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Substituent effects on stability, MEP, NBO analysis, and reactivity of 2,2,9,9-tetrahalosilacyclonona-3,5,7-trienylidenes, at density functional theory


Cyclonona-3,5,7-trienylidene appears as boat-shaped transition state for having a negative force constant, while its singlet state exhibits less stability than the corresponding triplet state. Succeeding the quest for the largest unsaturated stable carbene-like species, theoretical investigations coupled with suitable isodesmic reactions are used to examine the effects of α,αʹ-tetrahalo groups on the thermodynamic along with kinetic viabilities of nine-membered cyclic silylenes. All the singlet and triplet silylenes appear as boat-shaped minima for having positive force constants on their potential energy surfaces and singlet states emerge as ground state, exhibiting more stability than their corresponding triplet states. The order of stability estimated by singlet (S)–triplet (T) energy separation (ΔES–T = ET − ES) emerges as α,αʹ-tetrahydrocarbene < α,αʹ-tetrahydrosilylene < α,αʹ-tetrafluorosilylene < α,αʹ-tetraiodosilylene < α,αʹ-tetrachlorosilylene < α,αʹ-tetrabromosilylene. This research specifies band gap (ΔEHOMO–LUMO) of scrutinized silylenes with this order. Hence, singlet 2,2,9,9-tetrabromosilacyclonona-3,5,7-trienylidene exists as the most stable species. From both thermodynamic and kinetic points of view, this species is more stable than synthesized silylene by Kira. It shows the highest heat of dehydrogenation through isodesmic reaction. The NBO analysis provides significant evidences for the stability of it through positive hyperconjugation, negative hyperconjugation, as well as mesomeric effects.

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This research is financially supported by Technical and Vocational University of Tehran, Dr. Shariaty College, Tehran, and North Tehran Branch, Islamic Azad University, Tehran, Iran.

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Correspondence to Hajieh Bastami.

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Supplementary material 1 The calculated sum of electronic and thermal enthalpy (Htot), sum of electronic and thermal free energy (Gtot), changes of enthalpy (ΔHS-T), changes of free energy (ΔGS-T), polarizability (αxx, αyy, αzz, and <α>), nucleophilicity index (N), global electrophilicity (ω), chemical potential (μ), global hardness (η), electronegativity (χ), global softness (S), maximum electronic charge (ΔNmax), the second order perturbation theory analysis of Fock Matrix in NBO basis including stabilization energies E(2) corresponding to the most important charge transfer interactions (donor-acceptor), C-D-C angle (D being the divalent, carbene-like atom), bond lengths, XYZ Cartesian coordinates, MEP maps, and shapes of selected frontier molecular orbitals for scrutinized silylenes (27 pages) (DOCX 34341 kb)

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Koohi, M., Bastami, H. Substituent effects on stability, MEP, NBO analysis, and reactivity of 2,2,9,9-tetrahalosilacyclonona-3,5,7-trienylidenes, at density functional theory. Monatsh Chem 151, 11–23 (2020).

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  • Silylene
  • Stability
  • Carbene-like species
  • Isodesmic reaction