A density functional theory study of methyl shifts and methylcyclopropanol ion formation in C 4H 8O +· ions

Abstract

C 4H 8O +· ions undergo a number of skeletal isomerizations, including rearrangement to the 2-butanone ion before metastable dissociation. However, the mechanisms whereby these rearrangements take place are not all firmly resolved. To remedy this, we characterized potential steps in the skeletal isomerizations of C 4H 8O +· ions using density functional theory. We established that branched and straight chain C 4H 8O +· ions with the oxygen on the terminal carbon interconvert through methylcyclopropanol ions and β-distonic ions. The cyclic isomers were found to be more stable than their ring-opened β-distonic forms. Transformation of ions with the O on a terminal carbon to ions with O on the second carbon occurs by methyl shifts that convert CH 2 ·CH(CH 3)CHOH + and CH 3CH 2CHCHOH +· to CH 3CH(OH)CHCH 2 +·. The second methyl shift takes place by inversion of the methyl configuration, possibly in accord with predictions of Woodward–Hoffmann rules.