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The molecular structures of 2-[4-chloro-2-fluoro-5-(prop- 2-ynyloxy)phenyl]-1,3,4,5,6,7-hexahydroisoindole-1,3-dione, C17H13ClFNO3, (I). and the isomeric compound 2-[4-chloro-2-fluoro-5-(prop-2-ynyloxy)phenyl]-cis-1,3,3a,4,7,-7a-hexahydroisoindole-1,3-dione, (II), are, as anticipated, significantly different in their conformations and in the distances between the farthest two atoms. The six-membered ring of the 1,3,4,5,6,7-hexahydroisoindole-1,3-dione moiety in (I) adopts a half-chair conformation. The dihedral angle between the five-membered dione ring of (I) and the benzene ring is 50.96 (7)°. The six-membered ring of the cis-1,3,3a,4,7 ,7a-hexahydroisoindole-1,3-dione moiety in (II) adopts a boat conformation. The dihedral angle in (II) between the five-membered dione ring and the benzene ring is 61.03 (13)°. In the crystal structures, the molecules are linked by C—H··· hydrogen bonds and weak π-π interactions. Compound (I) is a much more potent herbicide than (II). The Cl···H distances between the farthest two atoms in (I) and (II) are 11.37 and 9.97 Å, respectively. [ABSTRACT FROM AUTHOR]