Experimental and Theoretical Electron Density Study of a Highly Twisted Polycyclic Aromatic Hydrocarbon: 4-Methyl-[4]helicene

Helicenes are molecules of considerable interest in view of their aromaticity which persists despite a marked
departure from planarity and because of the extreme potency of some of their metabolites as tumor and
mutation promoters. In this study, the electron density of 4-methyl-[4]helicene (or 4-methylbenzo[c]phenanthrene) is studied topologically with an emphasis on the fjord region since this region is where metabolic
activation is initiated. The molecule consists of four fused aromatic rings that assume a twisted geometry.
This geometry brings two hydrogen atoms into close proximity in the fjord region of the molecule accompanied
by the appearance of an intramolecular C−Hδ+···δ+H−C bond path (an interaction termed hydrogen−hydrogen
or H− H bonding to distinguish it from dihydrogen bonding from which it is qualitatively distinct). In addition
to the intramolecular H−H interaction, a number of intermolecular interactions are shown to be involved in
the packing of this molecule in the crystalline state. The effect of the nonplanarity of the molecule on the
local aromaticity of each ring is also discussed.