Fundamentals of Silicon Chemistry: Molecular States of Silicon-Containing Compounds

Silicon and its compounds have made possible the design of new materials, which, from computers to space travel, have helped to shape the technology of our 20th century. Conversely, the demands of new technology have stimulated the fast development of silicon chemistry as part of the “renaissance” of inorganic chemistry. This article uses selected examples of predominantly organosilicon compounds to discuss in simplified terms the measurement and assignment of suitable spectroscopic “molecular fingerprints” as well as the resulting benefit for the preparative chemist. The comparison of “equivalent” states of “chemically related” molecules is emphasized, based on perturbation arguments and supporting quantum-chemical models. Special attention is given to the relation between structure and energy, which allows us to understand and to predict the connectivity between and the spatial arrangement of silicon “building blocks”, the energy-dependent electron distribution over the effective nuclear potentials of a molecular framework, and, especially, the partly considerable effects of “silicon substituents” on molecular properties. Future-directed extensions and applications include polysilane band structures, Rydberg states of chromophores containing silicon centers, redox reactions and ion-pair formation of silicon-substituted π systems, and molecular dynamic phenomena in solution or on thermal fragmentation in the gas phase. The main objective is a set of clear and practical rules for interpreting measurements and planning experiments.     

An ab initio structural and vibrational analysis of gauche,trans,trans- and gauche,cis,trans-hexa-1,3,5-trienes

The results of complete geometry optimizations of the high energy stable gauche,Trans,trans- and gauche,Cis,trans- rotamers of hexa-1,3,5-trienes are reported at the RHF/6-31G//RHF/B-31G level. The angles of rotation around one of the single C-C bonds are found to be 33.7° and 45.5°, respectively. The corresponding harmonic force fields of these molecules are also reported at this level and corrected using scale factors transferred from buta 1,3-diene. Aspecial scale factor was used for the central C=C double bond stretching coordinate to take into account vibronic coupling. The theoretical vibrational frequencies, calculated with the scaled quantum mechanical (SQM) force fields, allow a complete interpretation of the experimental vibrational spectra of these molecules.Preliminary results were reported at the Austin XII Symposium on Molecular Structure, Austin, TX, February 28 through March 3, 1988, S 18, p. 111 (USA) and at the XIXth European Congress on Molecular Spectroscopy, Dresden, September 4 through September 8, 1989, p. 226 (GDR).     

Elektronentransfer und Ionenpaar-Bildung. 40. Mitteilung. Einkristall-Struktur von Bis(Natrium-1,1′-Biphenyl-2-thiolat-Diglyme): Ein Zwischenprodukt der reduktiven Ringöffnung von Dibenzothiophen

Electron Transfer and Ion Pair Formation Single Crystal Structure of Bis(sodium 1,1′-biphenyl-2-thiolate-diglyme): An Intermediate in the Reductive Ring Opening of Dibenzothiophene On Na-metal reduction of dibenzothiophene, the five-membered sulfur ring opens to form a colorless 1,1′-biphenyl-2-thiolate sodium salt, which, according to its single-crystal structure determination, is a dimer containing a four-membered, twice diglyme-solvated ring (diglyme···Na^⊕?SR)₂. Additional measurements provide the following information: cyclic voltammetry in aprotic MeCN solution shows one quasi-reversible electron transfer at E = ?2.58 V. The dibenzothiophene radical anion can be generated in aprotic THF solution at a K mirror and characterized by an 81-line ESR spectrum and its simulation. This blue species is also the first UV/VIS detectable one before the solution changes via green (due to blue + yellow color mixing) to yellow, yielding across an isosbestic point a second and diamagnetic compound. All of the above results suggest a consecutive two-electron reduction followed by an intersystem protonation, M + (e^?) → M^.? (blue) + (e^?) → (M^??, yellow?) + (H^⊕) → MH^? (colorless), to yield the crystallized and structurally characterized reaction intermediate. The diglyme-solvated sodium-salt dimer provides a basis for a quantum-chemical discussion of some facets of the most likely microscopic reduction pathway.     

Rotation about the C?N bond in 2-aza-1,3-butadiene and the N?N bond in 2,3-diaza-1,3 butadiene: A molecular orbital study

The geometry and energy of 2-aza-1,3-butadiene and 2,3-diaza-1,3-butadiene have been calculated using the 6-31G* basis set as a function of the CNCC and CNNC dihedral angles, respectively. With the 2-aza derivative potential minima are located at 0° (trans) and at about 130° for a gauche structure approximately 9.5 kJ mol^?1 less stable than the trans. Potential maxima are at about 75° giving a gauche barrier height of approximately 19 kJ mol^?1 relative to the trans structure, and at 180° (cis) giving a barrier height of approximately 14.5 kJ mol^?1 relative to the 130° gauche structure. With the 2,3-diaza derivative the gauche barrier has disappeared and there are a series of gauche structures in the region 70°–100° of almost equal energy 12.5-15 kJ mol^?1 less stable than the trans. In addition the cis barrier is much greater, nearly 70 kJ mol^?1 relative to the trans structure. Inclusion of electron correlation, accounting for about 50% of the correlation energy, produces no significant changes in the shape of the potential energy curves. There are systematic and progressive changes in almost all the geometrical parameters as the ?CH? groups in butadiene are replaced by ?N? . The outward tilt and compression within the methylene groups show adverse steric interactions to be operative in the cis structures. The values of V_nn indicate that gauche structures of both the 2-aza and the 2,3-diaza derivatives near the cis structure are more compact (as with butadiene), and gauche structures of the 2-aza derivative near the trans structure are less compact (as with butadiene). Originating in the changes in bond lengths and bond angles, rotation-independent nuclear–nuclear interactions again play an important role.     

Intramolecular interaction effects and the structure of N2OS and N2O2: An Ab initio study

An ab initio study of O?N? N?S with full geometry optimization has been carried out to corroborate the presence of an interaction between the terminal atoms in this type of structure, which, in O?N? N?O, apparently stabilizes the cis conformer. Using the unscaled 4–31G basis set with a full set of d functions on the sulfur, there is a potential minimun at the trans but not the cis geometry. A gauche conformer with a torsional angle of 77.2° is the most stable. With N₂O₂ this basis set gives potential minima at both the cis and trans geometries, but the trans conformer is slightly more stable, contrary to experiment and the results of (7,3) basis-set calculations reported in the literature in which Gaussian lobe functions were employed. Using a (9,5) basis set there is no longer a potential minimum at the cis geometry, and a gauche structure is more stable than the cis conformer as in the case of N₂OS with the less-extended basis set. Force constants (harmonic and anharmonic), compliance constants, relaxed force constants, and interaction-displacement coordinates for both molecules are compared for key structural elements.     

Photoelektronen-Spektren und Moleküleigenschaften. 133 [1, 2]. Trifluormethyldisulfan und Derivate F3CSSX (X  CF3, F,Cl, Br)

Photoelectron Spectra and Molecular Properties. 133. Trifluoromethyldisulfane and Derivates F₃CSSX (X?CF₃, F, Cl, Br) The He(I) photoelectron spectra of trifluoromethyldisulfane F₃CSSH and its derivatives F₃CSSX (X?CF₃, F, Cl, Br) are assigned by Koopmans correlations, IE = ?ε, with MNDO eigenvalues and by radical cation state comparison. Of special interest are the n/n splittings, which amount to 1.15 eV F₃C? SS? F or 0.87 eV in F₃? SS? Cl, and the dependance of which on the dihedral angle ω(XS? SX), on the SS bond length and on the acceptor effect of the F₃C substituents is discussed.     

Improved determination of flavour compounds in butter by solid-phase (micro)extraction and comprehensive two-dimensional gas chromatography

The practicability and potential of comprehensive two-dimensional gas chromatography (GC x GC) coupled to both conventional flame ionisation (FID) and time-of-flight mass spectrometric (TOF-MS) detection, were compared with those of conventional one-dimensional (1D) GC, with the determination of flavour compounds in butter as an application. For polar flavour compounds, which were collected from the aqueous fraction of butter by means of solid-phase extraction (SPE), it was found that GC x GC dramatically improves the overall separation. Consequently, quantification and preliminary identification based on the use of ordered structures, can be performed more reliably. The improvement effected by replacing 1D-GC by GC x GC is considerable also in the case of TOF-MS detection, as illustrated by the high match factors generally obtained during identification. GC x GC was also used successfully for the characterisation of volatile flavour compounds in the headspace of butter collected by solid-phase microextraction (SPME) and to study the effect of heat treatment on the composition of butter samples in more detail.     

The use of 90°-1,3-butadiene as a reference structure for the evaluation of stabilization energies for benzene and other conjugated cyclic hydrocarbons

Reactions are described that employ 90°-1,3-butadiene as a reference structure for the evaluation of the stabilization energyof the benzenoid and other cyclic conjugated hydrocarbons. The unique benefits of this rotamer of butadiene as a reference molecule within the homodesmotic conceptual framework are discussed. Experimental stabilization energies are presented for a number of cyclic hydrocarbons.     

Limitations and perspectives in the determination of carbofuran with various liquid chromatography-mass spectrometry interfacing systems

Column liquid chromatography with mass spectrometric detection (LC-MS) has been widely accepted as the preferred technique for the identification and quantification of polar and thermally labile compounds at trace levels. Over the last decade many different types of LC-MS interfacing techniques have been used for the determination of carbamate pesticides and especially for the N-methylcarbamate carbofuran. This article addresses the difficulties encountered with the various types of LC-MS interface and discusses recent alternatives for the determination of carbofuran. With thermospray and particle beam interfaces the quantification of carbofuran is affected by both the ion source pressure and temperature, whereas quantification using the recently developed atmospheric pressure ionization interfaces, atmospheric pressure chemical ionization, electrospray, and ionspray, is less dependent on these parameters.     

Upper limit for thermal direct photon production in heavy-ion collisions at 60 and 200A·GeV

The production of direct photons has been investigated in reactions ofp and¹⁶O projectiles at 60 and 200A·GeV with C and Au nuclei. Photon and ⁰ spectra have been measured in the pseudorapidity range 1.52.1 for the transverse momentum region 0.4 GeV/cp _T 2.8 GeV/c employing the lead-glass spectrometer SAPHIR. An upper limit of 15% at the 90% confidence level for the direct photon signal relative to the neutral pion production is obtained from the comparison of measured photon spectra with Monte Carlo simulations of the hadronic background based on the reconstructed yield of ⁰ and mesons. Consequences for a possible phase transition to a quark-gluon plasma are discussed.