Inclusion compounds in perhydrotriphenylene. Part II. The conformation of the included molecules

Abstract

X-Ray rotating-crystal photographs of perhydrotriphenylene (PHTP) inclusion compounds show the presence of continuous streaks due to interference from the included molecules; from their spacings the molecular repeat distance can be measured. The chain conformation of n-hydrocarbons is not completely trans-planar, although it tends to planarity with increasing molecular weight. This phenomenon has been interpreted quantitatively by assuming a statistical distribution of the molecules between the trans-planar model and a twisted conformation represented by the … TGTG′TGTG′… sequence of internal rotation angles (T =trans, G =gauche+, G′=gauche^–). An analogous investigation has shown that included monocarboxylic acids are dimeric, and that in dialkyl ether molecules the C–O–C groups enter largely into the molecular twisting. In both cases, the alkyl groups have conformations similar to those in the pure hydrocarbons. An eclipsed conformation is suggested for included 2,2,4-trimethylpentane.