Conglomerate crystallization in organic compounds. Part 3. the X-ray diffraction crystal structure of triphenyl methane, molecular mechanics, and quantum mechanical calculations of the structure and energetics of this molecular propeller
The motivation for this study was the interesting, and contrasting, reported crystallization modes of triphenylmethane and triphenylsilane, the former said to crystallize as a conglomerate [Riche and Pascard-Billy,Acta Crystallog. B1974,30, 1874], whereas the latter was reported to crystallize as a racemate [Allemand and Gerdi,Cryst. Struct. Commun.1979,8, 927]. As we show in what follows, we had planned to justify these observations via molecular mechanics and semiempirical (AM1) calculations. However, in the process of doing some geometrical calculations unavailable in the original, we found that it was impossible for the reported structure [Riche and Pascard-Billy,op. cit.] to be correct. Therefore, we redetermined the structure and found that triphenylmethane also crystallizes as a racemate. The compound was recrystallized from toluene and its structure was solved in space groupPna21 (No. 33). The cell constants area=25.593(14),b=14.804(19),c=7.521(15) å;V=2849.5(6) å3,d(MW=244.32 g mol−1;z=8)=1.139 g cm−3. A total of 2183 independent reflections were collected over the range 4‡ ≤ 2θ ≥40‡. The structure was solved by direct methods and refined using rigid-body constraints for the three phenyl rings as a result of the large, and unfavorable, ratio of variables/observed data had the individual atoms been refined. Molecular mechanics (HyperChem) calculations were carried out to ascertain the degree of agreement between the observed structural parameters and those obtained from an energy-minimized structure. The agreement is remarkably good, especially given the quality of the X-ray data. Similar comments can be applied to the results obtained by semiempirical (AM1) calculations (in HyperChem). Thus, this is a satisfying agreement lending weight to the suggestion that the X-ray data are very reasonable, even if flawed. Calculations were also carried out to determine a conformational potential surface for the molecule as a function of rotation of the phenyl rings around their HC-C(phenyl) vector. These calculations indicate a shallow minimum around the conformation with C3 symmetry with torsional angles [H-C-C(head)-C(ortho) of 30‡, 30‡, 30‡], as well as possible low-energy pathways between molecules of opposite axial chirality.
Key wordsTriphenylmethane structure energetics molecular propellers conformations
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