Cocrystallization with acetylene. The 1:1 Complex with benzene: Crystal growth, X-ray diffraction and molecular simulations.
The crystal structure of an unusual 1:1 mol complex between benzene and acetylene, two very small and apolar mols., was detd. by x-ray-anal. of crystals grown by first mixing the two liqs. under conditions of low temp. and high pressure in a capillary, followed by repeated zone melting to form crystals directly on the goniometer head of a diffractometer. Each acetylene mol. is clamped between two parallel benzene rings, with its mol. axis apparently perpendicular to the benzene planes. Closer inspection of thermal-motion data from the diffraction expt. suggests that the acetylene mol. undergoes a wobbling, or precession, motion between the two rings so that it is perpendicular to them only in a time-averaged sense. The results of quantum-chem. calcns. on isolated mol. dimers and trimers support this conclusion. The calcn. of sep. coulombic, dispersion, polarization and repulsion contributions to intermol. bonding reveals that the C-H ... p-bond interaction between acetylene and benzene in a T-shaped dimer consists of a mixt. of coulombic and polarization interactions. In the benzene-acetylene cocrystal, its magnitude is quant. comparable with that of other dispersive interactions. 5.4 Ns Mol.-dynamics simulations of the liq. mixt. reveal that the two components are persistently miscible, a possible key to the formation of the cocrystal. No structure is, however, obsd. in the soln. during the relatively short simulation time.
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