Abstract
Dissociation pathways of the global minimum geometry of Si2C5H2 with a planar tetracoordinate carbon (ptC) atom, 2,7-disilatricyclo[4.1.0.01,3]hept-2,4,6-trien-2,7-diyl (1), have been theoretically investigated using density functional theory and coupled-cluster (CC) methods. Dissociation of Si-C bond connected to the ptC atom leads to the formation of 4,7-disilabicyclo[4.1.0]hept-1(6),4(5)-dien-2-yn-7-ylidene (4) through a single transition state. Dissociation of C-C bond connected to the ptC atom leads to an intermediate with two identical transition states and leads back to 1 itself. Simultaneous breaking of both Si-C and C-C bonds leads to an acyclic transition state, which forms an acyclic product, cis-1,7-disilahept-1,2,3,5,6-pentaen-1,7-diylidene (19). Overall, two different products, four transition states, and an intermediate have been identified at the B3LYP/6-311++G(2d,2p) level of theory. Intrinsic reaction coordinate calculations have also been done at the latter level to confirm the isomerization pathways. CC calculations have been done at the CCSD(T)/cc-pVTZ level of theory for all minima. Importantly, all reaction profiles for 1 are found be endothermic in Si2C5H2. These results are in stark contrast compared to the structurally similar and isovalent lowest-energy isomer of C7H2 with a ptC atom as the overall reaction profiles there have been found to be exothermic. The activation energies for Si-C, C-C, and Si-C/C-C breaking are found to be 30.51, 64.05, and 61.85 kcal mol−1, respectively. Thus, it is emphasized here that 1 is a kinetically stable molecule. However, it remains elusive in the laboratory to date. Therefore, energetic and spectroscopic parameters have been documented here, which may be of relevance to molecular spectroscopists in identifying this key anti-van’t-Hoff-Le Bel molecule.
Highlights
(i) what is the energetic stability of the proposed molecule from a given elemental composition on the molecular potential energy surface (PES)? and (ii) what is its kinetic stability? A firm answer obtained from these two questions either directly or indirectly informs the experimentalists whether the theoretically proposed molecule could possibly be identified in the laboratory or not
It was outlined in the past that molecules with a planar tetracoordinate carbon (ptC) atom can be enormously stabilized by the cooperative influence of metal pairs (Zr/Al or Zr/Zr+ ) of atoms [22]
In 2017, isomers of X2 C5 H2 (X = Si, Ge, Sn, and Pb) with a ptC atom have been theoretically proposed as global minimum geometries but to date they are yet to be identified in the laboratory [51]
Summary
Apart from chemical curiosity, interest in molecules with a planar tetracoordinate [1,2,3,4,5,6,7,8]. In 2017, isomers of X2 C5 H2 (X = Si, Ge, Sn, and Pb) with a ptC atom have been theoretically proposed as global minimum geometries but to date they are yet to be identified in the laboratory [51] This indirectly indicates that mere thermodynamic stability is not the only governing factor in the successful identification of molecules in the laboratory [52,53,54,55,56,57]. The thermodynamic stabilities of various isomers have been examined at length, to the best of our knowledge, the kinetic stability of 1, which contains a ptC atom, is yet to be studied Though it was reported as a global minimum nearly three years ago, the experimental evidence is completely absent on 1 and on all other low-lying Si2 C5 H2 isomers. The kinetic stability of 1 has been examined here, which may possibly aid the detection of this peculiar molecule using infrared or rotational spectroscopy in the laboratory
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