Photoionization of CO2 -CO-O2 Mixtures. Formation and Reactions of Ion Clusters.

Various mixtures containing combinations of CO2, O2, or CO have been photoionized at 16.7 and 21.2 eV at pressures up to 1.5 torr in the NBS high pressure photoionization mass spectrometer. In CO2-CO mixtures the interactions of CO 2 + ions eventually lead to the formation of (CO) 2 + and [(CO)2-CO2]+ cluster ions, while photoionization of CO2-CO-O2 mixtures yields mainly oxygen-containing clusters at higher pressures. The investigation of O2-CO mixtures also revealed reactions between O 4 + and CO. The role of impurity reactions involving H2O is considered in some detail, and the implications of all of these data to the vapor phase radiolysis of CO2 is discussed.

Photoionization of C 4 H 8 + Isomers. Unimolecular and Bimolecular Reactions of the C 4 H 8 + Ions.

1-Butene, cis-2-butene, isobutene and methylcyclopropane have been photoionized with the resonance lines of krypton (10.0-10.6 eV) and argon (11.6-11.8 eV). We have determined that the internally excited 1 - C 4 H 8 + ion and, to a much lesser extent, the i - C 4 H 8 + ion isomerizes to the 2 - C 4 H 8 + structure. In both cases the extent of isomerization increases, approximately by a factor of ten when the photon energy is increased from 10 to 11.7 eV. An inert gas, neon, quenches the isomerization of the i - C 4 H 8 + ion and, to a much lesser rextent, that of the 1 - C 4 H 8 + ion. The unimolecular fragmentation of the C 4 H 8 + isomeric ions has been examined at 11.61-11.8 eV. In this energy range the dissociative lifetime of i - C 4 H 8 + was found to be at least 5 × 10-6 s, and collisional quenching of the dissociative process is already noticeable at pressures in the 10-3 torr range. The rate coefficients for the reaction C 4 H 8 + (  thermal  ) + C 4 H 8 → ( C 8 H 16 + ) * occurring in the isomeric C4H8 systems have been determined under conditions where the structure of the reacting C 4 H 8 + ion is established. The values in cm3/molecule · second are 1-C4H8 - 6.0 ± 0.5 × 10-l0, cis-2-C4H8 - 0.37 ± 0.1 × 10-10, i-C4H8 - 5.4 ± 0.4 × 10-10. At pressure below 10-3 torr, the internally excited ( C 8 H 16 + ) * produced in the reaction dissociates along various channels with relative probabilities depending upon the structure of both the ionic and neutral reactant. Above 10-3 torr collisional quenching of ( C 8 H 16 + ) * is noted.

Photoionization of Propylene, Cyclopropane, and Ethylene. The Effect of Internal Energy on the Bimolecular Reactions of C 2 H 4 + and C 3 H 6 + .

The ion-molecule chemistry occurring in photoionized ethylene, propylene and cyclopropane has been investigated in the NBS high pressure mass spectrometer. Emphasis has been placed on determining the overall effect of internal excitation on the absolute rate coefficients for the various reactions. This was accomplished by comparing the behavior found when ionization was induced by photon absorption at the ionization threshold with that found at higher photon energies. The values for the absolute rate coefficients for the formation of excited compound ions at various photon energies were as follows (units of 10-10 cm3/molecule · s); C 2 H 4 + , 9.6 (10.6 eV photons); 8.5 (11.7 eV photons);. C 3 H 6 + , 8.4 (10.0 eV photons); 6.8 (11.7 eV photons); C 3 H 6 + , 7.8 (10.0 eV photons), 6.1 (11.7 eV photons); c -C 3 H 6 + and c -C 3 H 6 + , 2.2 (10.0 eV photons); 2.2 (11.7 eV photons). The distributions of product ions obtained from the unimolecular decomposition of the compound ion in each system have also been determined at various photon energies, and considerable variation has been observed. In general, the variations are similar to those found elsewhere when the kinetic energy of the reactant ion is increased. The bimolecular reactions of the C 6 H 12 + compound ion obtained from C 3 H 6 + > have been investigated at higher pressures, and separate experiments involving isotopically labeled ethylenes have provided new information concerning the nature of the compound ion in that system. Results obtained from other laboratories are discussed in some detail, and in most cases the differences found for the same system are explicable in terms of the energy content (internal and/or kinetic) of the reactant ion.

Ion-Molecule Reactions in Isopropanol: Implications for Its Radiolysis.

The photoionization of isopropanol (i-PrOH) at 11.7 eV has been investigated in the NBS high pressure photoionization mass spectrometer over the pressure range 0.1 to approximately 200 millitorr. The major primary ion, CH3CHOH+, reacts with a total rate coefficient of 1.1 ±0.1× 10-9 cm3/mole-cule-s to yield mainly (i-PrOH)H+ and CH3(i-C3 H7)COH+. At pressures in excess of a few millitorr consecutive reactions of (i-PrOH)H+ and CH3(i-C3H7)COH+ were also detected, including the overall dehydration process ( i -PrOH ) H + + i -PrOH → H 2 O + ( i -C 3 H 7 ) 2 OH + These data are interpreted in light of a recent radiolysis study, and a thermal cationic chain mechanism for production of (I-C3H7)2O consistent with those results is proposed. The value for the proton affinity of SF6 is also discussed in some detail.