The chemistry of some low energy C5H9O+ oxonium ions

The reactions of the low energy oxonium ions CH3CH=CH-C+ (H)OCH3, b(1)(+), CH2=CH-C+(CH3)OCH3, b(2)(+), and CH2=C(CH3)-C+(H)OCH3, b(3)(+), are reporte d and compared with those of their 1,3-H shift isomers CH3CH=CHCH2OCH2+ b(4 )(+), CH2=CHCH(CH3)OCH2+, b(5)(+), and CH2=C(CH3)CH2OCH2+, b(6)(+). The met astable ion (MI) spectrum of the C4H6OCH3+ ions b(1)(+) - b(3)(+) is domina ted by loss of CH2O. Elimination of C3H6, which is associated with a compos ite metastable peak, is also significant from b(2)(+) and b(3)(+). From (mu ltiple) collision experiments and analysis of D- and C-13-labeled isotopolo gues, it follows that b(1)(+)-b(3)(+) 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]. Los s of CH2O from b(4)(+)-b(6)(+) also occur via an INC. This reaction is asso ciated with a very small kinetic energy release, indicating that it generat es the most stable C4H7+ ion, CH2=CHC+ (H)CH3, at the thermochemical thresh old. However, this process is only prominent for ions b(5)(+), which also u ndergo 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 b(4)(+) and b(6)(+) and these reactions, which also occur from b(5)(+), are proposed to take place from 1,2-H shift isomer s of the cyclic counterparts of b(4)(+)-b(6)(+) The behavior of b(1)(+)-b(3 )(+) and b(4)(+)-b(6)(+) differs considerably from their C4H7O+ homologues, CH2=CH-C+(H)OCH3, a(1)(+), and CH2=CHCH2OCH2+, a(2)(+), Differences in the dissociation characteristics of the CnH2n-2OCH3+ species (n = 3-5) are dis cussed in terms of the energetics of the products that may be formed. Delta H(f) (298 K) values for the key ions in this study were obtained from CBS-Q B3 calculations and thermochemical estimates. (C) 2001 Elsevier Science B.V .