Triphenylmethyl perchlorate, structure

The hrst X-ray crystal structure of a carbocation salt was reported in 1965. Triphenylmethyl perchlorate (27) has a planar central carbon. The three phenyl rings are each twisted 30°, so that overall the cation has a propellor shape. Disordered perchlorate anions sit above and below the central carbon, with a C—Cl separation of 4.09 A. 

In contrast to triphenylmethyl chloride, which has the properties of a covalent compound, triphenylmethyl perchlorate behaves as an ionic compound. The presence of triphenylmethyl cations in this solid has been confirmed by an X-ray crystal structure determination.The central carbon is planar, but the three phenyl rings are at an angle of 54 to the plane of the trigonal carbon so that the overall cation has a propeller-like shape. The temperature-dependent NMR spectrum of the carbocation also indicates that it has this structure in solution.The twisting of the aromatic rings with respect to each other is evidently the result of van der Waals repulsions between the ortho hydrogens. 

We now examine the theory recently put forward by Penczek [6]. Strictly, we would not need to concern ourselves with it, as the system to which it is said to apply does not involve initiation by perchloric acid, but by triphenylmethyl salts. None the less, it is useful to consider it briefly. Penczek believes that the propagating species in his systems is an oxy-carbenium ion stabilised by co-ordination of two oxygens from a polymer molecule, as shown in structure 6 ... 

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