Special Issue "Planar Tetracoordinate Carbon—Fifty Years and Beyond"

A special issue of Atoms (ISSN 2218-2004). This special issue belongs to the section "Quantum Chemistry, Computational Chemistry and Molecular Physics".

Deadline for manuscript submissions: 31 July 2022 | Viewed by 9053

Special Issue Editors

Dr. Venkatesan S. Thimmakondu
E-Mail Website
Guest Editor
Department of Chemistry and Biochemistry, San Diego State University, San Diego, CA 92182, USA
Interests: planar tetracoordinate carbon; astrochemistry; quantum chemistry; molecular spectroscopy
Dr. Krishnan Thirumoorthy
E-Mail Website
Guest Editor
Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Tamil Nadu 632014, India
Interests: chiral recognition; planar tetracoordinate carbon; computational chemistry; role of chirality in biomolecules

Special Issue Information

Dear Colleagues,

In 1968, Monkhorst first mentioned the idea of planar tetracoordinate carbon (ptC) in the literature for a transition-state geometry. Later, in 1970, Hoffmann and co-workers had shown ways to stabilize molecules containing ptC atom. In the last fifty years, a plethora of molecules containing ptC have been theoretically characterized, and some experimentally detected. The concept of molecules with a ptC atom still enthralls both experimentalists and theoreticians, as it is a fundamental deviation from the tetrahedral tetracoordinate carbon atom, defined independently by van’t Hoff and Le Bel in 1874. Thus, ptC molecules can collectively be defined as “anti-van’t Hoff–Le Bel molecules”. Over a period of time, the idea has not only been extended to carbon group elements (Si, Ge, etc.) but also to other elements such as B, N, Al, and P and very recently even to the F atom. Molecules with a planar hypercoordinate carbon (phC) and other elements have also been witnessed in the literature over time. The core chemistry-based motivation in identifying these special classes of molecules stems from the fundamental fact that no two structural isomers of a given elemental composition behave in the same way chemically. Additionally, the idea of ptC or phC is a helpful tool to develop potential new 2D materials. The purpose of this Special Issue is to collect some recent trends in this subject area, as the field is continuously emerging and would supplement the existing literature. Therefore, we warmly welcome contributions from both experimental and theoretical scientific communities working in this field.

Dr. Venkatesan S. Thimmakondu
Dr. Krishnan Thirumoorthy
Guest Editors

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Keywords

  • planar tetracoordinate carbon
  • planar tetracoordinate silicon
  • planar hypercoordinate carbon
  • planar hypercoordinate silicon
  • planar pentacoordinate carbon
  • planar pentacoordinate silicon
  • 2D materials
  • anti-van’t Hoff–Le Bel molecules

Published Papers (6 papers)

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Research

Article
Persistent Planar Tetracoordinate Carbon in Global Minima Structures of Silicon-Carbon Clusters
Atoms 2022, 10(1), 27; https://doi.org/10.3390/atoms10010027 - 28 Feb 2022
Cited by 1 | Viewed by 1039
Abstract
Recently, we reported a series of global minima whose structures consist of carbon rings decorated with heavier group 14 elements. Interestingly, these structures feature planar tetracoordinate carbons (ptCs) and result from the replacement of five or six protons (H+) from the [...] Read more.
Recently, we reported a series of global minima whose structures consist of carbon rings decorated with heavier group 14 elements. Interestingly, these structures feature planar tetracoordinate carbons (ptCs) and result from the replacement of five or six protons (H+) from the cyclopentadienyl anion (C5H5) or the pentalene dianion (C8H62−) by three or four E2+ dications (E = Si–Pb), respectively. The silicon derivatives of these series are the Si3C5 and Si4C8 clusters. Here we show that ptC persists in some clusters with an equivalent number of C and Si atoms, i.e., Si5C5, Si8C8, and Si9C9. In all these species, the ptC is embedded in a pentagonal C5 ring and participates in a three-center, two-electron (3c-2e) Si-ptC-Si σ-bond. Furthermore, these clusters are π-aromatic species according to chemical bonding analysis and magnetic criteria. Full article
(This article belongs to the Special Issue Planar Tetracoordinate Carbon—Fifty Years and Beyond)
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Article
Exploring the Excited-State Nonadiabatic Effects in the Semisaturated Planar Tetracoordinated Carbon Molecule C7H4
Atoms 2022, 10(1), 10; https://doi.org/10.3390/atoms10010010 - 19 Jan 2022
Viewed by 712
Abstract
We theoretically study the nonadiabatic relaxation dynamics of low-lying singlet excited-states of semisaturated planar tetracoordinated carbon molecule, C7H4. This molecule possesses a stable C2v ground-state equilibrium geometry. The three low-lying singlet states, S1, S2 [...] Read more.
We theoretically study the nonadiabatic relaxation dynamics of low-lying singlet excited-states of semisaturated planar tetracoordinated carbon molecule, C7H4. This molecule possesses a stable C2v ground-state equilibrium geometry. The three low-lying singlet states, S1, S2 and S3, lie in the energy gap of about 1.2 eV. The potential energy surfaces constructed within the quadratic vibronic coupling formalism reveal multiple conical intersections in the Franck-Condon region. Upon photoexcitation to S3, the wavepacket decays rapidly to lower states via these conical intersections. We also observe the wavepacket transfer to S3 during the initial wavepacket evolution on lower states, suggesting the nonadiabatic behavior of photoexcited planar C7H4. Full article
(This article belongs to the Special Issue Planar Tetracoordinate Carbon—Fifty Years and Beyond)
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Article
BAl4Mg−/0/+: Global Minima with a Planar Tetracoordinate or Hypercoordinate Boron Atom
Atoms 2021, 9(4), 89; https://doi.org/10.3390/atoms9040089 - 27 Oct 2021
Cited by 1 | Viewed by 1278
Abstract
We have explored the chemical space of BAl4Mg/0/+ for the first time and theoretically characterized several isomers with interesting bonding patterns. We have used chemical intuition and a cluster building method based on the tabu-search algorithm [...] Read more.
We have explored the chemical space of BAl4Mg/0/+ for the first time and theoretically characterized several isomers with interesting bonding patterns. We have used chemical intuition and a cluster building method based on the tabu-search algorithm implemented in the Python program for aggregation and reaction (PyAR) to obtain the maximum number of possible stationary points. The global minimum geometries for the anion (1a) and cation (1c) contain a planar tetracoordinate boron (ptB) atom, whereas the global minimum geometry for the neutral (1n) exhibits a planar pentacoordinate boron (ppB) atom. The low-lying isomers of the anion (2a) and cation (3c) also contain a ppB atom. The low-lying isomer of the neutral (2n) exhibits a ptB atom. Ab initio molecular dynamics simulations carried out at 298 K for 2000 fs suggest that all isomers are kinetically stable, except the cation 3c. Simulations carried out at low temperatures (100 and 200 K) for 2000 fs predict that even 3c is kinetically stable, which contains a ppB atom. Various bonding analyses (NBO, AdNDP, AIM, etc.) are carried out for these six different geometries of BAl4Mg/0/+ to understand the bonding patterns. Based on these results, we conclude that ptB/ppB scenarios are prevalent in these systems. Compared to the carbon counter-part, CAl4Mg, here the anion (BAl4Mg) obeys the 18 valence electron rule, as B has one electron fewer than C. However, the neutral and cation species break the rule with 17 and 16 valence electrons, respectively. The electron affinity (EA) of BAl4Mg is slightly higher (2.15 eV) than the electron affinity of CAl4Mg (2.05 eV). Based on the EA value, it is believed that these molecules can be identified in the gas phase. All the ptB/ppB isomers exhibit π/σ double aromaticity. Energy decomposition analysis predicts that the interaction between BAl4/0/+ and Mg is ionic in all these six systems. Full article
(This article belongs to the Special Issue Planar Tetracoordinate Carbon—Fifty Years and Beyond)
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Article
Stereomutation in Tetracoordinate Centers via Stabilization of Planar Tetracoordinated Systems
Atoms 2021, 9(4), 79; https://doi.org/10.3390/atoms9040079 - 14 Oct 2021
Cited by 2 | Viewed by 650
Abstract
The quest for stabilizing planar forms of tetracoordinate carbon started five decades ago and intends to achieve interconversion between [R]- and [S]-stereoisomers without breaking covalent bonds. Several strategies are successful in making the planar tetracoordinate form a minimum on [...] Read more.
The quest for stabilizing planar forms of tetracoordinate carbon started five decades ago and intends to achieve interconversion between [R]- and [S]-stereoisomers without breaking covalent bonds. Several strategies are successful in making the planar tetracoordinate form a minimum on its potential energy surface. However, the first examples of systems where stereomutation is possible were reported only recently. In this study, the possibility of neutral and dications of simple hydrocarbons (cyclopentane, cyclopentene, spiropentane, and spiropentadiene) and their counterparts with the central carbon atom replaced by elements from groups 13, 14, and 15 are explored using ab initio MP2 calculations. The energy difference between the tetrahedral and planar forms decreases from row II to row III or IV substituents. Additionally, aromaticity involving the delocalization of the lone pair on the central atom appears to help in further stabilizing the planar form compared to the tetrahedral form, especially for the row II substituents. We identified 11 systems where the tetrahedral state is a minimum on the potential energy surface, and the planar form is a transition state corresponding to stereomutation. Interestingly, the planar structures of three systems were found to be minimum, and the corresponding tetrahedral states were transition states. The energy profiles corresponding to such transitions involving both planar and tetrahedral states without the breaking of covalent bonds were examined. The systems showcased in this study and research in this direction are expected to realize molecules that experimentally exhibit stereomutation. Full article
(This article belongs to the Special Issue Planar Tetracoordinate Carbon—Fifty Years and Beyond)
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Article
In Silico Studies on Selected Neutral Molecules, CGa2Ge2, CAlGaGe2, and CSiGa2Ge Containing Planar Tetracoordinate Carbon
Atoms 2021, 9(3), 65; https://doi.org/10.3390/atoms9030065 - 10 Sep 2021
Cited by 3 | Viewed by 1091
Abstract
Density functional theory (DFT) was used to study the structure, stability, and bonding in some selected neutral pentaatomic systems, viz., CGa2Ge2, CAlGaGe2, and CSiGa2Ge containing planar tetracoordinate carbon. The systems are kinetically stable, as [...] Read more.
Density functional theory (DFT) was used to study the structure, stability, and bonding in some selected neutral pentaatomic systems, viz., CGa2Ge2, CAlGaGe2, and CSiGa2Ge containing planar tetracoordinate carbon. The systems are kinetically stable, as predicted from the ab initio molecular dynamics simulations. The natural bond orbital (NBO) analysis showed that strong electron donation occurs to the central planar carbon atom by the peripheral atoms in all the studied systems. From the nucleus independent chemical shift (NICS) analysis, it is shown that the systems possess both σ- and π- aromaticity. The presence of 18 valence electrons in these systems, in their neutral form, appears to be important for their stability with planar geometries rather than tetrahedral structures. The nature of bonding is understood through the adaptive natural density partitioning analysis (AdNDP), quantum theory of atoms in molecules (QTAIM) analysis, and also via Wiberg bond index (WBI) and electron localization function (ELF). Full article
(This article belongs to the Special Issue Planar Tetracoordinate Carbon—Fifty Years and Beyond)
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Article
CAl4Mg0/−: Global Minima with a Planar Tetracoordinate Carbon Atom
Atoms 2021, 9(2), 24; https://doi.org/10.3390/atoms9020024 - 09 Apr 2021
Cited by 7 | Viewed by 2651
Abstract
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 [...] Read more.
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 CAl4Mg0/− 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
(This article belongs to the Special Issue Planar Tetracoordinate Carbon—Fifty Years and Beyond)
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