L. Wayne Sieck

Rate coefficients for ion‐molecule reactions I. Ions containing C and H

A compilation is presented of experimentally determined bimolecular and thi order rate coefficients for the reactions of hydrocarbon ions with neutral molecules in the vapor phase. The literature covered is from 1960 to the present, and both positive and negative ions are considered. Four hundred and fifty‐eight separate reaction‐pairs are tabulated, and the ionic reaction products and experimental conditions are specified wherever possible. Preferred values are suggested for a number of these processes.

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.

Chemical Kinetics Database and Predictive Schemes for Humid Air Plasma Chemistry. Part I: Positive Ion–Molecule Reactions

Reliable kinetic and thermodynamic data are required to model the evolution of electric discharge or electron-beam decomposition chemistry of gases in humid air streams. In this first segment of a continuing series, we provide a core database describing the initially dominant ion-neutral molecule reactions in humid air plasmas. Recommended reaction rate data and extrapolation tools are presented in a manner to facilitate prediction of reactivities and reaction channels as a function of temperature, pressure, and applied electric field.

Thermal decomposition of ions. III. Protonated ethanol and diethyl ether

Abstract Above 550 K, the C2H5OH+2 ion decomposes to yield H3O+ and C2H4. An alternative decomposition channel is hydrogen loss to give CH3CHOH+, and deuteration studies suggest that one hydrogen atom originates from hydroxyl and another from the ethyl group. The decomposition products regenerate C2H5OH+2 by proton transfer to C2H5OH, thus forming reaction ‘cycles’ that reach steady states. At higher concentrations of C2H5OH, condensation with C2H5OH+2 forms (C2H5)2OH+, which in turn decomposes with the loss of C2H4 to regenerate C2H5OH+2, which results in another steady-state cycle. Decomposition rate constants are calculated from steady-state ion ratios, and at 650–670 K are in the range of 1–10 × 103 s−1 for all the reactions. The decompositions of C2H5OH+2 are at the low-pressure limit and those of (C2H5)2OH+ are at the high-pressure limit at 2–8 torr of CH4. The decompositions show Arrhenius activation energies of 24–30 kcal mol−1.

The ionization energy of SF5 and the SF5–F bond dissociation energy

The techniques of Fourier transform spectroscopy and high pressure mass spectrometry have been used to investigate the ionization energy (IP) of SF5 via electron exchange reactions. An IP of 9.60±0.05 eV has been derived from this data, which is in disagreement with estimates from flowing afterglow measurements (IP≥10.15 eV). In addition, no evidence has been found for a fluoride transfer equilibrium involving CF+3 and SF6, which was thought to interrelate the heats of formation of CF+3 and SF+5. Our results are consistent with the recommended (SF5–F) bond strength of ∼4.0 eV, and suggest an energy threshold for SF+5 production from SF6 of 13.6 eV.

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.

Pulsed Electron-Beam Ionization of Humid Air and Humid Air/Toluene Mixtures: Time-Resolved Cationic Kinetics and Comparisons with Predictive Models

The technique of pulsed electron-beam high-pressure mass spectrometry wasused to investigate the sequential cationic chemistry in humid air streamsat 4.2×102 Pa and 380 K. The system was then modeled usingthe ACUCHEM program, incorporating thirty-five reactions taken from theformulations given in Part I of the new National Institute of Standards andTechnology (NIST) Chemical Kinetics Database for Humid Air Plasmas. Theresulting temporal ion profiles were found to be in qualitative agreementwith the laboratory data. Analogous pulsed electron-beam measurements werealso carried out with humid air samples containing low levels of toluene,and these results were also reproduced qualitatively by a model incorporatingforty-eight reactions after the inclusion of an unexpected, but crucial,channel involving the reaction of an intermediate air-generated cluster ionwith toluene. The benefits of laboratory validation of predictive databasesin systems for which the literature data are incomplete are emphasized.

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+).

Ion–Molecule Reactions in the Radiolysis of Ethane

The reactions of ions generated in ethane irradiated with gamma rays have been studied by analyzing the neutral products formed in reactions with ethane and with other molecules. In experiments in the presence of added (C2D5)2CDCD3, for example, it is shown that the following reactions take place: C2H5++C2H6→(C4H11+)*→sec‐C4H9++H2; sec‐C4H9++(C2D5)2CDCD3→n‐C4H9D+C6D13+. The intermediate (C4H11+)* ion can be stabilized by collisions and will then undergo an undetermined reaction (neutralization or proton transfer) to give n‐C4H10 as a product. The over‐all rate constant for reaction of the ethyl ion with ethane is shown to be ≤ 10−10 cm3/molecule·sec. Similarly, it is demonstrated that the reaction C2H3++C2H6→C4H9+ leads predominantly to the formation of t‐butyl ions under these conditions: C4H9++(C2D5)2CDCD3→(CH3)3CD+C6D13+. Supplementary experiments performed in a photoionization mass spectrometer demonstrate that ethylene ions undergo a “resonance H2− transfer” reaction with ethane: C2H4++C2D6→C2D4++C2H...