We report a comparative study of the photoinduced localized atomic reaction, LAR, of 1,2- and 1,4-dichlorobenzene (1,2- and 1,4-diClPh) on Si(111) 7x7, studied by scanning tunneling microscopy (STM). We observe different chlorine nearest-neighbor separations for the Cl-Si photoformed chemical "imprints" for the two adsorbates; the most probable separation of neighboring Cl's in the case of 1,2-dichlorobenzene being 8 +/- 3 Angstrom, whereas that for 1,4-dichlorobenzene is 14 +/-3 Angstrom. These differing separations can be understood if the two C-Cl bonds in the respective adsorbates extend roughly linearly to chlorinate a nearby Si dangling-bond; for 1,2 diClPh the two C-Cl bonds are at 60 degrees to one another leading to the chlorination of closely neighboring Si, whereas for 1,4 diClPh the two C-Cl bonds are at similar to 180 degrees leading to chlorination of sites twice as far apart, located to either side of the 1,4 diClPh adsorbate.