The crystal structures of methyl bromide and methyl iodide have been determined by the X-ray diffraction method at about −120°C and about −80°C respectively. Both the crystals are isomorphous, with orthorhombic space group D2h16–Pnma. Unit cells containing four molecules have the dimensions: a=4.474(1), b=6.420(2), and c=9.150(1) Å for methyl bromide, and: a=4.597(2), b=6.987(1), and c=10.117(1) Å for methyl iodide. These structures are quite different from that of methyl chloride, which has a symmetry of C2v12–Cmc21. In the three crystals, all the molecules are found on the mirror planes; the difference lies in the mutual orientations of the molecules.
The crystal structure of methylene dichloride has been determined by the X-ray diffraction method at about −120°C. The structure has a symmetry D2h14–Pbcn, and its orthorhombic tetramolecular cell has these dimensions: a=4.249±0.001, b=8.138±0.020, and c=9.492±0.002 Å. The positional parameters of the chlorine and carbon atoms are obtained by a three-dimensional Patterson function, followed by a least-squares refinement including the hydrogen atom. The structure thus obtained is in excellent agreement with the results of lattice-energy calculations. The crystal structure is quite different from that of methylene dibromide (and of methylene diiodide).
The crystal structures of methylene dibromide and methylene diiodide (Form II) have been determined by the X-ray diffraction method at about −90°C and about −20°C respectively. Both the crystals are isomorphous, with monoclinic space group C2h6–C2/c. Unit cells containing eight molecules have these dimensions: a=12.239(12), b=4.459(15), c=15.212(16) Å, and β=113.54(7)° for methylene dibromide, and a=13.346(3), b=4.720(90), c=16.479(5) Å, and β=114.48(8)° for methylene diiodide. The orientations of the methylene groups have been obtained by lattice-energy calculations.
The crystal structure of bromoform has been determined by the X-ray diffraction method at about -20 °C. There are two molecules per cell in the structure with hexagonal space group P63 (C6). Unit-cell constants are a= 6"335 + 0"003 and c= 7.214+ 0-003 /~,. The structure is statistically disordered, with molecules oriented upwards and downwards along the c axis at random. The order parameter w was found to be nearly ½ and hence the symmetry of the crystal can approximately be represented by
P63/m (C26~).The structure is isomorphous with that of iodoform, while it is quite different from that of chloroform.
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