Molecular Orbital Theory—Teaching a Difficult Chemistry Topic Using a CSCL Approach in a First-Year University Course

Collaboration is regarded as one of the core competences of the 21st century when it comes to complex problem solving. In response to high dropout rates among STEM students, we developed a digital-collaborative intervention on a difficult topic, MO theory, for first-year chemistry students. First, students work independently in a Digital Learning Environment (DLE). Afterwards, they collaborate in small groups and create Concept Maps on MO theory. We evaluate this intervention through knowledge tests, tests of attractiveness, cognitive load, and usability during the DLE and concept mapping process, as well as audio and screen recordings during the collaborative group processes. This paper presents the detailed study design together with results from a first study in January 2021, focusing on the practicability of the intervention and students’ feedback. Overall, each small group succeeded in creating a Concept Map. Students rated all phases of the intervention as attractive, with high usability and low cognitive load, although the interactive videos scored better for attractiveness and usability than the concept mapping process. On that basis, first adjustments for a second cycle of the intervention, which will be conducted in January 2022, were derived.

Valence Bond Theory—Its Birth, Struggles with Molecular Orbital Theory, Its Present State and Future Prospects

This essay describes the successive births of valence bond (VB) theory during 1916–1931. The alternative molecular orbital (MO) theory was born in the late 1920s. The presence of two seemingly different descriptions of molecules by the two theories led to struggles between the main proponents, Linus Pauling and Robert Mulliken, and their supporters. Until the 1950s, VB theory was dominant, and then it was eclipsed by MO theory. The struggles will be discussed, as well as the new dawn of VB theory, and its future.

2D-Map of Intermolecular Interactions in Organic Chemistry Reveals Underrepresented Area of Research

The molecular orbital (MO) theory is an indispensable model to describe the interaction between two molecular species, particularly during chemical bond formation. Here we show that by plotting the energy difference (E_DA) of the HOMO of an electron donor and the LUMO of an acceptor against their overlap integral (S_DA), many similar types of bimolecular interactions tend to be clustered near one another on the 2D map. Interestingly, in one of the six arbitrarily divided sections designated as “B2”, the interacting molecular pairs appear to present a type of interaction as a “hybrid” between chemical reaction and radical pair formation, a lesser explored area of research. We propose that such interactions could be crucial for the development of materials with unique optical, magnetic, and catalytic properties from purely organic molecules.<br>

2D-Map of Intermolecular Interactions in Organic Chemistry Reveals Underrepresented Area of Research

The molecular orbital (MO) theory is an indispensable model to describe the interaction between two molecular species, particularly during chemical bond formation. Here we show that by plotting the energy difference (E_DA) of the HOMO of an electron donor and the LUMO of an acceptor against their overlap integral (S_DA), many similar types of bimolecular interactions tend to be clustered near one another on the 2D map. Interestingly, in one of the six arbitrarily divided sections designated as “B2”, the interacting molecular pairs appear to present a type of interaction as a “hybrid” between chemical reaction and radical pair formation, a lesser explored area of research. We propose that such interactions could be crucial for the development of materials with unique optical, magnetic, and catalytic properties from purely organic molecules.<br>

Comprehending the quadruple bonding conundrum in C2 from excited state potential energy curves

The question of quadruple bonding in C2 has emerged as a hot button issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory.

Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces: A Molecular Orbital Perspective

The question of quadruple bonding in C₂ has emerged as a hot button issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory. Here, we have systematically studied the Potential Energy Curves (PECs) of low lying high spin sigma states of C₂, N₂ and Be₂ and HC≡CH using several MO based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the^2S+1Σ_g/u (with 2S+1=1,3,5,7,9) states of C₂ and comparisons with those of relevant dimers and the respective wavefunctions were conducted. We contend that unlike in the case of N₂ and HC≡CH, the presence of a deep minimum in the ⁷Σ state of C₂ and CN⁺ suggest a latent quadruple bonding nature in these two dimers. Hence, we have struck a reconciliatory note between the MO and VB approaches. The evidence provided by us can be experimentally verified, thus providing the window so that the narrative can move beyond theoretical conjectures.

Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces: A Molecular Orbital Perspective

The question of quadruple bonding in C₂ has emerged as a hot button issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory. Here, we have systematically studied the Potential Energy Curves (PECs) of low lying high spin sigma states of C₂, N₂ and Be₂ and HC≡CH using several MO based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the^2S+1Σ_g/u (with 2S+1=1,3,5,7,9) states of C₂ and comparisons with those of relevant dimers and the respective wavefunctions were conducted. We contend that unlike in the case of N₂ and HC≡CH, the presence of a deep minimum in the ⁷Σ state of C₂ and CN⁺ suggest a latent quadruple bonding nature in these two dimers. Hence, we have struck a reconciliatory note between the MO and VB approaches. The evidence provided by us can be experimentally verified, thus providing the window so that the narrative can move beyond theoretical conjectures.

The ‘Inverted Bond’ revisited. Analysis of ‘in silico’ models and of [1.1.1]Propellane using Orbital Forces

<div>This article dwells on the nature of “inverted bonds”, which make reference to the σ interaction between two s-p hybrids by their smaller lobes, and their presence in [1.1.1]propellane <b>1</b>. Firstly we study H 3 C-C models of C-C bonds with frozen HCC angles reproducing the constraints of various degrees of “inversion”. Secondly, the molecular orbital (MO) properties of [1.1.1]propellane <b>1</b> and [1.1.1]bicyclopentane <b>2</b> are analyzed with the help of orbital forces as a criterion of bonding/antibonding character and as a basis to evaluate bond energies. Triplet and cationic state of <b>1</b> species are also considered to confirm the bonding/antibonding character of MOs in the parent molecule. These approaches show an essentially non-bonding character of the σ central CC interaction in propellane. Within MO theory, this bonding is thus only due to π-type MOs (also called ‘banana’ MOs or ‘bridge’ MOs) and its total energy is evaluated to ca. 50 kcal/mol. In bicyclopentane <b>2</b>, despite a strong σ-type repulsion, a weak bonding (15-20 kcal/mol) exists between both central CC, also due to π-type interactions, though no bond is present in the Lewis structure. Overall, the so-called ‘inverted’ bond, as resulting from a σ overlap of the two s-p hybrids by their smaller lobes, appears highly questionable.</div>

The ‘Inverted Bond’ revisited. Analysis of ‘in silico’ models and of [1.1.1]Propellane using Orbital Forces

<div>This article dwells on the nature of “inverted bonds”, which make reference to the σ interaction between two s-p hybrids by their smaller lobes, and their presence in [1.1.1]propellane <b>1</b>. Firstly we study H 3 C-C models of C-C bonds with frozen HCC angles reproducing the constraints of various degrees of “inversion”. Secondly, the molecular orbital (MO) properties of [1.1.1]propellane <b>1</b> and [1.1.1]bicyclopentane <b>2</b> are analyzed with the help of orbital forces as a criterion of bonding/antibonding character and as a basis to evaluate bond energies. Triplet and cationic state of <b>1</b> species are also considered to confirm the bonding/antibonding character of MOs in the parent molecule. These approaches show an essentially non-bonding character of the σ central CC interaction in propellane. Within MO theory, this bonding is thus only due to π-type MOs (also called ‘banana’ MOs or ‘bridge’ MOs) and its total energy is evaluated to ca. 50 kcal/mol. In bicyclopentane <b>2</b>, despite a strong σ-type repulsion, a weak bonding (15-20 kcal/mol) exists between both central CC, also due to π-type interactions, though no bond is present in the Lewis structure. Overall, the so-called ‘inverted’ bond, as resulting from a σ overlap of the two s-p hybrids by their smaller lobes, appears highly questionable.</div>