The Quadruple Bonding in C2 Reproduces the Properties of the Molecule (original) (raw)
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The nature of the fourth bond in the ground state of C2: the quadruple bond conundrum
Chemistry (Weinheim an der Bergstrasse, Germany), 2014
Does, or doesn't C2 break the glass ceiling of triple bonding? This work provides an overview on the bonding conundrum in C2 and on the recent discussions regarding our proposal that it possesses a quadruple bond. As such, we focus herein on the main point of contention, the 4th bond of C2, and discuss the main views. We present new data and an overview of the nature of the 4th bond--its proposed antiferromagnetically coupled nature, its strength, and a derivation of its bond energy from experimentally based thermochemical data. We address the bond-order conundrum of C2 arising from generalized VB (GVB) calculations by comparing it to HC≡CH, and showing that the two molecules behave very similarly, and C2 is in no way an exception. We analyse the root cause of the deviation of C2 from the Badger Rule, and demonstrate that the reason for the smaller force constant (FC) of C2 relative to HC≡CH has nothing to do with the bond energies, or with the number of bonds in the two molecul...
Insights into the Perplexing Nature of the Bonding in C2 from Generalized Valence Bond Calculations
Journal of Chemical Theory and Computation, 2014
Diatomic carbon, C 2 , has been variously described as having a double, triple, or quadruple bond. In this article, we report full generalized valence bond (GVB) calculations on C 2. The GVB wave functionmore accurate than the Hartree−Fock wave function and easier to interpret than traditional multiconfiguration wave functionsis well-suited for characterizing the bonding in C 2. The GVB calculations show that the electronic wave function of C 2 is not well described by a product of singlet-coupled, shared electron pairs (perfect pairing), which is the theoretical basis for covalent chemical bonds. Rather, C 2 is best described as having a traditional covalent σ bond with the electrons in the remaining orbitals of the two carbon atoms antiferromagnetically coupled. However, even this description is incomplete as the perfect pairing spin function also makes a significant contribution to the full GVB wave function. The complicated structure of the wave function of C 2 is the source of the uncertainty about the nature of the bonding in this molecule.
The nature of the quadruple chemical bond in the dicarbon molecule
arXiv: Chemical Physics, 2019
The molecular dissociation energy has often been explained and discussed in terms of singlet bonds, formed by bounded pairs of valence electrons. In this work we use a highly correlated resonating valence bond ansatz, providing a consistent paradigm for the chemical bond, where spin fluctuations are shown to play a crucial role. Spin fluctuations are known to be important in magnetic systems and correspond to the zero point motion of the spin waves emerging from a magnetic broken symmetry state. Recently, in order to explain the excitation spectrum of the carbon dimer, an unusual quadruple bond has been proposed. Within our ansatz, a satisfactory description of the carbon dimer is determined by the magnetic interaction of two Carbon atoms with antiferromagnetically ordered S=1S=1S=1 magnetic moments, a picture that provide also a natural and much simpler explanation of the fourth bond.
Why is the bond multiplicity in C2 so elusive?
Computational and Theoretical Chemistry, 2015
We reexamine the full GVB (or spin-coupled) description of the bonding in the ground state of C 2 at its equilibrium geometry, prompted by recent controversy as to whether or not this system exhibits four bonds. We show that two apparently different interpretations (namely a single r bond plus antiferromagnetic coupling of high spin units on the two atoms as opposed to a conventional set of covalent r and p bonds) do in fact have a high overlap. Neither description is adequate. Further insights into the nature of the bonding in this system emerge from an analysis of spin correlation matrix elements. We also analyze domain-averaged Fermi holes and values of the QTAIM-generalized Wiberg-Mayer index. It proves important to assess the significance of the various numerical results by means of direct comparison with analogous calculations for HCCH. We suggest that an enhanced weight for triplet spin-coupling modes, relative to HCCH, increases the difficulty of describing the electronic structure of C 2 in terms of conventional bonding models that feature a whole number of two-center two-electron bonds.
The C–H⋯π bonds: strength, identification, and hydrogen-bonded nature: a theoretical study
Chemical Physics Letters, 2000
Using the MP2 method and, in some cases, a basis set-up to aug-cc-pVTZ quality, the properties of C-H PPP p bonds have been investigated in model dimers. Their strength goes from 0.55 up to 2.5 kcalrmol, depending on the C-H carbon hybridization and the p system. The presence of these bonds is identified by the presence of a bond critical point linking the H atom and atoms of the p system. The critical point characteristics and the vibrational shift of the C-H PPP p bonded dimers are similar to those present in C-H PPP O and O-H PPP p hydrogen-bonded dimers, thus indicating a hydrogenbonded nature. q 2000 Elsevier Science B.V. All rights reserved. 0009-2614r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved.
2020
The A-A dissociation energy with respect to geometry frozen fragments (BE) of has been calculated for AH n-AH n models (C 2 H 6 , Si 2 H 6 , Ge 2 H 6 and N 2 H 4) as a function of = H-A-A angles. Following a sigmoidal variation, BE decreases rapidly when decreases to yield "inverted bonds" for < 90° and finally nearly vanishes. On the contrary BE increases when increases with respect to the equilibrium value; we propose the term of "superdirect" to qualify such bonds. This behaviour has been qualitatively interpreted in the case of C 2 H 6 by the variation of the overlap of both s+p hybrids. The BE of one C-H bond in CH 3 behaves similarly as function of its H-C-H angle with the other three hydrogen atoms. The concept of inverted/direct/superdirect bond is generalized to any CC sigma bond in hydrocarbons and can be characterized by the mean angle value <> of this bond with substituents (multiple-bonded substituents are considered as several substituents). This applies as well to formal single bonds as to sigma bonds in a formally multiple bond. Using dynamic orbital forces (DOF) as indices, the intrinsic bond energies are studied as a function of <> for a panel of 33 molecules. In formally single bonds, this energy decreases from the "superdirect" bonds in butadiyne, tetrahedryltetrahedrane and related compounds (<> > 125°), to the "inverted bonds" (<> < 90°) in bicyclobutane and [1.1.1]propellane for which it is nearly vanishing. The ring strain in cyclopropane and cyclobutane can be interpreted in terms of directness/superdirectness of CC and C-H bonds. Sigma bonds in formally multiple bonds are found inverted or near inverted and thus are significantly weaker than standard single bonds.
Variations in the Nature of Triple Bonds: The N2, HCN and HC2H Series
The journal of physical chemistry. A, 2016
The inertness of molecular nitrogen and the reactivity of acetylene suggest there are significant variations in the nature of triple bonds. To understand these differences, we performed generalized valence bond as well as more accurate electronic structure calculations on three molecules with putative triple bonds: N2, HCN and HC2H. The calculations predict that the triple bond in HC2H is quite different than the triple bond in N2, with HCN being an intermediate case but closer to N2 than HC2H. The triple bond in N2 is a traditional triple bond with the spins of the electrons in the bonding orbital pairs predominantly singlet coupled in the GVB wave function (92%). In HC2H, on the other hand, there is a substantial amount of residual CH(a4Σ-) fragment coupling in the triple bond at its equilibrium geometry with the contribution of the perfect pairing spin function dropping to 82% (77% in a full valence GVB calculation). This difference in the nature of the triple bond in N2 and HC2H...
2021
The C-C dissociation energy with respect to geometry frozen fragments (BE) has been calculated for C2H6 as a function of = H-C-C angles. BE decreases rapidly when decreases from its equilibrium value to yield the so-called “inverted bonds” for < 90°; on the contrary BE increaseswhen increases to yield somehow “superdirect” bonds, following a sigmoidal variation. The central bond in Si2H6, Ge2H6 and N 2H4 as well as the C-H bond in CH3-H behaves similarly. The concept of “invertedness”/”directedness” is generalized to any CC sigma bond in hydrocarbons and characterized by the mean angle value of substituents. Using dynamic orbital forces (DOF) as indices, the intrinsic bond energies are studied as a function of for formally single bonds in apanel of 22 molecules. This energy decreases from the strongest “superdirect” bonds in butadiyne, ( = 180°) or tetrahedrylacetylene to the weakest “inverted bond” in cyclobutene, tetrahedrane, bicyclobutane and [1.1.1]propellane ( = 6...