This study compared the GC-MS and FTIR properties of all six regioisomeric benzoyl substituted-1-n-pentylindoles, which are compounds that have the benzoyl-group attached at each of the possible ring substituent positions of the indole ring.
In addition, the substituents attached to the indole ring, benzoyl- and 1-n-pentyl-groups, are identical for all six isomers. The study found that the electron ionization mass spectra show equivalent regioisomeric major fragments resulting from cleavage of the groups attached to the central indole nucleus. Fragment ions occur at m/z 77 and 105 for the phenyl and benzoyl cations common to all six regioisomeric substances. Fragmentation of the benzoyl and/or pentyl groups yields the cations at m/z 234, 220, 214, 186 and 144. Although the relative abundance of the ions varies among the six regioisomeric substances, the 1-n-pentyl-3-benzoylindole and 1-n-pentyl-5-benzoylindole share similar relative abundances for the major fragment ions. Chromatographic separations on a capillary column containing a 0.5 ìm film of 100 percent trifluoropropyl methyl polysiloxane (Rtx-200) provided excellent resolution of these six compounds. The elution order appears related to the relative distance between the two indole substituted groups. The latest eluting compounds (highest retention time) have the two substituents on opposite sides of the indole nucleus. Infrared absorption spectral data show the carbonyl absorption band for each of the benzoylindoles and provide distinguishing and characteristic information to individualize each of the regioisomers in this set of compounds. (Publisher abstract modified)