Search Results (5)

Here, five bonds to carbon through tri-coordination are theoretically established in the global minimum energy isomers of ${\mathrm{Al}}_3{\mathrm{C}}_3^{-}$ anion (1a) and ${\mathrm{Al}}_3{\mathrm{C}}_3$ neutral (1n) for the first time. Various isomers of ${\mathrm{Al}}_3{\mathrm{C}}_3^{-/0}$ are theoretically identified using density functional theory at the PBE0-D3/def2-TZVP level. Chemical bonding features are thoroughly analyzed for these two isomers (1a and 1n) with different bonding and topological quantum chemical tools, such as adaptive natural density partitioning (AdNDP), Wiberg Bond Indices (WBIs), nucleus-independent chemical shifts (NICS), and atoms in molecules (AIM) analyses. The structure of isomer 1a is planar with C_2v symmetry, whereas its neutral counterpart 1n is non-planar with C₂ symmetry, in which its terminal aluminum atoms are out of the plane. The central allenic carbon atom of isomers 1a and 1n exhibits tri-coordination and thus makes it a case of five bonds to carbon, which is confirmed through their total bond order as observed in WBI. Both the isomers show σ- and π-aromaticity and are predicted with the NICS and AdNDP analyses. Further, the results of ab initio molecular dynamics simulations reveal their kinetic stability at room temperature; thus, they are experimentally viable systems. Full article

The term hypercoordination refers to the extent of the coordination of an element by its normal value. In the hypercoordination sphere, the element can achieve planar and/or non-planar molecular shape. Hence, planar hypercoordinate carbon species violate two structural rules: (i) The highest coordination number of carbon is four and (ii) the tetrahedral orientation by the connected elements and/or groups. The unusual planar orientations are mostly stabilized by the electronic interactions of the central atom with the surrounding ligands. In this review article, we will talk about the current progress in the theoretical prediction of viable planar hypercoordinate carbon compounds. Primary knowledge of the planar hypercoordinate chemistry will lead to its forthcoming expansion. Experimental and theoretical interests in planar tetracoordinate carbon (ptC), planar pentacoordinate carbon (ppC), and planar hexacoordinate carbon (phC) are continued. The proposed electronic and mechanical strategies are helpful for the designing of the ptC compounds. Moreover, the 18-valence electron rule can guide the design of new ptC clusters computationally as well as experimentally. However, the counting of 18-valence electrons is not a requisite condition to contain a ptC in a cluster. Furthermore, this ptC idea is expanded to the probability of a greater coordination number of carbon in planar orientations. Unfortunately, until now, there are no such logical approaches to designing ppC, phC, or higher-coordinate carbon molecules/ions. There exist a few global minimum structures of phC clusters identified computationally, but none have been detected experimentally. All planar hypercoordinate carbon species in the global minima may be feasible in the gas phase. Full article

Forty-one isomers of Al₂C₄H₂ that lie within 50 kcal mol⁻¹ are theoretically identified in this work using density functional theory. Among these, isomers 3 and 14 contain a planar tetracoordinate carbon (ptC) atom that lies at 3.3 and 16.9 kcal mol⁻¹, respectively, and are above the global minimum geometry 1 at the ωB97XD/6-311++G(2d,2p) level of theory. The other ten isomers that also contain unique bonding features are isomers 4, 18, 20, 21, 22, 27, 28, 31, 34, and 40. Out of these isomers, 4, 18, 20, 22, 27, 28, and 34 contain planar tetracoordinate aluminum (ptAl) whereas isomers 31 and 40 contain both ptC and ptAl atoms. Chemical bonding characteristic features are thoroughly analyzed for all these eleven isomers with various bonding and topological quantum chemical tools, such as NBO, AdNDP, WBI, and ELF, except isomer 27 due to the observed elongated Al-Al bond length. The current results indicate that ptC isomer 3 is more stable than other isomers because electron delocalization is more prevalent and it also has double aromaticity as observed from the ELF, NICS, and AdNDP analysis. Further, the structural stability of these isomers is investigated through ab initio molecular dynamics (AIMD) simulation. Isomer 21 shows the planar pentacoordinate aluminum but it is observed as a kinetically unstable geometry from AIMD and, further, one could notice that it isomerizes to isomer 12. Full article

Isomers of CAl₄Mg and CAl₄Mg⁻ have been theoretically characterized for the first time. The most stable isomer for both the neutral and anion contain a planar tetracoordinate carbon (ptC) atom. Unlike the isovalent CAl₄Be case, which contains a planar pentacoordinate carbon atom as the global minimum geometry, replacing beryllium with magnesium makes the ptC isomer the global minimum due to increased ionic radii of magnesium. However, it is relatively easier to conduct experimental studies for CAl₄Mg^0/− as beryllium is toxic. While the neutral molecule containing the ptC atom follows the 18 valence electron rule, the anion breaks the rule with 19 valence electrons. The electron affinity of CAl₄Mg is in the range of 1.96–2.05 eV. Both the global minima exhibit π/σ double aromaticity. Ab initio molecular dynamics simulations were carried out for both the global minima at 298 K for 10 ps to confirm their kinetic stability. Full article

Dissociation pathways of the global minimum geometry of Si${}_2$C${}_5$H${}_2$ with a planar tetracoordinate carbon (ptC) atom, 2,7-disilatricyclo[4.1.0.0${}^{1,3}$]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 Si${}_2$C${}_5$H${}_2$. These results are in stark contrast compared to the structurally similar and isovalent lowest-energy isomer of C${}_7$H${}_2$ 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. Full article