X-Ray crystallographic study of guest–molecule orientations in the β-hydroquinone clathrates of acetonitrile and methyl isocyanide

Abstract

Redetermination of the crystal structure of the Type III β-hydroquinone–acetonitrile clathrate, 3C₆H₄(OH)₂·CH₃CN (1), from new Mo-K_α diffractometer data confirmed the principal finding of previous studies, namely that the three symmetry-independent acetonitrile molecules fit snugly inside clathration cavities in a pseudo-rhombohedral host lattice (space group P3), with one guest molecule aligned in the opposite sense to the other two. A parallel study of the potentially isomorphous β-hydroquinone–methyl isocyanide clathrate, 3C₆H₄(OH)₂·CH₃NC (2), showed that it is a Type II clathrate (space group R3), in which all three guest molecules in the unit cell are equivalent and point in the same direction parallel to the c axis. The effective molecular length (linear molecular skeleton plus the sum of appropriate van der Waals radii at both ends) of CH₃NC is inferred to be ca. 0.1 Å longer than that of CH₃CN, in keeping with the relative stabilities and unit-cell dimensions of (1) and (2). Lattice parameters are: for (1), a= 16.003(2), c= 6.245(2)Å; for (2), a= 15.946(2), c= 6.348(2)Å. The structures have been refined to R values of 0.080 (1) and 0.056 (2) using, respectively, 1 277 and 600 observed reflections.