The perchloro­triphenyl­methyl (PTM) radical

In spite of the considerable understanding and development of perchloro­triphenyl­methyl (PTM) radical derivatives, the preparation of crystals of the pure unsubstituted PTM radical, C₁₉Cl₁₅, suitable for single-crystal X-ray diffraction has remained a challenge since its discovery, and only two studies dealing with the crystal structure of the unsubstituted PTM radical have been published. In one study, the radical forms clathrates with aromatic solvents [Veciana, Carilla, Miravitlles & Molins (1987). J. Chem. Soc. Chem. Commun. pp. 812–814], and in the other the structure was determined ab initio from powder X-ray diffraction data [Rius, Miravitlles, Molins, Crespo & Veciana (1990). Mol. Cryst. Liq. Cryst. 187, 155–163]. We report here the preparation of PTM crystals for single-crystal X-ray diffraction and their resolution. The structure, which shows monoclinic symmetry (C2/c), revealed a nonsymmetric mol­ecular propeller conformation (D₃ symmetry) caused by the steric strain between the ortho-Cl atoms, which protect the central C atom (sp²-hybridization and major spin density) and give high chemical and thermal persistence to the PTM. The supra­molecular structure of PTM shows short Cl...Cl inter­molecular inter­actions and can be described in terms of layers formed by rows of mol­ecules positioned in a head-to-tail manner along the c axis.