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Thermodynamic, structural, and magnetic criteria, the properties of the charge distributions, and low-energy ionization processes are theoretically analyzed to learn about the role of π-electron delocalization in recently synthesized stable singlet carbenes (Arduengo et al. J. Am. Chem. Soc. 1991, 113, 361) and silylenes (Denk et al. J. Am. Chem. Soc. 1994, 116, 2691) of the imidazol-2-ylidene type and also in related model systems. The different approaches show consistently that cyclic electron delocalization does indeed occur in the CC unsaturated imidazol-2-ylidene systems, in particular with respect to the corresponding C−C saturated imidazolin-2-ylidenes. However, the conclusion regarding the degree of conjugation and aromaticity depends on the criteria used, being quite small according to the “atoms-in-molecules” charge analysis but more significant according to the energetic and the magnetic properties. According to all criteria, the aromatic character of imidazol-2-ylidenes is less pronounced compared to benzene or the imidazolium cation. π-Electron resonance is found to be less extensive in the silylenes compared to their carbene analogs.

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Theoretical ab initio calculations were carried out to simulate the hydrolysis of the siloxane bond under neutral and acidic conditions. The most important factors reducing the energy barrier of the reaction are the protonation of the siloxane oxygen (acid catalysis) and the basic assistance to the nucleophile (responsible for withdrawal of the proton from a nucleophile). The basic assistance may be effectively accomplished by hydrogen bond complexes, which help to transfer a proton from the nucleophile to the leaving group. Such complexes may consist of water, silanol, and acid. Solvation lowers the barrier of acidcatalyzed hydrolysis, due to better stabilization of the charged transition states. The enhanced reactivity of the terminal siloxane bond in siloxanols is connected partly with the ability of the SiOH group to participate in hydrogen bond structures facilitating the intramolecular proton transfer to the siloxane oxygen. Scheme 6. Structures of the Reactive Complexes, 2(2)RCn, Transition States, 2(2)TSn, and Product Complexes, 2(2)PCn, for Hydrolysis of Disiloxanol 2 by a Water Dimer

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A series of isomeric silenes H2C=SiHR (1) and H2Si=CHR (2) where R = CH3, SiH3, F, OH, OSiH3, CN, and N02 were studied at the RHF/3-21G and 6-31G* levels. The two basis sets give in general similar results. The calculated r(C=Si) are (3-21G, values (in A) for 1 followed by 2) 1.

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