Preparative isolation of (+)-beta-eudesmol fromAmyris balsamifera

Summary Optically pure (+)-beta-eudesmol is a possible starting material for the synthesis of several termite defense compounds. A two step procedure for the isolation of gram quantities of (+)-beta-eudesmol from commercially availableAmyris balsamifera oil (syn. West Indian sandalwood oil), containing 8% beta-eudesmol, was developed. Step one consisted of an efficient vacuum distillation of the total oil. Step two was a medium pressure LC separation with an AgNO₃ impregnated silica gel stationary phase. Several other separation procedures failed due to the presence of many closely related sesquiterpene alcohols (75% of the oil).     

Spin–Spin Coupling Between Two meta-Benzyne Moieties In a Quinolinium Tetraradical Cation Increases Their Reactivities

The reactivity of a carbon-centered σ,σ,σ,σ-type singlet-ground-state tetraradical containing two meta-benzyne moieties was examined in the gas phase. Surprisingly, the tetraradical showed higher reactivity than its individual meta-benzyne counterparts. The reactivity of meta-benzynes is controlled by their (calculated) distortion energy ΔE_2.3, singlet–triplet spitting ΔE_S–T, and electron affinity (EA_2.3) of the meta-benzyne moiety at the transition state geometry for hydrogen-atom abstraction reactions. The addition of a second meta-benzyne moiety to a meta-benzyne does not significantly change EA_2.3. However, ΔE_2.3 is substantially decreased for both meta-benzyne moieties in the tetraradical, and this explains their higher reactivities. The decrease in ΔE_2.3 for each meta-benzyne moiety in the tetraradical is rationalized by stabilizing spin–spin coupling between one radical site in each meta-benzyne moiety. Therefore, spin–spin coupling between the meta-benzyne moieties in this tetraradical increases its reactivity, whereas spin–spin coupling within each meta-benzyne moiety decreases its reactivity.     

The chemisorption and decomposition of NO on the (110) surface of iridium

The chemisorption of NO on Ir(110) has been studied with thermal desorption mass spectrometry (including isotopic exchange experiments), X-ray and UV-photoelectron spectroscopies, Auger electron spectroscopy,LEED and CPD measurements. Chemisorption of NO proceeds by precursor kinetics with the initial probability of adsorption equal to unity independent of surface temperature. Saturation coverage of molecular NO corresponds to 9.6 × 10¹⁴ cm^?2 below 300 K. Approximately 35% of the saturated layer desorbs as NO in two well separated features of equal integrated intensity in the thermal desorption spectra. The balance of the NO desorbs as N₂ and O₂ with desorption of N₂ beginning after the low-temperature peak of NO has desorbed almost completely. Molecular NO desorbs with activation energies of 23.4–28.9 and 32.5–40.1 kcal mole^?1, assuming the preexponential factor for both processes is between 10¹³–10¹⁶ s^?1. At low coverages of NO, N₂ desorbs with an activation energy of 36–45 kcal mole^?1, assuming the preexponential factor is between 10^?2 and 10 cm²s^?1. Levels at 13.5, 10.4 and 8.5 eV below the Fermi level are observed with HeI UPS, associated with the 4σ, 5σ and 1π orbitals of NO, respectively. Core levels of NO appear at 531.5 eV [O(1s)] and 400.2 eV [N(1s)], and do not shift in the presence of oxygen. Oxygen overlayers tend to stabilize chemisorbed NO as reflected in thermal desorption spectra and a downshift in the 1π level to 9.5 eV.     

Laser-induced acoustic desorption (LIAD) mass spectrometry

Large thermally labile molecules were not amenable to mass spectrometric analysis until the development of atmospheric pressure evaporation/ionization methods, such as electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), since attempts to evaporate these molecules by heating induces degradation of the sample. While ESI and MALDI are relatively soft desorption/ionization techniques, they are both limited to preferential ionization of acidic and basic analytes. This limitation has been the driving force for the development of other soft desorption/ionization techniques. One such method employs laser-induced acoustic desorption (LIAD) to evaporate neutral sample molecules into mass spectrometers. LIAD utilizes acoustic waves generated by a laser pulse in a thin metal foil. The acoustic waves travel through the foil and cause desorption of neutral molecules that have been deposited on the opposite side of the foil. One of the advantages of LIAD is that it desorbs low-energy molecules that can be ionized by a variety of methods, thus allowing the analysis of large molecules that are not amenable to ESI and MALDI. This review covers the generation of acoustic waves in foils via a laser pulse, the parameters affecting the generation of acoustic waves, possible mechanisms for desorption of neutral molecules, as well as the various uses of LIAD by mass spectrometrists. The conditions used to generate acoustic or stress waves in solid materials consist of three regimes: thermal, ablative, and constrained. Each regime is discussed, in addition to the mechanisms that lead to the ablation of the metal from the foil and generation of acoustic waves for two of the regimes. Previously proposed desorption mechanisms for LIAD are presented along with the flaws associated with some of them. Various experimental parameters, such as the exact characteristics of the laser pulse and foil used, are discussed. The internal and kinetic energy of the neutral desorbed molecules are also considered. Our research group has been instrumental in the development and use of LIAD. For example, we have systematically examined the influence of many parameters, such as the type of the foil and its thickness, as well as the analyte layer's thickness, on the efficiency of desorption of neutral molecules. The coupling of LIAD with different instruments and ionization techniques allows for broad use of LIAD in our research laboratories. The most important applications involve analytes that cannot be analyzed by using other mass spectrometric methods, such as large saturated hydrocarbons and heavy hydrocarbon fractions of petroleum. We also use LIAD to characterize lipids, peptides, and oligonucleotides. Fundamental research on the reactions of charged mono-, bi-, and polyradicals with biopolymers, especially oligonucleotides, also requires the use of LIAD, as well as thermochemical measurements for neutral biopolymers. These are but a few of the uses of LIAD in our research group.     

The reduction of NO with D2 over the (110) surface of iridium

The heterogeneously catalyzed reaction between NO and D₂ to produce N₂, ND₃ and D₂O over Ir(110) was investigated under ultra-high vacuum conditions for partial pressures of the reactants between 5 × 10^?8 and 1 × 10^?6Torr, total pressures between 10^?7 and 10^?6 Torr, and surface temperatures between 300 and 1000 K. Mass spectrometry, LEED, UPS, XPS and AES measurements were used to study this reacting system. In addition, the competitive coadsorption of NO and deuterium was investigated via thermal desorption mass spectrometry and contact potential difference measurements to gain further insight into the observed steady state rates of reaction. Depending on the ratio of partial pressures ($\text{R }\text{P}{}_{\mathrm{<mtext>D</mtext>}}{}_2\text{P}{}_{\mathrm{<mtext>NO</mtext>}}$), the rate of reduction of NO to N₂ shows a pronounced enhancement when the surface is heated above a critical temperature. As the surface is cooled, the rate maintains a high value independent of temperature until a lower critical temperature is reached, where the rate drops precipitously. This hysteresis is due to a change in the structure and composition of the surface. For sufficiently large values of R and for an “activated” surface, N₂ and ND₃ are produced competitively between 470 and 630 K. Empirical models of the different regimes of the steady state reaction are presented with interpretations of these models.     

Investigations into beam monitors at the AEg ̄ IS experiment

Detailed diagnostic of antiproton beams at low energies is required for essentially all experiments at the Antiproton Decelerator (AD), but will be particularly important for the future Extra Low ENergy Antiproton ring (ELENA) and its keV beam lines to the different experiments. Many monitors have been successfully developed and operated at the AD, but in particular beam profile monitoring remains a challenge. A dedicated beam instrumentation and detector test stand has recently been setup at the AE \(\bar {g}\) IS experiment (Antimatter Experiment: Gravity, Interferometry, Spectroscopy). Located behind the actual experiment, it allows for parasitic use of the antiproton beam at different energies for testing and calibration. With the aim to explore and validate different candidate technologies for future low energy beam lines, as well as the downstream antihydrogen detector in AE \(\bar {g}\) IS, measurements have been carried out using Silicon strip and pixel detectors, a purpose-built secondary emission monitor and emulsions. Here, results from measurements and characterization of the different detector types with regard to their future use at the AD complex are presented.     

The interaction of hydrogen and cyclopropane with the (110) surface of iridium

The interaction of cyclopropane with hydrogen and the residue resulting from the decomposition of the former on the reconstructed Ir(110)-(1×2) surface has been studied with thermal desorption mass spectrometry. Although hydrogen will not adsorb onto the saturated overlayer of dissociatively adsorbed cyclopropane, the preadsorption of hydrogen into the β₂ adstate inhibits the decomposition of cyclopropane on the surface. Desorption of the hydrogen from the saturated overlayer of the dissociatively adsorbed cyclopropane partially regenerates the reactivity of the surface.