The chemistry of the non-metallic elements has in recent years passed through a period of rapid development, often referred to as its “renaissance”. To emphasize just one of the key facets: numerous short-lived molecules containing multiple bonds to elements of the third and higher periods have been discovered, often accompanied by the planned synthesis of derivatives which are sterically shielded by bulky groups and thus kinetically stabilized. Thus today molecules such as silabenzenes H6C6−nSin and silaethenes H2SiCH2 or R2SiCR2, disilenes R2SiSiR2 and diphosphenes RPPR, silaphenylisonitrile H5C6NSi, or methylidyne-phosphanes RCP, are all well-known species. Sandwich compounds with P6 rings or silicon centers demonstrate that there are now hardly any barriers to impede the imagination of the non-metal chemist. In sharp contrast is our lack of knowledge regarding the “microscopic” pathways of chemical reactions: thus apart from information provided for example by molecular beam experiments, or from exact numerical calculations involving species consisting of only a few atoms, it remains largely unknown from which directions medium-sized molecules must approach each other to successfully collide and form a “reaction complex”, in which way their structures are changed in such a process or which role is played by molecular dynamics in the energy transfer.–The pyrolysis of azides XN3, i.e. compounds which tend to explode violently when ignited in the condensed phase but can be heated in low-pressure gas flow systems without much risk, illustrates that studies of reactive intermediates are of interest not only because novel molecules may be discovered and isolated, and thereby possibilities for synthesis expanded. Moreover, some aspects of the “microscopic” pathways of these azide pyrolyses can be described satisfactorily on the basis of calculated energy hypersurfaces, and the influence of molecular dynamics becomes experimentally visible in the “chemical activation” of intermediates which leads to their “thermal explosion”.

The Pyrolysis of Azides in the Gas Phase

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.

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

Ab initio molecular orbital theory has been applied to the structure of hexasilabenzene (Si6H6), the silicon analog of benzene (C6H6). The minimum energy structure is calculated to be planar with D6h symmetry, as in the case of benzene, with flexible basis sets at the Hartree–Fock level of theory. Through the estimation of a significant effect of electron correlation, however, a chair‐like puckered structure of D3d symmetry is proposed for hexasilabenzene. The preference of D3d over D6h is explained in terms of some ‘‘multiradical’’ character of the skeletal bonding (i.e., the antiferromagnetic‐like localization of electrons onto each Si).

Hexasilabenzene (Si6H6). Is the benzene-like D6h structure stable?

Zur Analyse und zur Optimierung zahlreicher Gasreaktionen in stromenden Systemen hat sich die Photoelektronen-Spektroskopie bestens bewahrt: So gelingt es mit dieser Echtzeit-Messonde bei den meisten Pyrolysen – in Millimol-Ansatzen und innerhalb weniger Stunden –, die Temperaturen fur die verschiedenen Zersetzungskanale zu ermitteln, die Hauptprodukte zu charakterisieren und gegebenenfalls deren Ausbeute zu verbessern. Bei geeigneter Versuchsfuhrung lassen sich kurzlebige und/oder reaktive Molekule wie P2, Thioformaldehyd oder Silabenzol nachweisen. Von Vorteil ist die PE-spektroskopische Gasanalytik insbesondere bei der Suche nach Heterogen-Katalysatoren, die innerhalb eines Tages zwischen 300 und 1300 K mit Gasgemischen wechselnder Zusammensetzung getestet werden konnen. PE-Spektrometer eignen sich zum ”on line„-Anschlus an Rechner; tragbare Gerate fur den Laborbetrieb befinden sich in Entwicklung.

Photoelektronen-Spektren und Moleküleigenschaften: Echtzeit-Gasanalytik in strömenden Systemen†‡

We have incorporated a pulsed, hyperthermal nozzle with a cryostat to study the matrix-isolated infrared spectroscopy of organic radicals. The radicals are produced by pyrolysis in a heated, narrow-bore (1-mm-diam) SiC tube and then expanded into the cryostat vacuum chamber. The combination of high nozzle temperature (up to 1800 K) and near-sonic flow velocities (on the order of 104 cm s−1) through the length of the 2 cm tube allows for high yield of radicals (approximately 1013 radicals pulse−1) and low residence time (on the order of 10 μs) in the nozzle. We have used this hyperthermal nozzle/matrix isolation experiment to observe the IR spectra of complex radicals such as allyl radical (CH2CHCH2), phenyl radical (C6H5), and methylperoxyl radical (CH3OO). IR spectra of samples produced with a hyperthermal nozzle are remarkably clean and relatively free of interfering radical chemistry. By monitoring the unimolecular thermal decomposition of allyl ethyl ether in the nozzle using matrix IR spectroscopy, we have derived the residence time (τnozzle) of the gas pulse in the nozzle to be around 30 μs.

Intense, hyperthermal source of organic radicals for matrix-isolation spectroscopy

Analysis and optimization of gas-phase reactions. 13. Silatoluene

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Analysis and optimization of gas-phase reactions. 13. Silatoluene