Cyclische diazastannylene : XIV. Das tetramere eines 1-oxa-2-sila-3-aza-4λ2-stannetidins mit S4-symmetrie

1,3-Di-t-butyl-2,2-dimethyl-1,3,2,4λ²-diazasilastannetidine (II) reacts with 3,4,5-trimethoxybenzaldehyde to yield 3,4,5-trimethoxybenzaldehyde-t-butylimine (IV) and the title compound 2,2-dimethyl-3-t-butyl-1,2,3,4λ²-oxasilazastannetidine (III). III combines four different Main Group elements in one ring and is tetrameric in solvents as well as in the tetragonal crystalline state. An X-ray structure determination (R-value: 0.037) reveals the tetramer to have crystallographic S₄-symmetry, and that the molecules of III interact via oxygen-tin bonds, which are perpendicular to the plane of the ring. The oxygen atoms thus adopt trigonal-planar coordination, while the tin atoms have grigonal-pyramidal environments. The tin atoms are substituted by three different ligands: one nitrogen and two chemically non-equivalent oxygen atoms (SnN: 209.5(9), SnO: 218.8(6), SnO′: 213.7(6) pm). Each tin atom thus represents a center of chirality, which due to symmetry must be divided into two R- and two S-enantiomers. As a consequence the whole tetrameric molecule belongs to the optically inactive “meso”-form.     

Control of atomic collisions by laser polarization

Atomic collision pairs in a light field form a microscopic interferometer. The light acts as the beam splitter and controls at the same time the amplitudes and phases of the interfering waves. We demonstrate the complete tunability using linear and elliptic polarization.     

Über das Pferderennen in Siena

Zusammenfassung. In Siena findet zweimal j?hrlich ein Pferderennen statt, der sogenannte Palio, bei welchem jeweils zehn Contraden (Stadtbezirke) gegeneinander antreten. Nun hat Siena aber siebzehn Contraden, soda? ein gerechtes Auswahlverfahren n?tig ist. Traditionellerweise haben die sieben nichtteilnehmenden Contraden das Recht, im n?chsten Jahr mitzumachen, und die restlichen drei zur Komplettierung des Starterfelds werden dann durch das Los bestimmt. Dazu wird im folgenden ein mathematisches Modell (und zwar ein stochastischer Proze?) entwickelt, wobei im speziellen Fragen nach der Verteilung der Anzahl der Teilnahmen, nach der mittleren Teilnahmerate und nach der station?ren Verteilung beantwortet werden. Im Zuge dieser Untersuchungen finden auch lineare Differenzengleichungen Verwendung. Im letzten Abschnitt werden grundlegende Aspekte der Theorie der Markoff-Ketten am Beispiel des Auswahlverfahrens des Palio demonstriert. Eingegangen am 24.05.1996 / angenommen am 28.07.1998     

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Cyclische Diazastannylene. XIX. Zur Reaktion eines Bis(amino)germylens, -stannylens und -plumbylens mit Phosphortrichlorid und 2, 3-Dimethyl-1, 3-butadien

Cyclic Diazastannylenes. XIX. Reaction of a Bis(amino)germylene, -stannylene, and -plumbylene with Phosphorus Trichloride and 2, 3-Dimethyl-1, 3-butadiene The cyclic bis(amino)germylene 1 reacts with PCl₃ by a threefold insertion into the P? Cl bonds and forms [Me₂Si(N^tBu)₂Ge(Cl)]₃P( 4 ). 4 crystallizes in the triclinic space group P1 with cell dimensions: a = 1955.2(9), b = 1378.3(6), c = 1074.3(5) pm, α = 90.4(1), β = 121.6(1), γ = 97.9(1)° and Z = 2. X-ray structure analysis was used to show, that the molecule 4 has approximately C_3h point symmetry. All germanium, chlorine, and silicon atoms are quite accurately situated in a plane, perpendicular to which the GeN₂Si-rings are erected. The only heavy atom which disturbs the mirror symmetry is the phosphorus, which is on the top of a flat pyramide (Ge? P? Ge = 115.0°). Important bond lengths (mean values) are: Ge? P = 231.0(4), Ge? N = 182.4(7), Ge? Cl = 217.9(2) and Si? N = 173.6(7) pm. The unusual nearly planar coordination of the phosphorus atom can be explained by the particular steric requirements of the substituents. PCl₃ oxidizes the tin atom in the bis(amino)stannylene 2 by the formation of Me₂Si(N^tBu)₂SnCl₂ ( 5 ); as additional product originates an amorphous solid of analytical composition (PCl)_n. In contrast to 1 and 2 the lead atom     

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Book reviews

     

Comparing the structural topology of integral and peripheral membrane proteins utilizing electron paramagnetic resonance spectroscopy

The alignment of membrane proteins provides pertinent structural and dynamic information. Structural topology data gleaned from such studies can be used to determine the functional mechanisms associated with a wide variety of integral membrane proteins. In this communication, we successfully demonstrate, for the first time, the determination of the structural topology and helical tilt of an antimicrobial peptide magainin 2 using aligned X-band spin-label EPR spectroscopic techniques. This novel comparison unlocks many possibilities utilizing EPR spectroscopy to probe antimicrobial peptide topologies with increased sensitivity and may also give further clues to elucidate their corresponding mechanisms.     

Electrical performance of Ti/Pt thin films on glass-ceramic substrates after high temperature loading

In this study, the influence of post-deposition annealings (PDA) up to temperatures of T _PDA=700°C on the room-temperature resistivity of e-beam evaporated titanium/platinum (Ti/Pt) bi-layers on low temperature co-fired (LTCC) substrates covered with a glass encapsulate is investigated. The thickness of the platinum top layer is varied between 24 and 95 nm (titanium film thickness: 5 nm) and between 23 and 90 nm (titanium film thickness: 15 nm), respectively. In the “as-deposited” state and up to post-deposition annealing temperatures of T _PDA=450°C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. When applying, however, solely the Fuchs-Sondheimer model for evaluation, the effective mean free path for electrons is substantially above the value reported for crystalline platinum at room temperature. Compared to similar investigations on smooth Si/SiO₂ substrates yielding interpretable results within this theoretical approach, this is due to the increase of the thickness-dependent fraction in film resistivity which is strongly affected by the enhanced LTCC/glass surface roughness. At T _PDA>600°C, diffusion of titanium into the platinum top layer and the roughening of the LTCC/glass substrate dominate the electrical behavior, both causing an increase in film resistivity above average. In contrast to Si/SiO₂ substrates, thermal induced grooving effects in the Pt top layer play a minor role as the temperature coefficients of expansion of metallization and glass-ceramic substrate match better and the effective temperature difference for stress generation is lower due a glass softening temperature of about 450°C.