On the axial chirality of leucoindigo.

The diagonal components and the trace of two tensors which account for chiroptical response of the leucoindigo molecule C 16 H 12 N 2 O 2 $$ {\mathrm{C}}_{16}{\mathrm{H}}_{12}{\mathrm{N}}_2{\mathrm{O}}_2 $$ that is, static anapole magnetizability, and dynamic electric dipole-magnetic dipole polarisability depending on the frequency of impinging light, are a function of the ϕ $$ \phi $$ dihedral angle of torsion about the central CC bond, assumed to lie in the y $$ y $$ direction of the coordinate system. They vanish for symmetry reasons at ϕ = 0 ∘ $$ \phi ={0}^{\circ } $$ and ϕ = 180 ∘ $$ \phi ={180}^{\circ } $$ , corresponding respectively to C 2 v $$ {C}_{2v} $$ and C 2 h $$ {C}_{2h} $$ 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 ϕ = 90 ∘ $$ \phi ={90}^{\circ } $$ , 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 ϕ $$ \phi $$ 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 x $$ x $$ , y $$ y $$ and z $$ z $$ directions.