Silylenes electronic structure

Silylene 132 showed absorption maxima at 260 and 440 nm in its UV-vis spectra, the latter of which is assignable to the (Si)-3p(Si) transition and close to those observed for dimethylsilylene (453 nm) and l-silacyclopentane-l,l-diyl (436 nm). Thus, the electronic structure of silylene 132 is not much per-... 

Electronic structure of stable carbenes, silylenes and germylenes... 

The electronic structures of the l,3,2(A2)-diazasiloles 83-85 and 89 have been probed by He(i) and He(n) photoelectron spectroscopy. The experimental ionization potentials are summarized in Table 3. The molecular orbitals were assigned with the help of quantum mechanical computations for model compounds and in each case the HOMO was found to be a Jt-type MO of b, symmetry. In the case of the benzol] and pyiido[/4 fused compounds 85 and 89 also the HOMO-1 is a Jt-type MO. The next band was assigned to a lone pair-type MO of aj symmetry, which is mostly located at the dicoordinated silicon atom. Therefore, the HOMOs of the cyclic silylenes 83 and 84 differ in nature from those of the homologues V-heterocyclic carbenes which consist of an essentially aj lone pair-type MO at... 

The success of the GIAO calculations in predicting correctly the NMR spectrum of 4a and 5a suggests that this method can be used to study also the NMR spectra of transient silylenes which cannot yet be studied experimentally. Such studies can provide important ftmdamental information on the electronic structure of silylenes. 

Calculations of the chemical shifts and the CSA of a variety of silylenes are in progress in our laboratory, and we are confident that they will provide valuable information as well as important insights into the electronic structure of these interesting species. We also hope that the calculations will prompt the experimental measurements of chemical shift anisotropies for stable silylenes. 

Summary Answers are suggested to the questions what have we learned from silylene chemistry and what do we look forward to learning in the future Included are initial observations of the chemistry of supersilylenes , four-valence electron monovalent silicon cations R-Si whose electronic structure endows them with the potential for forming as many as three new bonds in a single reactive collision. 

In an early study, Arduengo et al. conducted XPS and DFT studies on a model carbene, and on the analogous silylene and germylene [45]. While such studies showed a localized lone pair in the plane of the imidazolylidene ring, with an empty p orbital perpendicular to this, the electronic structures of the silylene and germylene analogs were quite different. The HOMOs in these compounds... 

The electronic structure of silylenes has been discussed in depth since the first matrix isolation of dimethylsilylene by West and coworkers , who observed that dimethylsilylene produced by the irradiation of dodecamethylcyclohexasilane in an argon matrix at 10 K or in a 3-methylpentane glass matrix at 77 K showed a characteristic absorption maximum at 450 nm. All the silylenes investigated so far are known to be ground state singlets and the absorption due to the excitation of a lone-pair electron to the vacant pjr orbital of the silylene (equation 13) is observed in the UV-VIS region °. The absorption maxima depend strongly on the electronic and steric effects of the substituents. Absorption maxima... 

The electronic structure, including the spectroscopic aspects and the chemistry of silylenes, were the subject of some recent reviews. Here, our aim is to show, using silylenes as a case study, that it is possible to plan systematically new stable systems with unusual bonding, by using the methodology of computational chemistry in a systematic way. For other aspects we refer to the previous reviews, when appropriate. The procedure applied was the following ... 

The silylene precursor LSi L=CH(C=CH2)(CMe)(NAr)2, Ar = 2,6-jPr2C6H3 possesses a ylide-like (zwitterionic) electronic structure and exhibits an electron-rich butadiene moiety in the backbone that can be utilized for the metal-free activation of E—H bonds or the addition of Lewis acids. By virtue of this ability and the basic 5-donation by the silicon atom the arsasilene functionality can be obtained by a straightforward reaction procedure. 

The chemical potential, chemical hardness and sofmess, and reactivity indices have been nsed by a number of workers to assess a priori the reactivity of chemical species from their intrinsic electronic properties. Perhaps one of the most successful and best known methods is the frontier orbital theory of Fukui [1,2]. Developed further by Parr and Yang [3], the method relates the reactivity of a molecule with respect to electrophilic or nucleophilic attack to the charge density arising from the highest occupied molecular orbital or lowest unoccupied molecular orbital, respectively. Parr and coworkers [4,5] were able to use these Fukui indices to deduce the hard and soft (Lewis) acids and bases principle from theoretical principles, providing one of the first applications of electronic structure theory to explain chemical reactivity. In essentially the same form, the Fukui functions (FFs) were used to predict the molecular chemical reactivity of a number of systems including Diels-Alder condensations [6,7], monosubstituted benzenes [8], as well as a number of model compounds [9,10]. Recent applications are too numerous to catalog here but include silylenes [11], pyridinium ions [12], and indoles [13]. 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

The stability of molecules depends in the first place on limiting conditions. Small, mostly triatomic silylenes and germylenes have been synthesized successfully at high temperatures and low pressures, 718). Their reactions can be studied by warming up the frozen cocondensates with an appropriate reactant, whereas their structures are determined by matrix techniques 17,18). In addition, reactions in the gas phase or electron diffraction are valuable tools for elucidating the structures and properties of these compounds. In synthetic chemistry, adequate precursors are often used to produce intermediates which spontaneously react with trapping reagents 7). The analysis of the products is then utilized to define more accurately the structure of the intermediate. 

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