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

In the series of silatranes XSi(OCH2CH2)(3)N, 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 SiMIDLINE HORIZONTAL ELLIPSISN distance (d(SiN)) in 1a turned out to be similar to 0.06 angstrom 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)(3)NMe2 (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 angstrom. 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 angstrom). 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 angstrom. A reasonable hierarchy of computationally accessible theory levels for studying the physicochemical manifestation of the non-covalent intramolecular SiMIDLINE HORIZONTAL ELLIPSISN interactions can be constructed only at d(SiN) > 2.3 angstrom: MP2 < PBE0 similar to B3PW91 similar to SCS-MP2 < CCSD < CCSD(T).

Automated summary

The research revised the geometry of silatranes and resolved the contradiction regarding the sensitivity to the medium effect. Despite studying a series of silatranes, quantum chemical methods struggled to accurately reproduce the experimental values. A reasonable hierarchy of computational theory levels for studying non-covalent intramolecular interactions involving silicon and nitrogen can only be constructed at specific distances.