Physi al achirality in geometrically hiral rotamers of hydrazine and boranylborane molecules

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

JOURNAL OF COMPUTATIONAL CHEMISTRY

JOHN WILEY & SONS INC

Lugar: New York; Año: 2021

0192-8651

The diagonal components and the trace of tensors which account for chiroptical response of the hydrazine molecule N2H4, i.e., static anapole magnetizability and frequency-dependent electric dipole-magnetic dipole polarisability, are a function ofthe phi ≡ H-N-N-H dihedral angle. They vanish for symmetry reasons at phi = 0◦ and phi = 180◦, corresponding respectively to C2v and C2h point group symmetries, i.e., cis and trans conformers characterized by the presence of molecular symmetry planes. Nonetheless, vanishing diagonal components have been observed also in the proximity of H-N-N-H = 90◦, in which the point group symmetry is C2 and hydrazine is unquestionably chiral. In the boranylborane molecule B2H4, assumingthe B-B bond in the y direction, the ayy component of the anapole magnetizability tensor approximately vanishes for dihedral angles H-B-B-H corresponding to chiral rotamers which belong to D2 symmetry. Such anomalous effects have been ascribed to physical achirality of these conformers, i.e., to their inability to sustain electronic current densities inducing either anapole moments, or electric andmagnetic dipole moments, about the chiral axis connecting heavier atoms, as well as perpendicular directions. In other terms, the structure of certain geometricallychiral rotamers may be such that neither toroidal nor helical flow, which determinechiroptical phenomenology, can take place in the presence of perturbing fieldsparallel or orthogonal to the chiral axis.