and CH., chemical ionhation mass spectra of thirteen Cs to Cll akylbenzenes are reported. Characteristic hydride and aWde ion abstraction reactions are observed with all reagent gases. The major fragmentation reactions of [MI%]+ are olefin elimination to form a protonated arene and arene elimination to form an alkyl ion. From the effect of structure and protonation exothermicity it is concluded that rearrangement of primary akyl groups to the more stable secondary or tertiary structure occurs prior to alkyl ion formation. A detailed fragmentation mechanism for protonated arenes is proposed. The 'effective' proton affinity of the methane-derived reagent system is estimated to be -556kJmol-'.

A series of formate (methyl through butyl) and acetate (methyl through pentyl) esters have been protonated in the gas phase by the Brgnsted acids H3+, N2H+, C02H+, N20H+, and HCO+. Carbonyl oxygen protonation is 87-97 kcal mol-' exothermic for H,+ and 47-57 kcal mol-' exothermic for the weakest acid HCO+, permitting a study of the effect of protonation exothermicity on the decomposition modes of the protonated esters. With the exception of protonated methyl formate, three decomposition modes, (a) to (c) are observed.Reaction (a) is unimportant for formates; for acetates it is the sole decomposition channel for the methyl ester, but is less important for higher acetates. The dependence of the relative importance of this reaction mode on the protonation exothermicity indicates an activation energy considerably in excess of AHo, presumably because the reaction involves a symmetry-forbidden 1,3-H shift for the carbonyl protonated ester. For the higher acetates where the difference in the proton affinities of the carbonyl and ether oxygens is less, acyl ion formation results, in part, from protonation at the ether oxygen. For protonated methyl formate the major fragmentation reaction yields CH30H2+ + CO; this reaction also appears to have an activation energy considerably in excess of the AH,. For the remaining esters either reaction (b) or (c) is the major decomposition mode. The competition between these two channels depends strongly on the protonation exothermicity and the relative activation energies. From the reaction competition we conclude that 1,2-H shifts occur in the case of primary alkyl esters yielding more stable secondary or tertiary alkyl ions. This rearrangement appears to occur after the excess energy has been partitioned between the alkyl ion and the neutral acid since the extent of further fragmentation of the alkyl ion reflects the original structure of the alkyl group.

. Can. J. Chem. 59,2125Chem. 59, (1981. The chemical ionization mass spectra of eight C5 alkanols and fourteen C, alkanols have been obtained using H3+, N,H+, CO,H+, N20H+, and HCO+ as reactant ions. This choice of reactant ions allows the exothermicity of the protonation reaction to be varied from -90 kcal mol-I (H3+) to -50 kcal mol-' (HCO+). The major fragmentation reaction in all cases was H 2 0 elimination from the protonated alcohol forming the appropriate C,H,,+ or C6H1,+ alkyl ion. The extent of further fragmentation of the alkyl ions decreased with decreasing exothermicity of the protonation reaction and was greatest for alkyl ions derived from primary alcohols, less for alkyl ions derived from secondary alcohols, and very small for alkyl ions derived from tertiary alcohols. The results indicate that there is negligible rearrangement to more stable alkyl ions prior to attaining the critical configuration which determines the energy partitioning between R+ and H,O in the fragmentation of ROH,+. Other less important reaction modes in the CI spectra involved formation of (M -H)+ ions and formation of oxycarbonium ions by alkane elimination from protonated alcohols.

The ion-molecule reactions in vinyl fluoride have been studied as a function of pressure and electron energy. The C2H,-and C2HF+ fragment ions react predominantly by charge transfer while C2HBL produces C2H,F+ and C4H5'. The C2H2F7 fragment forms C2H4F+, CHFZ-, CzH3Fz+, C4H4F+, and probably C2H3 +. The rate constants for the individual reactions have been measured. The C2H3F+ ion reacts to form C3H5 +, C3H4F+, C3H3F, +, and C4H5F7 (in minor yield), both by a second order and by a third order reaction. The rate constants and product distributions from the individual reactions have been evaluated. A number of consecutive reactions have been identified and shown to be third order processes.