Sterically-crowded pyrroles

The syntheses in good yields of a considerable number of moderately to severely sterically-crowded 1,2,5-trisubstituted pyrroles by means of modifications of the Knorr-Paal condensation are described. Notwithstanding reports to the contrary, the classical procedure; succeeded in moderately sterically-crowded cases, e.g., R² = R⁵ = Me, R¹ = i-Pr, Me₂CHMeCH-, Me₂N-, Ph₂N-, 9-carbazolo, subst. 4-morpholino, 1-piperazino, 1-piperidino, etc. A product-water azeotroping procedure succeeded in more severely sterically-crowded cases e.g., R² = R⁵ = Et, R¹ = i-Pr; R² = R⁵ = Me, R¹ = Me₃CCH₂-, R² = R⁵ = i-Pr, R¹ = PhCH₂-. Highly sterically-crowded pyrroles, e.g., R² = R⁵ = Me, R¹ = t-Bu, adamantyl, PhCH₂CMe₂; R² = R⁵ = Et, R¹ = t-Bu; R¹ = R² = R⁵ = i-Pr; R² = R⁵ = t-Ru, R¹ = Me, Et, PhCH₂-, were obtained using titanium tetrachloride as a Lewis acid catalyst and as a water scavenger. Infrared spectra of the reacting mixtures provided evidence pertinent to the mechanism of the reaction, which is discussed.     

Studies directed toward the total synthesis of azaspiracid: stereoselective construction of C(1)-C(12), C(13)-C(19), and C(21)-C(25) fragments

[reaction: see text] The efficient entry to the C(1)-C(12), C(13)-C(19), and C(21)-C(25) fragments of azaspiracid is outlined. The C(1)-C(12) portion is constructed using a key asymmetric allenyl borane addition to the corresponding alpha,beta-unsaturated aldehyde. The synthesis of the C(13)-C(19) portion utilizes an Evans asymmetric alkylation followed by Sharpless asymmetric dihydroxylation. In addition, a novel solution to the mismatched effects of a neighboring chiral oxazolidinone during a Sharpless dihydroxylation is detailed.     

The emission spectra of H35Cl+, H37Cl+, D35Cl+, and D37Cl+ in the region 2700–4100 Å

The $\text{A}{}^2\text{Σ}{}^{\mathrm{+}}\text{-X}{}^2\text{Π}$ emission spectrum of HCl⁺ has been measured and analyzed for four isotopic combinations. These analyses extend previous work and provide rotational constants for the v = 0–2 levels of the ground state and for the v = 0–9 levels of the excited state. RKR potentials have been determined for both states, although the upper state could not be fitted precisely to such a model. Calculated relative intensities based on these potentials demonstrated that the electronic transition moment must change rapidly with lower state vibrational quantum number. Although considerable caution should be exercised in applying the concept of equilibrium constants to the $\text{A}{}^2\text{Σ}{}^{\mathrm{+}}$ state, the following are the best estimates of these constants (in cm^?1) for the $\text{X}{}^2\text{Π}$ state of H³⁵Cl⁺: B_e = 9.9406, ω_e = 2673.7, A_e = ? 643.7, and $\text{r}{}_{\mathrm{e}}\text{= 1.315}\text{A}\text{?}$. For the $\text{A}{}^2\text{Σ}{}^{\mathrm{+}}$ state of H³⁵Cl: T_e = 28 628.08, B_e ～ 7.505, ω_e ～ 1606.5, and $\text{r}{}_{\mathrm{e}}\text{= 1.514}\text{A}\text{?}$.     

A pulsed-leak valve for use with ion trapping mass spectrometers

A pulsed-leak valve that allows the introduction of a prolonged, flat, and controllable pulse of gas is described. Test results from the valve that utilized a Fourier transform ion cyclotron resonance mass spectrometer with Ar and C₂H₆ as the sample gases indicate that the valve functions as expected and yields basically rectangular pressure profiles in the cell region. The rise and fall times are similar to those of just the stand-alone pulsed valve and are believed to be determined mainly by the design of the vacuum system, rather than the design of the pulsed-leak valve. Kinetic data for the reaction of Nb⁺ with C₂H₆, acquired with the use of the pulsed-leak valve to introduce the C₂H₆ gas, demonstrates the practical application of this valve for kinetic and other analogous studies. Use of the pulsed-leak valve greatly reduces the loss of the reactant ion signal during the cooling period.     

The surface science of semiconductor processing: gate oxides in the ever-shrinking transistor

Due to the extreme dimensional scaling required by Moore's law, Si device technology is increasingly subject to the limitations imposed by the intrinsic physics and chemistry of surfaces and interfaces. In this review we outline ways in which fundamental surface science has contributed an understanding to the microelectronics community and discuss areas where surface science may impact future development. We focus on the example of silicon dioxide (SiO₂) on silicon, since this interface lies at the heart of modern transistor technology and has therefore received a great deal of attention in recent years. We highlight a number of experimental and theoretical approaches that have elucidated the fundamental phenomena associated with the formation and evolution of this critical technological interface, revealing the remarkable interdependence of science and technology that now characterizes this rapidly evolving industry.     

A novel discrete mode narrow linewidth laser diode for spectroscopic based gas sensing in the 1.5 μm region

The characteristics of a novel Discrete Mode laser diode are described with particular reference to its capability in spectroscopic based gas sensing. In this regard, its emission linewidth, Side Mode Suppression Ratio and tuneability are of particular note. The measured narrow linewidth of 550?kHz has applications with respect to spectrally narrow absorption features found, for example, in saturated absorption lines, in hyperfine structure of some rotational manifolds and in whispering gallery mode features associated with optical microspheres. Other notable measured characteristics show mode-hop-free temperature tuning of 11 nm at a linear tuning rate of 0.1 nm/°C and a Side Mode Suppression Ratio in the range 56 to 46 dB and a long term (72?hours) wavelength stability of 1?pm per day. In essence, this novel Discrete Mode laser displays excellent potential for diode laser based sensing applications in the near infrared.