THE EFFECTS OF SATURATION AND SUBSTITUTION ON THE MOBILITY OF PROTONATED CYCLIC-COMPOUNDS

The mobility in air of protonated cyclic compounds was measured by ion mobility spectrometry techniques. From the mobility measurements it was found that the collisional cross-sections of saturated ions were much larger than those of their unsaturated analogs. This was attributed to elongation of bonds on saturation and to the additional hydrogen atoms. On substitution of CH2 by NH in saturated compounds, or of CH by N in unsaturated compounds, the mobility of the protonated compounds decreased. This was attributed to changes in the internal charge distribution: in the monosubstituted analogs the charge is localized mainly on the site of the heteroatom, while in the hydrocarbons the ionic charge is largely delocalized in the ring. Thus, the interaction of the ion with the drift gas molecules is stronger in the heterocyclic ions, leading to larger collision cross-sections and lower mobilities. Subsequent substitution had little or no further effect. Ions possessing a cyclic structure were observed to have higher mobilities than their noncyclic isomers. © 1990.