Ion-molecule reactions by a photoionization mass spectrometer. IV. C5-unsaturated hydrocarbons

Abstract

The reactions of parent ions of five pentene isomers and cyclopentene with their corresponding neutral molecules have been studied at 1.7 eV ion exit energy using a photoionization mass spectrometer in the pressure range between 1 and 10 mtorr. Pentene-1 was the most reactive system and 3-methylbutene-1 and 2-methylbutene-2 were much less reactive than others. C5H11+ formation in 2-methylbutene-1 had an exceptionally large cross section and this made the system much more reactive than other branched chain pentenes. Dimeric ions (C10 ions) were detected in all of the systems and no C9 ions were observed in any of the systems. Distribution of C6, C7, and C8 product ions in pentene isomers can be explained by the mechanism proposed by Henis except in the case of 2-methylbutene-2 where some rearrangements in the intermediate are necessary to explain the product distribution. Parent-minus-two reactions occurred in normal chain pentenes but did not in branched chain isomers. Parent-minus-one and parent-plus-one reactions occurred more or less in all of the pentene isomers. In cyclopentene, product distribution was very simple in contrast with the noncyclic pentenes, product ions C6 through C9 being absent. The cross sections of the observed reactions [(M−2)+, (M−1)+, and C10 ion formation], however, were fairly large.