar.1 Despite the potentially large set of possible binding sites and conformations, only two singly hydrated complexes are formed in the free jet expansion. Tentative structural assignments are made on the basis of comparisons with those already established for related systems; comparisons between the observed O−H vibrational frequencies and those computed for structures optimized at the B3LYP/6-31+G(d) level of theory, and in light of the relative energies of these structures, calculated at the single-point MP2/6-311+G(d,p) level are made. A discrepancy between the latest revision of the Gaussian 98 package (revision A.11) and earlier revisions, which can lead to different computed structures when optimizing noncovalently bound molecular complexes, is discussed in an appendix.

Unusual polycyclic aromatic hydrocarbons in Lower Cretaceous Ostracode Zone sediments and related oils of the Western Canada sedimentary basin

Analyses of the aromatic hydrocarbon fractions of extracts of Lower Cretaceous Ostracode Zone sediments from the Western Canada sedimentary basin indicate the predominance of polycyclic hydroaromatic components. Most of these have not been previously reported in the literature, but are obviously diagenetically related to a new class of molecule whose natural distribution and biological significance remain to be determined. The components whose molecular structures have been confirmed, together with those for which tentative structural assignments are given, provide strong support for a pathway of progressive aromatisation of the presumed polyprenol precursors. These compounds appear to be specific markers for the Ostracode Zone sourced oils in the Lower Mannville Group reservoirs of the Provost area and may serve as excellent oil-source correlation parameters.

High resolution infrared molecular beam spectroscopy of cyanoacetylene clusters

High resolution infrared optothermal spectroscopy has been used to study small clusters of cyanoacetylene, (HCCCN)n, containing three or more monomer units. For the linear HCCCN trimer the fundamental of the free C–H stretch vibration has been rotationally resolved and analyzed, yielding a ground vibrational state rotational constant (B0) of 94.031(13) MHz. In addition, an inner C–H stretch fundamental band of the linear trimer (rotationally unresolved) has been identified by observing the effect of large electric fields on the band shape. This assignment is also consistent with spectral intensity measurements as a function of molecular beam stagnation pressure. Predissociation lifetimes of the upper states of these two bands were determined from the observed homogeneous linewidths. Several other rotationally unresolved vibrational bands have also been observed. The stagnation pressure and electric field dependence of these spectra are used to estimate the cluster sizes and to make tentative structural assignments.

Analysis of Carbohydrates on IgG Preparations

Characterization of monoclonal antibodies (MAbs) produced for therapeutic or diagnostic purposes increasingly includes an assessment of their carbohydrate content. Using high‐performance anion exchange chromatography with pulsed amperometric detection (HPAEC/PAD), we have analyzed the PNGase F released oligosaccharides of several IgG preparations including human polyclonal IgG, a humanized monoclonal IgG (MAb M115), and a murine monoclonal IgG (MAb MY9‐6) derived respectively from serum, hybridoma cultures, and ascites fluid. The N‐linked oligosaccharides released by PNGase F treatment of the above IgGs were found to consist mainly of neutral, fucosylated, biantennary species. Comparison of glycosylation of human polyclonal IgG, MAb M115, and MAb MY9‐6 revealed differences in the levels of galactosylation and in the levels as well as the form of sialic acid present. HPAEC/PAD oligosaccharide profiling, combined with the use of enzymes (PNGase F, endoglycosidase F2, endoglycosidase H, neuraminidase, β‐galactosidase, and β‐N‐acetylhexosaminidase), and monosaccharide analysis allowed making of tentative structural assignments. By performing monosaccharide analysis directly on PVDF electroblotted heavy and light chain bands separated by SDS‐PAGE, it was verified that IgGs used in this study were glycosylated predominantly in their heavy chain.

Ab initio CI study of the electronic structure and geometry of neutral and cationic hydrogenated lithium clusters. Predictions and interpretation of measured properties

The ground state geometries of neutral and cationic Li n H ( n = 1–7) and Li n H 2 ( n = 2–6) clusters hae been determined in the framework of the SCF and CASSCF procedures. Differences and similarities between topologies of neutral and charged structures have been found. Large-scale CI calculations for valence electrons have been carried out to determine stabilities and ionization potentials (IP). A comparison between predicted and measured observables from the companion Letter allows for tentative structural assignments. Structural electronic properties of small hydrogenated lithium systems have been compared with those obtained for homogeneous Li n clusters.

ECP-CI study of electronic structure and geometry of small neutral and charged Ag n clusters; Predictions and interpretation of measured properties

The ground state geometries of small neutral Agn(n=2–9) and charged Agn± (n=2–9) clusters have been determined in the framework of the SCF procedure employing a relativistic pseudopotential accounting for core-valence correlation effects (RECP-CVC). Similarities and differences between neutral and charged clusters have been found. Large scale CI for 5s electrons only has been carried out for determining stabilities, ionization potentials (IP) and vertical detachment energies (VDE) of anions. A comparison between predicted and measured observables allows for the tentative structural assignments. In addition, the low lying energies of excited states for the neutral species at the anionic geometries have been calculated to account fully for geometrical and spectroscopic assignment to the photodetachment measurements.

Quantum molecular interpretation of the absorption spectra of Na5, Na6, and Na7 clusters

The configuration-interaction study of the excited states of the most-stable structures of Na5, Na6, and Na7 clusters employing ab initio effective-core potential corrected by the core polarization potential predicts spectroscopic patterns which are in good agreement with the measured depletion spectra. A comparison of the transition energies and the oscillator strengths with the experimental data makes possible tentative structural assignments. Planar Na5 and Na6 structures and the three-dimensional pentagonal bipyramid for Na7 appear responsible for the recorded spectra. The full agreement between theory and experiment is present for Na6 and Na7. The measured cross sections and calculated oscillator strengths for Na5 compare better for lower transition energies than for higher ones. A many-electron description of the excited states of Na5, Na6, and Na7 yields a complete quantum molecular interpretation of the absorption spectra. From a comparison of the experimental and theoretical results the conclusion has been drawn that a transition from planarity to the three dimensionality takes place for Na7.

CHARACTERIZATION AMD POTENTIAL UTILIZATION OF WHITEROCKS (DTAB) TAR SAND BITUMEN II. PYROLYSIS MASS SPECTROMETRY AND NUCLEAR MAGNETIC RESONANCE ANALYSES

ABSTRACT The proton and carbon-13 nuclear magnetic resonance characteristics of the native Whiterocks bitumen were determined by comparing the spectra of the samples to literature assignments. The tentative structural assignments of pyrolysis products were also obtained from the spectra of Curie-point low voltage mass spectrometric analysis. The results obtained from proton and carbon-13 nuclear magnetic resonance and pyrolysis-mass spectrometry analysis suggested that the native bitumen consists mostly of short- and long-chain (i.e., normal and isoprenoid chains) alkylsubstituents attached to polycyclic naphthenic, naphtheno-aromatic and/or aromatic moieties.

Gas chromatographic—mass spectrometric identification of impurities in 1,2-diethoxybenzene

Impurities in a 1,2-diethoxybenzene sample were identified based on mass spectral interpretation. To confirm the tentative structural assignment and also to identify the isomers of ethyl(diethoxy)benzenes, authentic 3,4-ethyl(diethoxy)benzene was synthesized. The suspected presence of an unreacted OH group in one of the compounds was confirmed by derivatization. Retention indices of the compounds present were measured.

Polycyclic aromatic hydrocarbons in Australian coals. I. Angularly fused pentacyclic tri- and tetraaromatic components of Victorian brown coal

Analysis of the tri- and tetraaromatic hydrocarbon fractions of a brown coal sample from the Latrobe Valley, Victoria, Australia indicate the predominance of pentacyclic hydroaromatic components. Many of these have not been previously reported in the literature, but are obviously diagenetically related to triterpenoids naturally occurring in the biosphere. The components whose molecular structures have been confirmed, together with those for which tentative structural assignments are given, offer strong support for a theory of progressive diagenetic aromatization of C-3 oxygenated triterpenoids, commencing from ring A. Other compounds present in smaller amounts suggest that 1,2-methyl shift reactions also occur prior to or during aromatization. There is a notable absence of polycyclic aromatic hydrocarbons (PAH's) which can be diagenetically related to the steroid or extended-side-chain hopane skeletons.

High-performance liquid chromatographic separation of high-molecular-weight polycyclic aromatic compounds in carbon black

High-molecular-weight (> 300 daltons) polycyclic aromatic compounds which were extracted from a carbon black were separated by reversed-phase high-performance liquid chromatography using gradient elution. Fractions were collected and analyzed by mass spectrometry and spectrofluorimetry. While only 15 compounds could be positively identified, 38 others were given tentative structural assignments, among which were several large sulfur heterocycles.

Techniques for the isolation and identification of steroids in insects and algae.

Analytical techniques, methods and instrumentation employed for the extraction, isolation separation, purification and identification of steroids from algae and insects are presented. The techniques include adsorption and argentation column chromatography and counter-current distribution for separating the individual steroids, and thin layer chromatography and gas liquid chromatography for monitoring the purification process. Double bond, steric and alkyl substituent separation factors and relative retention times are reported for a large number of sterols on 4 different column systems. Their use permits the tentative structural assignment of sterols. Ultraviolet, infrared, nuclear magnetic resonance (NMR) and mass spectral analyses are discussed in light of their significance in the isolation and identification of steroids from insects and algae. Numerous examples are presented, including the use of 220 MHz NMR spectrometry, which permits the differentiation and characterization of C-24 epimetric sterols and allows for a semiquantitative estimate of the 24α- and 24β-epimers present in a mixture.

Acetyl migration in Urs-12-ene-3α,24-diol

A previous tentative structural assignment of a monoacetate of urs-12- ene-3α,24-diol as the 3-mono-ester has been shown to require revision. Authentic urs-12-ene-3α,24-diol 3-monoacetate has been prepared as well as the 24-monoacetate and interconversion of the two monoacetates has been demonstrated. The possibility that this acetyl migration occurs through conversion of ring A to a boat form is supported by the ready isolation of a 3α,24-O-isopropylidene derivative.