Ab initio and DFT study of the structure of pentacoordinated silicon compounds: Si-substituted (O–Si)dimethyl( N-acetylacetamidomethyl)silanes

Abstract

The structure and thermodynamic stability of Si-substituted (O–Si)dimethyl( N-acetylacetamidomethyl)silanes were studied at the ab initio (HF, MP2 with different basis sets) and DFT (B3LYP/6-31G*) levels of theory. Performed calculations have demonstrated that these compounds exist explicitly in chelate forms with pentacoordinate silicon atom with energetic advantage of these forms over nonchelate structures equal to ∼3–6 kcal/mol (at T=0 and 298 K) depending on the nature of axial substituent at the silicon atom (X=OCOCH 3, OCOCF 3, F, Cl, Br). According to the Bader analysis of electron distribution, chelate cycles SiCNCO in the compounds studied are closed by weakly covalent Si–O bonds. A decrease in the Si–O bond distance upon variation of X is accompanied by increasing the stability of the (O–Si)chelate complexes and the extent of electron density transfer from the donor oxygen atom to the acceptor XSiC 3 fragment. The negative sign of the multi-center bond indices calculated for the X–Si–O fragment of Si-substituted (O–Si)dimethyl( N-acetylacetamidomethyl)silanes is suggestive of the (3c-4e) character of bonding in this fragment.