Single crystals of 2-iodo-3′-chlorodibenzoyl peroxide (P) undergo several competitive and consecutive topotactic transformations: upon standing in the laboratory atmosphere or during X-ray photography at ∼22°C, crystalline P is transformed to pseudomorphs containing preferentially oriented single crystal phases of o-iodosobenzoic acid (E), m-chlorobenzoic acid (D), and an as yet unidentified novel phase, "X." (Topotactic rearrangement of P to the known acicular polymorph of its benzoxiodole isomer, 3-oxo-3H,2,1-benzoxiodol-1-yl m-chlorobenzoate, occurs only at elevated temperatures, ∼55°C.) In a much slower, consecutive transformation, during further continuous exposure to X-rays, "X" is photochemically reduced to o-iodobenzoic acid (C), which also is formed as a single crystal phase. The resulting single crystal domains of D and E in decomposed pseudomorphs of P are mutually oriented in a manner which reflects their strikingly similar crystal structures. The structure of D has been determined by single crystal X-ray analysis in order to compare the aligned molecular packing arrangements of D and C, and P and D. The mutual alignment of P and E is also presented in this report. All of these topotactic products are aligned so as to conserve the point group symmetry of the parent crystal, P (conservative twinning.). The essentially flat molecular structure of D, C7H5ClO2, crystallizes in centrosymmetric hydrogen bonded "dimer" units in a monoclinic cell with a = 3.845, b = 16.038, c = 11.203 Å, β = 94.88°, Z = 4, P21 c. All hydrogen atoms, including the acidic proton, were located during the analysis which converged to R = 0.10 for the 1083 observed intensities. The X-ray diffraction pattern of m-chloroperoxybenzoic acid (a = 4.035, b = 5.969, c = 30.55 Å, β = 91.70°, Z = 4, P21 c) is different from that of "X.". © 1975.
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