Bent-bond versus separated-bond models: A spin-coupled survey for a few organic and inorganic systems

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The bonding in [1.1.1]propellane, bicyclo[1.1.0]butane, bicyclo[1.1.1]pentane, tetrahedrane, and cyclopropane is investigated by analyzing changes in the off-nucleus isotropic magnetic shielding within the space surrounding each of these molecules and, for [1.1.1]propellane, by examining also the diamagnetic and paramagnetic contributions to this shielding. Any shielding arising from the two "exo" sp 3 -like hybrid atomic orbitals on the bridgehead carbon atoms that have been used to support the idea of an inverted bond between these two atoms is found to be almost entirely contained within the [1.1.1]propellane cage and to contribute to a strongly shielded central region. This strongly shielded region suggests the establishment of a mainly covalent bonding interaction involving all carbon atoms that cannot be straightforwardly decomposed into contributions from individual carbon−carbon bonds. The emergence of the strongly shielding central region is traced by comparing the shielding variations in and around molecules with one three-membered carbon ring (cyclopropane), two fused three-membered carbon rings (bicyclo[1.1.0]butane), and three fused three-membered carbon rings ([1.1.1]propellane).

Section: Discussionmentioning

confidence: 64%

Section: Discussionmentioning

confidence: 73%

Magnetic Shielding Analysis of Bonding in [1.1.1]Propellane

Karadakov

Stewart

2023

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“…There has been considerable discussion in the literature on the nature of the multiple bonds in C 2 H 4 and C 2 H 2 , beginning with the early suggestion of Slater that these bonds were bent bonds . This topic has been discussed in detail by Karadakov et al and Ogliaro et al who reported SCGVB calculations on these two molecules. Penotti and Cooper provided additional insights into the nature of the multiple bonds in C 2 H 2 (as well as N 2 ).…”

Section: Theoretical and Computational Methodsmentioning

confidence: 99%

Orbital Hybridization in Modern Valence Bond Wave Functions: Methane, Ethylene, and Acetylene

Dunning

2019

The concept of hybrid orbitals is one of the key theoretical concepts used by chemists to explain the structures and other properties of molecules. Recent work found that the hybrid orbitals from modern ab initio valence bond wave functions differ significantly from traditional hybrid orbitals. We report a detailed analysis of the orbitals of methane, ethylene, and acetylene from spin-coupled generalized valence bond (SCGVB) wave functions, a variationally optimized valence bond wave function that places no constraints on the orbitals and spin function. The carbon-centered orbitals in the SCGVB wave functions are found to be 2s−2p hybrid orbitals largely localized on the carbon atom and pointed directly at the hydrogen atoms to which they are bonded. However, the SCGVB orbitals for methane, ethylene, and acetylene differ markedly from the sp 3 , sp 2 , and sp hybrid orbitals traditionally associated with these molecules. It is now clear that the orbitals in modern valence bond wave functions do not follow the hybridization rules of traditional valence bond theory. These findings imply that, in modern valence bond theories, other factors are responsible for the structures and properties of molecules that are traditionally attributed to orbital hybridization.

“…Alongside the use of the full spin space, which proves so useful, for example, in distinguishing clearly between aromatic, antiaromatic and nonaromatic π-electron systems, a key feature of SCGVB theory is the variational flexibility of the orbitals, given that they are expanded in the full molecular basis set without any constraints. It has been shown (see ref and references therein) that the relative energetic importance of the different numbers of spin and orbital degrees of freedom leads, in some instances but not in others, to a preference for bent bond descriptions over the corresponding σ–π separated solutions. Bent bonds have also been observed in a somewhat different context, namely for the in-plane σ bonding in strained ring systems.…”

Section: Insights Into the Electronic Structure Of Moleculesmentioning

confidence: 99%

Spin-Coupled Generalized Valence Bond Theory: New Perspectives on the Electronic Structure of Molecules and Chemical Bonds

Dunning

Cooper

et al. 2021

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Spin-Coupled Generalized Valence Bond (SCGVB) theory provides the foundation for a comprehensive theory of the electronic structure of molecules. SCGVB theory offers a compelling orbital description of the electronic structure of molecules as well as an efficient and effective zero-order wave function for calculations striving for quantitative predictions of molecular structures, energetics and other properties. The orbitals in the SCGVB wave function are usually semi-localized and, for most molecules, can be interpreted using concepts familiar to all chemists (hybrid orbitals, localized bond pair, lone pairs, etc.). SCGVB theory also provides new perspectives on the nature of the bonds in molecules such as C2, Be2 and SF4/SF6. SCGVB theory contributes unparalleled insights into the underlying cause of the first-row anomaly in inorganic chemistry as well as the electronic structure of organic molecules and the electronic mechanisms of organic reactions. The SCGVB wave function accounts for nondynamical correlation effects and, thus, corrects the most serious deficiency in molecular orbital (RHF) wave functions. Dynamical correlation effects, which are critical for quantitative predictions, can be taken into account using the SCGVB wave function as the zero-order wave function for multireference configuration interaction or coupled cluster calculations.* In prior articles in the literature the Spin-Coupled Generalized Valence Bond wave function has usually been referred to as the Spin-Coupled Valence Bond (SCVB) wave function or the full Generalized Valence Bond (full GVB) wave function. These wave functions are identical; to prevent confusion, the authors have adopted the allencompassing SCGVB name. Spin-Coupled Generalized Valence Bond Theory: New Perspectives on the Electronic Structure of Molecules and Chemical Bonds3 and recoupled pair bond dyad. 36 The SCGVB wave function describes molecules in terms of orbitals that are optimized at each geometry, allowing the atomic orbitals to change as a result of molecule formation. In addition to the changes in the orbitals, optimization of the general spin function used in the SCGVB wave function allows the spin function to change from that appropriate for the separated atoms or fragments to that appropriate for the molecule. SCGVB theory addresses a major limitation of RHF theory, providing an excellent zero-order description of molecules that require multiconfiguration descriptions, the making/breaking of bonds, and many chemical reactions and excited states. For most molecules, the SCGVB wave function captures the essential features of the FORS/CASSCF wave functions, with much reduced complexity compared to these latter wave functions.