On the axial chirality of leucoindigo

PAGOLA, G.I.; PROVASI, P.F.; FERRARO, M.B.; LAZZERETTI, P.

JOURNAL OF COMPUTATIONAL CHEMISTRY

JOHN WILEY & SONS INC

Año: 2023

0192-8651

The diagonal components and the trace of two tensors which account for chiroptical response of the leucoindigo molecule (Formula presented.) that is, static anapole magnetizability, and dynamic electric dipole-magnetic dipole polarisability depending on the frequency of impinging light, are a function of the (Formula presented.) dihedral angle of torsion about the central C-C bond, assumed to lie in the (Formula presented.) direction of the coordinate system. They vanish for symmetry reasons at (Formula presented.) and (Formula presented.), corresponding respectively to (Formula presented.) and (Formula presented.) point group symmetries, that is, cis and trans conformers characterized by the presence of molecular symmetry planes. Nonetheless, diagonal components and average value of static anapole polarizability and optical rotation tensors vanish at (Formula presented.), where leucondigo is unquestionably chiral from the geometrical viewpoint. Vanishing values of the average chiroptical properties have been observed also in the proximity of other (Formula presented.) angles. Attempts have been made to explain the occurrence of accidental zeros of chiroptical properties in terms of transition frequencies and scalar products appearing in the numerator of their quantum mechanical definitions. Within the electric dipole approximation, the presence of anomalous vanishing values of tensor components of anapole magnetizability and electric-magnetic dipole polarizability is ascribed to physical achirality, arising from the lack of either toroidal or spiral electron flow along the (Formula presented.), (Formula presented.) and (Formula presented.) directions.