Stabilities of isomeric halonium ions C2H4X+ (X = Cl, Br) by photoionization mass spectrometry and ion cyclotron resonance spectroscopy. General considerations of the relative stabilities of cyclic and acyclic isomeric onium ions

At least two noninterconverting structural isomers of the halonium ions C2H4X+ (X = Cl, Br) are shown to exist in the gas phase. Threshold measurements of their formation through halide loss in the photoionization and fragmentation of corresponding dihaloethanes yield enthalpies of formation for these species. For X = Cl, the cyclic halonium ion is 5.6 kcal/mol less stable than the acyclic species CH3CHCl+. These stabilities are reversed for X = Br, with the cyclic isomer being more stable by 1.4 kcal/mol. In addition, relative stabilities of a larger class of onium ions are inferred from ion cyclotron resonance studies of exchange reactions between C2H4X+ and HY to give C2H4Y+ and HX (X,Y = Cl, Br, OH, SH, NH2, PH2). From the preferred direction of these processes, stability is inferred to increase in the order X = OH < Cl < Br < SH, PH2 < NH2 for the cyclic onium ions and X = Cl < Br < SH, PH2 < OH < NH2 for their acyclic isomers