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Correlation Between Optical Activity and the Helical Molecular Orbitals of Allene and Cumulenes

preprint
submitted on 04.09.2020 and posted on 04.09.2020 by Marc Hamilton Garner, Clemence Corminboeuf

Helical frontier molecular orbitals (MOs) appear in disubstituted allenes and even-n cumulenes. Chiral molecules are optically active, but while these molecules are single-handed chiral, π-orbitals of both helicities are present. Here we computationally examine whether the optical activity of chiral cumulenes is controlled by the axial chirality or the helicity of the electronic structure. We exploit hyperconjugation with alkyl, silaalkyl, and germaalkyl substituents to adjust the MO helicity without altering the axial chirality. For the same axial chirality, we observe an inversion of the helical MOs contribution to the electronic transitions and a change of sign in the electronic circular dichroism and optical rotation dispersion spectra. While the magnitude of the chiroptical response also increases, it is similar to that of chiral cumulenes without helical π-orbitals. Overall, Helical π-orbitals correlate with the big chiroptical response in cumulenes, but are not a prerequisite for it.

Funding

Independent Research Fund Denmark, case no. 9056-00009B

History

Email Address of Submitting Author

marc.garner@epfl.ch

Institution

Ecole Polytechnique federale de Lausanne (EPFL)

Country

Switzerland

ORCID For Submitting Author

0000-0002-7270-8353

Declaration of Conflict of Interest

No conflict of interest.

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