Isomorphism problem for a class of point-symmetric structures

Compounds represented by the following general formula: wherein W represents a phenyl group, a phenyl group substituted with a member selected from the group consisting of a halogen atom, a nitro group, a cyano group, an alkyl group containing 1 to 3 carbon atoms and an alkoxy group containing 1 to 4 carbon atoms, with the number of the substituents being 1 or 2 when said substituent is a halogen atom and 1 when said substituent is other than halogen, or an unsubstituted naphthyl group, and said phenyl group may take the form of X represents a hydrogen atom, an unsubstituted phenyl group or an alkyl group containing 1 to 3 carbon atoms, Y represents a halogen atom, and n represents an integer of 1 to 3. These compounds are useful as photo-initiators.