Stuart K. Searles

High Pressure Photoionization Mass Spectrometry. III. Reactions of NO+(X 1Σ+) with C3–C7 Hydrocarbons at Thermal Kinetic Energies

The vapor phase reaction of NO+(X 1Σ+) with C3 through C6 normal, branched, or cyclic alkanes was found to proceed exclusively via an H− transfer mechanism, NO+(X 1Σ+)+RH2→RH++HCO. In addition to (I), C4H9+ was also formed by a second‐order process in the reaction with 3‐methylhexane. Absolute rate constants were determined for all systems at thermal kinetic energies. Isomers containing tertiary H atoms were found to be the most reactive, exhibiting rate constants on the order of 10+9 cm3/molecule·sec. Isotopic labeling has verified that the tertiary site is involved in the H− transfer reaction in those molecules having both secondary and tertiary H atoms. The rate constants found for n‐alkanes and nonsubstituted cycloaklanes fall in the range 10−12–10−10 cm3/molecule·sec. The bimolecular reaction cyclo‐C6H11++NO→C6H11NO+ was also noted at higher pressures. No further reaction of the RH+ species generated in (I) was found in any other RH2–NO combination at pressures up to 0.5 torr.

High‐Pressure Photoionization Mass Spectrometry. Photoionization of Propane at 11.6–11.8 eV. Formation and Reactivity of the (C3H8)2+ Dimer Ion

The major reaction path of the propane molecular ion with propane was found to be the formation of the dimer ion (C3H8)2+ via a termolecular mechanism, C3H8++C3H8→ lim C3H8(C3H8)2++C3H8. In addition, C3H6+ and C3H7+ were also found as minor reaction products at lower pressures. The reactions of the dimeric ions with ethylene and NO were also investigated. The charge exchange reaction, (C3H8)2++NO→NO++ 2C3H8, was found in propane–NO mixtures, suggesting a recombination energy in excess of 9.24 eV. The formation of C3H8NO+ was also detected at higher total pressures. The dimeric ion was also found to transfer H2 to ethylene without affecting the structural integrity of the carbon skeleton, (C3H8)2++C2H4→C6H14++C2H6, indicating that this species exhibits the chemical behavior of a saturated hydrocarbon ion.

Irradiation of Cyclohexene with 8.4, 10.0, and 11.6–11.8‐eV Photons. Dissociation of Neutral Excited Cyclohexene and Reactions of C6H10+

The photolysis of cyclohexene has been investigated at photon energies above (10.0, 10.6–11.8 eV) and below (8.4 eV) its ionization potential (8.9 eV). The ionic processes were studied in a mass spectrometer equipped with a photon source. The parent C6H10+ ion was seen to undergo the following reactions with cyclohexene: C6H10++cyclo‐C6H10→cyclo‐C6H12+C6H8+, k = 0.7 × 10−10cm3/molecule·sec →C12H20+, k = 4.1 × 10−10cm3/molecule·sec. Analysis of the yields of cyclohexane formed in the first of these reactions in the photolysis at 10.0 eV over the pressure range 1–10 torr indicates that in this pressure range the relative importance of the two reactions is the same as in the ion source of the mass spectrometer at a pressure of 10−2 torr. At 11.6–11.8 eV the parent ion also dissociates, mainly to form C5H7+ and CH3. Evidence is presented which indicates that at these energies, the majority of the C6H10+ ions retain a cyclic structure. The neutral, electronically excited cyclohexene molecule dissociates mainly...

Modes and Rates of Reaction of Cyclopentene and Methylcyclopentene Ions with Their Parent Molecules

Cyclopentene and methylcyclopentene ions were generated by irradiating the respective parent compounds with 10.0‐eV photons; cyclopentene was also irradiated with 11.6–11.8‐eV photons. The ionic products of ion–molecule reactions were observed in the NBS high‐pressure photoionization mass spectrometer, while the neutral products of these reactions were determined by chemical analysis of products formed in photolytic experiments in a closed system. The cyclopentene ion, which at 10.0 eV retains its cyclic structure, undergoes an H2 transfer reaction (c‐C5H8++c‐C6H8→c‐C5H10+c‐C5H6+, k = 3.3 × 10−10cm3/molecule·sec) and a condensation reaction (c‐C5H8++c‐C5H8→C10H16, k = 2.7 × 10−10cm3/molecule·sec) with the parent molecule. The same reactions are observed for ions formed at 11.6–11.8 eV, but at the higher energy, approximately 20% of the ions are observed to undergo ring opening to form 1,3‐C5H8+ ions; the latter ions undergo an H2 transfer reaction with the cyclopentene molecule (1,3‐C5H8++c‐C5H8→2‐C5H10+C...

High‐Pressure Photoionization Mass Spectrometry. Reactions of Alkane and Cycloalkane Molecular Ions with Water Vapor at Thermal Kinetic Energies

The reactions of alkane molecular ions (RH2+) with water vapor were found to proceed via a bimolecular mechanism in both ethane and propane. Parent ions from cyclohexane, cyclopentane, i‐butane, n‐butane, i‐pentane, n‐pentane, and n‐hexane were observed to react exclusively via a termolecular mechanism involving two water molecules: RH2++2H2O→H+(H2O)2+RH. Thermal rate constants of 1.2 and 1.4 × 10−9 cm3/molecule·sec, respectively, were derived for the bimolecular reactions of C2H6+ and C3H8+ with H2O. The termolecular rate constants found in other RH2+–H2O combinations were quite high, falling in the range 10−25–10−27 cm6/molecule2·sec. The nature of the collision complex is discussed, and new limits are estimated for δHf(H3O+).