Helical electronic transitions of spiroconjugated molecules

Garner, Marc H.; Corminbœuf, Clémence

doi:10.1039/d1cc01904j

Cited by 18 publications

(37 citation statements)

References 41 publications

(29 reference statements)

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“…[26] Earlier, [30] the interdependence of steric-electronic factors was reported to be more complex than that discernable from molecular structures alone, as was also the case for the helical electronic transitions of spiroconjugated molecules. [31] The unknown chirality-helicity equivalence was recently located, but not quantified, for chiral compounds by some of the current authors, [28] who were able to differentiate the S and R stereoisomers consistent with the naming schemes from optical experiments. The insufficiency of conventional (scalar) QTAIM [32] (quantum theory of atoms in molecules) for distinguishing S and R stereoisomers was also demonstrated.…”

Section: Introductionmentioning

confidence: 90%

Chirality without Stereoisomers: Insight from the Helical Response of Bond Electrons

Nie

et al. 2021

ChemPhysChem

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The association between molecular chirality and helical characteristics known as the chirality-helicity equivalence is determined for the first time by quantifying a chirality-helicity measure consistent with photoexcitation circular dichroism experiments. This is demonstrated using a formally achiral S N 2 reaction and a chiral S N 2 reaction. Both the achiral and chiral S N 2 reactions possess significant values of the chirality-helicity measure and display a Walden inversion, i. e. an inversion of the chirality between the reactant and product. We also track the chirality-helicity measure along the reaction path and discover the presence of chirality at the transition state and two intermediate structures for both reactions. We demonstrate the need for the chirality-helicity measure to differentiate between steric effects due to eclipsed conformations and chiral behaviors in formally achiral species. We explain the significance of this work for asymmetric synthetic reactions including the intermediate structures where the Cahn-Ingold-Prelog (CIP) rules cannot be used.

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Section: Introductionmentioning

confidence: 90%

Chirality without Stereoisomers: Insight from the Helical Response of Bond Electrons

Nie

et al. 2021

ChemPhysChem

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“…Next, we examine the simple spiroconjugated molecules 1,6-dimethylspiro[4.4]nonatetraene and 1,4-dimethylspiro[2.2]pentadiene. − As with the even [ n ]cumulenes, metallacumulenes, and polyynes, the two orthogonal π-systems may interact and mix. , However, only half of the frontier MOs of spiroconjugated molecules become helical when the molecules are appropriately substituted . As the helicality of their MOs might influence optical properties, the quantification of helicality is of interest. The carbon chain is defined as the carbon atoms in the shortest through-bond path between the methyl groups.…”

Section: Resultsmentioning

confidence: 99%

“…Given the interest in helical MOs as a means to understand related properties and the range of molecular systems in which these MOs appear, − it is problematic that the assessment of helical MOs relies on visual inspection alone. It is thus altogether binary; if an MO is shaped like a coil, we may deem it to be helical.…”

Section: Introductionmentioning

confidence: 99%

Quantification of the Helicality of Helical Molecular Orbitals

2021

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The frontier molecular orbital (MO) topology of linear carbon molecules, such as polyynes, can be visually identified as helices. However, there is no clear way to quantify the helical curvature of these π-MOs, and it is thus challenging to quantify correlations between the helical curvature and molecular properties. In this paper, we develop a method that enables us to compute the helical curvature of MOs based on their nodal planes. Using this method, we define a robust way of quantifying the helical nature of MOs (helicality) by their deviation from a perfect helix. We explore several limiting cases, including polyynes, metallacumulenes, cyclic allenes, and spiroconjugated systems, where the change in helical curvature is subtle yet clearly highlighted with this method. For example, we show that strain only has a minor effect on the helicality of the frontier orbitals of cycloallenes and that the MOs of spiroconjugated systems are close to perfect helices around the spiro-carbon. Our work provides a well-defined method for assessing orbital helicality beyond visual inspection of MO isosurfaces, thus paving the way for future studies of how the helicality of π-MOs affects molecular properties.

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“…Earlier [1], the interdependence of steric‐electronic factors for spiroconjugated molecules [2] was determined to be more complex than that discernable from molecular structures alone, thereby calling into question the use of the Cahn–Ingold–Prelog (CIP) priority rules for determining the chiral configuration (R/S) [3, 4]. The simplest example of axial chirality, that results in the ability to exploit axial chirality for asymmetric synthesis, is possessed by certain allenes that consist of a linear 3‐carbon skeleton formed by two π‐bonds that are orthogonal to each other and joined by a sp‐hybridized carbon.…”

Section: Introductionmentioning

confidence: 99%

“…The simplest example of axial chirality, that results in the ability to exploit axial chirality for asymmetric synthesis, is possessed by certain allenes that consist of a linear 3-carbon skeleton formed by two π-bonds that are orthogonal to each other and joined by a sp-hybridized carbon. The condition for axial chirality in allenes is that the two terminal sp 2 -hybridized carbons bear different substituents [5]. Cumulenes are a class of linearly conjugated π-systems terminated by tri-coordinate carbon atoms [6].…”

Section: Introductionmentioning

confidence: 99%

Chirality–helicity of cumulenes: A non‐scalar charge density derived perspective

Xing

Azizi

Momen

et al. 2022

Int J of Quantum Chemistry

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We investigate the presence of helical character and chirality using a vector‐based charge density perspective instead of energetic or structural measures. The vector‐based perspective of the chemical bonding, constructed using the most preferred direction of charge density accumulation, finds the presence of induced symmetry‐breaking for α,ω‐disubstituted [4]cumulenes as the end groups are torsioned. The stress tensor trajectories Tσ(s) are used to provide the additional symmetry‐breaking required to quantify the degree and nature of the chirality and helical character. The S‐1,5‐dimethyl‐[4]cumulene contains a very low degree of chiral character but significant axiality(helicity) resulting in a weakly helical morphology of the corresponding Tσ(s). The (‐)S(‐), (+)S(‐) and (+)S(+) conformations of S‐1,5‐diamino‐[4]cumulene contain very significant degrees of both chirality and helical character resulting in helical morphology of the corresponding Tσ(s). We discuss the consequences for the Tσ(s) in locating chiral character in these molecules in future experiment investigations.

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Helical electronic transitions of spiroconjugated molecules

Abstract: The two π-systems of allene can combine into helical molecular orbitals (MOs), yet the helicity is lost in the π-π∗ transitions due to excited state mixing. In spiroconjugated molecules the...

Cited by 18 publications

References 41 publications

Chirality without Stereoisomers: Insight from the Helical Response of Bond Electrons

Chirality without Stereoisomers: Insight from the Helical Response of Bond Electrons

Quantification of the Helicality of Helical Molecular Orbitals

Chirality–helicity of cumulenes: A non‐scalar charge density derived perspective

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