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The crystal and molecular structures of S-isopropyl O-methyl O-(3,5,6-trichloro-2-pyridyl) phosphoramidothioate, C9H12N2O2PSC13, and dimethoate [ O,O-dimethyl S-(N-methylcarbamoyl)methyl phosphorodithioate], C5HI2NO3PS2, have been determined by three-dimensional X-ray analysis. The former compound crystallizes in the triclinic space group P1 with a = 10.319 (5) Å, b = 10.730 (6) Å, c = 8.44 (4) Å, a = 99.01 (2)°, β = 114.02 (l)°, and y = 62.64 (1)° with Z = 2, and the latter compound crystallizes in the monoclinic space group P21 with a = 6.574 (2) Å, b = 9.354 (2) Å, c = 9.885 (2) Å, and β = 107.4 (2)° with two molecules per unit cell. The structures were refined by a full-matrix, least-squares procedure to final residual indices of R = 0.058 and 0.068, respectively, using reflections with F° > 3σ(F°). Atomic charge densities were calculated via CNDO/2 methods in order that positive charge center separations as well as geometric factors could be compared between these insecticides, acetylcholinesterase, and previous insecticides studied in this series.