The hierarchy of ab initio and DFT methods for describing an intramolecular non-covalent Si⋯N contact in the silicon compounds using electron diffraction geometries

Abstract

In the series of silatranes XSi(OCH₂CH₂)₃N, 1 (X = Me, 1a; H, 1b; F, 1c) with the known gas electron diffraction (GED) structures, the problematic geometry of 1-methylsilatrane 1a has been revised. In particular, the new value of the Si⋯N distance (d_SiN) in 1a turned out to be ∼0.06 Å longer than the generally accepted one. This d_SiN resolves the long-standing contradiction between the data of the structural and spectral experiments regarding the sensitivity of 1 to the medium effect. We also performed the ab initio and DFT study of the combined series of silatranes 1a–c, silylalkylamines H₃Si(CH₂)₃NMe₂ (2a) and F₃SiCH₂NMe₂ (2b), silylhydrazines F₃SiN(Me)NMe₂ (2c) and F₃SiN(SiMe₃)NMe₂ (2d), and silyloxyamines ClH₂SiONMe₂ (2e,f), (F₃C)F₂SiONMe₂ (2g,h) and F₃SiONMe₂ (2i), in which the GED d_SiN values are in a wide range of 2–3 Å. None of the involved quantum chemical methods has succeeded in reproducing all the experimental gas-phase d_SiN values in 1a–c, 2a–i with an acceptable accuracy (0.01–0.03 Å). The problems of the used methods, primarily CCSD with the Pople basis sets, are caused by four molecules with the geminal SiNN and SiON fragments (2d,f–i) and d_SiN < 2.3 Å. A reasonable hierarchy of computationally accessible theory levels for studying the physicochemical manifestation of the non-covalent intramolecular Si⋯N interactions can be constructed only at d_SiN > 2.3 Å: MP2 < PBE0 ∼ B3PW91 ∼ SCS-MP2 < CCSD < CCSD(T).