Variations in the Nature of Triple Bonds: The N2, HCN and HC2H Series (original) (raw)
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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 Quadruple Bonding in C2 Reproduces the Properties of the Molecule
Chemistry (Weinheim an der Bergstrasse, Germany), 2016
Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C2 is bonded by a quadruple bond that can be distinctly characterized by valence-bond (VB) calculations. We demonstrate that the quadruply-bonded structure determines the key observables of the molecule, and accounts by itself for about 90 % of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply-bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ-type bond, the bond strength of which is estimated as 17-21 kcal mol(-1) . Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol(-1) , respectively, above the quadruply-bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of t...
This chapter outlines the basic elements of Spin-Coupled Generalized Valence Bond (SCGVB) theory, including the structure of the SCGVB wavefunction as well as its optimization and analysis. The SCGVB wavefunctions accounts for non-dynamical electron correlation yet still provides a compelling orbital description of the electronic structures of the ground and excited states of molecules and of the electronic mechanisms of chemical reactions. The insights provided by SCGVB theory are illustrated using a representative set of examples: the “traditional” HCN molecule, and its fragmentation into CH and N; the formation of “hypervalent” SF4 and SF6 from SF(n–1) + F; resonance/antiresonance in benzene and tropylium cation (C7H7+ ); the excited states of benzene, CH and O3/SO2 ; and the reaction of ground state methylene with H2 . The effects of dynamical electron correlation, which must be included to make quantitative predictions of molecular properties, can be taken into account by using...
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...
The Journal of Physical Chemistry A, 2007
The electronic and structural properties of dihydronitroxide/water clusters are investigated and compared to the properties of formaldehyde/water clusters. Exploring the stationary points of their potential energy surfaces (structurally, vibrationally, and energetically) and characterizing their hydrogen bonds (by both atoms in molecules and natural bond orbitals methods) clearly reveal the strong similarity between these two kind of molecular systems. The main difference involves the nature of the hydrogen bond taking place between the X-H bond and the oxygen atom of a water molecule. All the properties of the hydrogen bonds occurring in both kind of clusters can be easily interpreted in terms of competition between intermolecular and intramolecular hyperconjugative interactions.
A theoretical study of 1:1 and 1:2 complexes of acetylene with nitrosyl hydride
Ab initio calculations at MP2 computational level using aug-cc-pVTZ basis set were used to analyze the interactions between 1:1 and 1:2 complexes of acetylene and nitrosyl hydride. The structures obtained have been analyzed with the atoms in molecules and the density functional theory-symmetry adapted perturbation theory methodologies. Four minima were located on the potential energy surface of the 1:1 complex. Twenty-four different structures have been obtained for the 1:2 complexes. Five types of interactions are observed, CHÁÁÁO, CHÁÁÁN, NHÁÁÁp hydrogen bonds and orthogonal interactions between the p clouds of triple bond, or the lone pair of oxygen with the electron-deficient region of the nitrogen atom. Stabilization energies of the 1:1 and 1:2 clusters including basis set superposition error and ZPE are in the range 3-8 and 6-17 kJ mol-1 at MP2/aug-cc-pVTZ computational level, respectively. Blue shift of NH bond upon complex formation in the ranges between 18-30 and 20-96 cm-1 is predicted for 1:1 and 1:2 clusters, respectively. The total nonadditive energy in the 1:2 cluster, calculated as the sum of the supermolecular nonadditive MP2 energy and the three-body dispersion energy, presents values between-1.48 and 1.20 kJ mol-1 .
Interaction of C3H3+ isomers with molecular nitrogen: IR spectra of C3H3+–(N2)n clusters (n=1–6)
International Journal of Mass Spectrometry, 2002
The intermolecular interaction and microsolvation process of isomeric C 3 H 3 + ions in molecular nitrogen are characterized by infrared (IR) photodissociation spectroscopy of C 3 H 3 +-(N 2) n complexes (n = 1-6) and quantum chemical calculations (n = 0-4). The rovibrational analysis of the C 3 H 3 +-N 2 spectrum unambiguously reveals the presence of (at least) two C 3 H 3 + isomers in the ion source, namely the propargyl (H 2 CCCH +) and the cyclopropenyl (c-C 3 H 3 +) cations. Analysis of the cluster size-dependent vibrational frequency shifts and splittings, the photofragmentation branching ratios, and the results of density functional calculations provides a consistent picture of the microsolvation of cC 3 H 3 + and H 2 CCCH + in inert nitrogen. In the most stable cC 3 H 3 +-(N 2) n complexes, the first three N 2 ligands form (nearly) linear and equivalent proton bonds to the three protons of cC 3 H 3 + , leading to highly symmetric planar structures with C 2v (n = 1, 2) and D 3h symmetry (n = 3). After completion of this first solvation subshell at n = 3, further N 2 ligands form weaker intermolecular bonds to the C atoms of the nearly planar cC 3 H 3 +-(N 2) 3 ion core. The dissociation energies of the H-bonds and C-bonds in cC 3 H 3 +-(N 2) n are estimated as D 0 (H) = 900 ± 130 cm −1 and D 0 (C) = 860 ± 170 cm −1 , respectively. In the most stable H 2 CCCH +-N 2 complex, the N 2 ligand forms a linear ionic H-bond to the acetylenic C-H group of H 2 CCCH + , leading to a planar structure with C 2v symmetry. The calculations suggest that the next two ligands bind to the protons of the CH 2 group giving rise to planar structures with C s (n = 2) and C 2v symmetry (n = 3), and these structures are compatible with the observed IR spectra.
Journal of Chemical Theory and Computation, 2015
The ground state, X 1 Σ g + , of N 2 is a textbook example of a molecule with a triple bond consisting of one σ and two π bonds. This assignment, which is usually rationalized using molecular orbital (MO) theory, implicitly assumes that the spins of the three pairs of electrons involved in the bonds are singlet coupled (perfect pairing). However, for a six-electron singlet state, there are five distinct ways to couple the electron spins. The generalized valence bond (GVB) wavefunction lifts this restriction, including all of the five spin functions for the six electrons involved in the bond. For N 2 we find that the perfect pairing spin function is indeed dominant at R e , but that it becomes progressively less so from N 2 to P 2 and As 2. Although the perfect pairing spin function is still the most important spin function in P 2 , the importance of a quasi-atomic spin function, which singlet couples the spins of the electrons in the σ orbitals, while high spin