Classical planar doubly bonded Si-substituted silenes—a boundary system between olefins and heavier Group 14 analogs

Directed Gas Phase Formation of Silene (H2 SiCH2 )

The silene molecule (H₂ SiCH₂ ; X¹ A₁ ) has been synthesized under single collision conditions via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with silane (SiH₄ ). Exploiting crossed molecular beams experiments augmented by high-level electronic structure calculations, the elementary reaction commenced on the doublet surface through a barrierless insertion of the methylidyne radical into a silicon-hydrogen bond forming the silylmethyl (CH₂ SiH₃ ; X² A') complex followed by hydrogen migration to the methylsilyl radical (SiH₂ CH₃ ; X² A'). Both silylmethyl and methylsilyl intermediates undergo unimolecular hydrogen loss to silene (H₂ SiCH₂ ; X¹ A₁ ). The exploration of the elementary reaction of methylidyne with silane delivers a unique view at the widely uncharted reaction dynamics and isomerization processes of the carbon-silicon system in the gas phase, which are noticeably different from those of the isovalent carbon system thus contributing to our knowledge on carbon silicon bond couplings at the molecular level.

Thermochemistry of germanium and organogermanium compounds

This article reviews the current state of thermochemistry (enthalpies of formation) of germanium and organogermanium compounds.

Reexamination of the π-bond strengths within H2C=XHn systems: a theoretical study

The accurate determination of π-bond energies, D(π), in doubly-bonded species has been an important issue in theoretical chemistry. The procedure using the divalent state stabilization energy defined by Walsh has been suggested, and the procedure seems to be conceptually reasonable and applicable to all kinds of doubly-bonded species. Therefore, the aim of this study was to examine whether the procedure could be a reliable methodology for estimating the D(π) values for a variety of H(2)C=XH(n) species. To achieve a higher accuracy, the D(π) values were estimated at QCISD(T)/6-311++G(3df,2p) level of theory combined with isogyric correction. The D(π) values estimated in this work were in excellent agreement with the extant literature values. On the other hand, in determining accurate D(π) values for doubly bonded species, especially in species with lone-pair electrons such as H(2)C=O, it has been found that consideration of highly sophisticated electron correlation effects could be important. However, sufficiently accurate D(π) values have been obtainable at QCISD(T) or CCSD(T) levels with a 6-311++G(3df,2p) basis set on geometries at relatively inferior correlated levels such as MP2 and B3LYP levels with a 6-31+G(d) basis set.

An Alternative Interpretation of the C−H Bond Strengths of Alkanes

A new model based on 1,3 repulsive steric interactions (geminal repulsion) is proposed for explaining the variation in the C-H bond strengths of the alkanes. The model builds from the assumption that 1,3 repulsive interactions are the major factor in determining the stability of a C-C or C-H bond in an alkane. From this simple premise, the model successfully reproduces the effect of branching on the stability of alkanes, alkyl radicals, and alkenes. The results suggest that geminal repulsion can provide a simple, unified explanation for these fundamental stability trends. Although previous explanations have been widely accepted, it is shown that the theoretical support for them is relatively shallow and that the current hyperconjugative stabilization model is inconsistent with several experimental and computational results concerning alkyl radicals. In contrast, an explanation based on geminal repulsion provides a general conceptual framework for rationalizing each of these stability trends and is based on a physical effect that is known to play a role in the stability of alkanes and related species.