Organosilicon compounds field effects

The ultimate analysis of organosilicon compounds is an important subject to every worker in the field of siloxane polymers and their intermediates, for without dependable analytical methods the research chemist gropes blindly, at a loss concerning the composition of his products and unable to evaluate the effects of chemical attack. It is the purpose Of this chapter to trace very briefly the development of adequate analytical procedures for organosilicon compounds, with particular emphasis upon those methods which may be used for investigating the composition of silicone polymers. 

Because this chapter is a follow-up of previous work in the field it is not necessary to repeat the basics of ab initio methods. This has been done in detail by Basch and Hoz, who also discuss the most important atomic properties of Ge, Sn and Pb. We also recommend the theoretical section in the chapter by Apeloig about organosilicon compounds in this series who gave an excellent overview about the most important aspects of ab initio, semiempirical and force-field methods. The reader will find there an explanation of the most common standard methods which will be mentioned in this review without further explanation. We will focus in the following on those theoretical and computational aspects of methods which are particularly important for heavy-atom molecules that have been advanced in the last decade, i.e. ECPs and DFT. We also briefly discuss relativistic effects. We point out that semiempirical methods" and force field parameters are available for the elements Ge, Sn and Pb. However, the application of the two methods has not gained much popularity and not many papers have been published in the field. Most reports are restricted to special problems. ... 

The double pulsed field gradient spin echo (DPFGSE) pulse sequence has been used to improve the measurement of proton-proton nuclear Overhauser effect (NOE) [28]. The DPFGSE NOE does not rely on difference spectroscopy and very small NOEs can be measured. This technique has been used to determine the structure of organosilicon compounds [28]. 

We have shown in this paper that molecular orbital calculations at the ab initio level can be used to predict reliably the spectral transitions in silylenes, to evaluate the effects of substituents on the Si=Si multiple bond, to shed new light on existing experimental data and to direct future work towards the synthesis of novel isomers of disilenes. Although carbon and silicon are isoelectronic, multiple bonds to silicon differ dramatically from multiple bonds to carbon and analogies from carbon chemistry might therefore be entirely misleading when applied to silicon compounds. We believe that our studies have demonstrated the enormous power of modem computational methods and hope that this paper will prompt future theoretical studies and more importantly, theoretical-experimental collaborations in the field of organosilicon chemistry. 

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