The chemistry of some low energy C5H9O+ oxonium ions

The reactions of the low energy oxonium ions CH3CHCHC+(H)OCH3, b1+, CH2CHC+(CH3)OCH3, b2+, and CH2C(CH3)C+(H)OCH3, b3+, are reported and compared with those of their 1,3-H shift isomers CH3CHCHCH2OCH2+, b4+, CH2CHCH(CH3)OCH2+, b5+, and CH2C(CH3)CH2OCH2+, b6+. The metastable ion (MI) spectrum of the C4H6OCH3+ ions b1+– b3+ is dominated by loss of CH2O. Elimination of C3H6, which is associated with a composite metastable peak, is also significant from b2+ and b3+. From (multiple) collision experiments and analysis of D- and 13C-labeled isotopologues, it follows that b1+–b3+ do not readily interconvert. Loss of CH2O is proposed to involve a 1,5-H shift followed by a (dipole-assisted) 1,3-H shift into an energy-rich ion-neutral complex (INC) [C4H7+/CH2O]. Loss of CH2O from b4+–b6+ may also occur via an INC. This reaction is associated with a very small kinetic energy release, indicating that it generates the most stable C4H7+ ion, CH2CHC+(H)CH3, at the thermochemical threshold. However, this process is only prominent for ions b5+, which also undergo a facile loss of H2O, via rearrangement in the INC [C4H7+/CH2O], to yield C5H7+ (most probably the cyclopentenyl cation). Loss of C3H6 and CO dominates the MI spectra of b4+ and b6+ and these reactions, which also occur from b5+, are proposed to take place from 1,2-H shift isomers of the cyclic counterparts of b4+–b6+. The behavior of b1+–b3+ and b4+–b6+ differs considerably from their C4H7O+ homologues, CH2CHC+(H)OCH3, a1+, and CH2CHCH2OCH2+, a2+. Differences in the dissociation characteristics of the CnH2n−2OCH3+ species (n = 3–5) are discussed in terms of the energetics of the products that may be formed. ΔHf (298 K) values for the key ions in this study were obtained from CBS-QB3 calculations and thermochemical estimates.