W. B. Knighton

Electron attachment to SF5CF3 (296–563 K) and calculations of the neutral and anion thermochemistry

Moller–Plesset (MP) perturbation theory and density functional theory (DFT) were used to examine the structure and bonding of trifluoromethyl sulfurpentafluoride, SF5CF3, and the corresponding anion, SF5CF3−. The structural parameters, charge analysis, and energetics are all consistent with the anion having ion-dipole character (i.e., SF5−–CF3). Results from G2(MP2) theory yield a neutral D2980(SF5–CF3)=301 kJ mol−1 (3.12 eV), anion D2980(SF5−–CF3)=21.5 kJ mol−1 (0.22 eV), EA(SF5CF3)=119 kJ mol−1 (1.24 eV), ΔfH2980(SF5CF3)=−1639 kJ mol−1, and ΔfH298(SF5CF3−)=−1750 kJ mol−1. The calculated value for the standard enthalpy of formation for SF5CF3 differs from the previous estimate by 78 kJ mol−1. DFT was found to perform poorly for quantities related to the neutral SF5–CF3 bond. Calculations were also carried out for SF5, SF5−, CF3, and CF3− fragments, and both DFT and G2(MP2) methods performed well for these open-shell species. Rate constants for electron attachment to SF5CF3 were measured over the temperat...

Effect of buffer gas and pressure variations on the formation of Br−2 in reactions of thermal electrons with dibrominated hydrocarbons and fluorocarbons

Abstract The relative amounts of Br−2 and Br− formed from the attachment of thermal electrons to ten dibrominated hydrocarbons and fluorocarbon have been measured under a range of buffer gas conditions, including pressure variations from 1 Torr to 1 atm. Measurements by a pulsed e-beam high pressure mass spectrometer have been made at 1–4 Torr and 50°–150°C, using seven different buffer gases. Measurements by an atmospheric pressure ionization mass spectrometer have been made in nitrogen buffer gas at 100°–200°C. The relative abundances of Br−2 and Br− produced in these electron capture reactions are found to change significantly with variations in buffer gas pressure and, in the low-torr pressure range, with changes in the composition of the buffer gas. A model for the EC reactions of dibrominated compounds is proposed in which the branching ratio for the two pathways leading to Br−-2 and Br− depends on the extend of collisional quenching of vibrationally excited reaction intermediates by the buffer gas molecules.

Cluster-Assisted Decomposition Reactions of the Molecular Anions of SF6 and C7F14

The cluster ions formed by the attachment of dimethylsulfoxide (DMSO) and methanol to the molecular negative ions of C7F14 and SF6 have been studied by a pulsed e-beam high pressure mass spectrometer (PHPMS) and by an atmospheric pressure ionization mass spectrometer (APIMS). The free energy change (ΔG°) for the clustering equilibria reaction, M−+S⇌M−S, at 35°C are found to be −7.7 and −7.2 kcal/mol for S = DMSO and M−=C7F14− and SF6− respectively, and −6.4 and −4.5 kcal/mol for S = methanol and M−=C7F14− and SF6− respectively. While the cluster ions formed by DMSO are found to be stable against side reactions, those formed by methanol undergo decomposition processes in which the central core ion is fragmented. At 35 °C, the rate law for the decomposition of the SF6−(CH30H)1 ion is second-order, involving the M−(CH30H)1 cluster ion and another methanol molecule. While the C7F14−(CH30H)1 ion also decomposes through this second-order process, a competing unimolecular mechanism is also operative at 35°C. With increases in the PHPMS ion source temperature to 150°C, the unimolecular decomposition process becomes progressively dominant for both of the M−(CH30H)1 cluster ions of C7F14 and SF6. Methanol cluster ions of the type M−S2 are not observed under any of the conditions examined here. When methanol or water partial pressures of a few torr or higher are present in the buffer gas of the APIMS ion source, the decomposition reactions are very fast and only the fragment ions produced by these reactions are observed in the electron-capture (EC)-APIMS spectra of C7F14 and SF6 . Also, in the methanol-containing APIMS ion source, the course of the SF6− decomposition reaction is altered so that fragment ions of the type F−(S)n dominate the EC-APIMS spectrum of SF6 at all ion source temperatures. For C7F14, fragment ions of the type F−(S)n become dominant at lower ion source temperatures. These previously unknown reactions are expected to be important in the analysis of perfluorinated compounds by mass spectrometric methods that utilize ionization by electron capture or negative chemical ionization. The nature of the fragment ions produced in these cluster-assisted reactions may also provide a new source of information concerning the structures of the molecular negative ions of SF6 and C7F14.

Electron attachment to PSCl3

Electron attachment to PSCl3 was studied in 133-Pa pressure of helium gas at temperatures from 298-550 K. Measurements of rate constants and branching fractions were made in a flowing-afterglow Langmuir-probe (FALP) apparatus. These experiments yielded an electron attachment rate constant of 5.1 x 10(-8) cm3 s(-1) that was found not to change significantly in the 298-550 K temperature range. This rate constant represents an attachment efficiency of about 14%. Attachment in 133 Pa of He gas yielded only the dissociative ion products PSCl2- and Cl-. The FALP data suggest that there is an activation energy of about 17 meV for production of PSCl2-. Attachment to PSCl3 was also studied at high pressure (9-93 kPa) of N2 in an ion mobility mass spectrometer, at 298 K. In contrast to the low-pressure data, the parent anion product channel (PSCl3-) was observed (along with the dissociative channels), and increased in importance with N2 pressure. Gaussian-3 (G3) calculations were carried out for PSCl3 and PSCl2 neutrals and anions to aid in interpretation of the experimental results. The calculations indicate that the electron affinity EA(PSCl2) is slightly smaller than EA(Cl), which may account for the observed branching fractions for PSCl2- and Cl- in the low-pressure experiments. A natural population analysis was performed to obtain the charges associated with each atom in the molecules in order to estimate how the attached electron is distributed. Comparison is made between the present study of electron attachment to PSCl3 and our earlier work on attachment to POCl3, and G3 calculations are reported here for neutral and anionic POCl2 and POCl3. In contrast to PSCl2, the calculations imply that EA(POCl2) is slightly greater than EA(Cl). For both PSCl3 and POCl3, the calculations show that the dissociative electron attachment process is close to thermoneutral.

Effect of pressure and temperature on the competition between nondissociative and dissociative electron attachment to POCl3

Branching between the nondissociative, POCl3−, and dissociative, POCl2−, products of the POCl3 electron attachment reaction has been examined as a function of buffer gas pressure and temperature. A strong positive pressure dependence is observed for the nondissociative channel, where at 303 K, the %POCl3− increases from 31% at 1 Torr to 94% at 675 Torr total pressure. Conversely, the dissociative channel displays a strong positive temperature dependence. The effect of pressure and temperature on the relative amounts of POCl3− and POCl2− observed is discussed in terms of the competition between the collisional stabilization and dissociation rates of the POCl3−* excited intermediate. The decomposition of POCl3−* is modeled with Rice–Ramsperger–Kassel decomposition kinetics weighted by the vibrational energy distribution of POCl3 neutrals. This model provides an excellent simulation of the experimental pressure and temperature dependencies of the electron attachment process.

Effect of buffer gas alterations on the thermal electron attachment and detachment reactions of azulene by pulsed high pressure mass spectrometry

Abstract A principal motivation for the present study is to determine the ion source conditions required for achievement of the high pressure limit (HPL) of kinetic behavior for the resonance electron capture (REC) reaction of azulene (Az), Az + e → Az−. This goal is accomplished here by measuring rate constants for the reverse process, thermal electron detachment by molecular anions of azulene, Az− → Az + e, by pulsed high pressure mass spectrometry by using a variety of buffer gases, methane, argon, nitrogen, and helium, over a range of pressures, from 1 to 6 Torr, over a range of temperatures, from 150 to 200 °C. From these measurements, it is shown that the ion source conditions commonly used in electron capture mass spectrometry for the trace analysis of REC-active molecules would not be sufficient for achievement of the HPL of the REC reaction of azulene and, therefore, would likely result in significantly reduced sensitivity to this compound. The problem highlighted here for the case of azulene is undoubtedly shared by many other REC-active compounds. The resolution of this problem is expected to require accommodation of several relevant factors shown here to be important in the case of azulene, including the choice of buffer gas, pressure, and ion source temperature.

Concentration enrichment in the ion source of a pulsed electron beam high pressure mass spectrometer

Abstract The magnitude of concentration enrichment of methyl iodide in methane, helium, and argon buffer gases within the ion source of a pulsed e-beam high pressure mass spectrometer (PHPMS) is characterized here by a theoretical model and by kinetic measurements of the gas phase ion/molecule reaction, F− + CH3I → CH3F + I−, at 150°C. A relatively simple PHPMS ion source is used in which the magnitude of enrichment is related only to the ventilation of individual molecules out of the ion source by passage through its ion exit and electron entrance slits. It is shown that significant enrichment does occur under typical operating conditions of this source and that the magnitude of enrichment is strongly dependent on the choice of ion source pressure. These observations are explained in terms of the flow conditions thought to exist in the ion source. Recommendations for improving the accuracy of future kinetic and equilibrium measurements by the PHPMS are provided.

Competition between nondissociative and dissociative electron attachment to halogenated cyclic alkenes in the gas phase

Abstract A flowing-afterglow Langmuir probe apparatus with mass spectral analysis was used to measure rate constants, identify reaction products, and measure branching fractions for electron attachment to 1,2-dichlorohexafluorocyclopentene ( c -C 5 F 6 Cl 2 ) and 1,2-dichlorooctafluorocyclohexene ( c -C 6 F 8 Cl 2 ) over the temperature range 295–555 K and buffer gas density range of 1.3–2.1 × 10 16 cm −3 . Electron attachment to both of these compounds is efficient over this temperature range and the rate coefficient for attachment is relatively independent of temperature. At 295 K the electron attachment rate coefficient for c -C 5 F 6 Cl 2 is 2.5 ± 0.6 × 10 −7 cm 3 s −1 and for c -C 6 F 8 Cl 2 is 3.5 ± 0.9 × 10 −7 cm 3 s −1 . At 300 K, electron attachment to both neutral reactants is predominantly nondissociative. At higher temperatures, a dissociative reaction channel forming Cl − is observed for both reaction systems. Significant branching fractions for Cl − (≥0.05) are observed at temperatures greater than 425 K for c -C 5 F 6 Cl 2 and at temperatures greater than 375 K for c -C 6 F 8 Cl 2 . Over the temperature range of 300–550 K the branching fractions for formation of the nondissociative and dissociative products were not dependent on buffer gas density in the range of 1.3–2.1 × 10 16 cm −3 .

Pulsed high pressure mass spectrometry with near-viscous flow ion sampling

Abstract Rate constants for the reaction, F − + CH 3 I → CH 3 F + I − , are determined at 150°C by pulsed electron beam high pressure mass spectrometry (PHPMS) under conditions where the ions within the source are sampled by near-viscous flow through ion-exit apertures of relatively large diameter. It is shown that the rate constants thereby determined are of high accuracy and are not subject to a common source of measurement error that is intrinsic to PHPMS measurements made under the molecular or near-molecular flow conditions that are generally used. The success of ion sampling by near-viscous flow demonstrated here also suggests that the PHPMS technique can be successfully extended to the study of ion/molecule reaction rates at significantly greater buffer gas pressures than previously considered feasible within the conventional view of the method.

Lewis acid–base interactions of SiF4 with molecular anions formed by electron capture reactions

Abstract Lewis acid–base interactions between SiF 4 and a wide range of molecular negative ions are reported here for the first time. The molecular anions include those formed by simple electron attachment to p -benzoquinone, benzophenone, nitrobenzene, and 21 substituted nitrobenzenes and also include the o - and p -nitrophenoxy anions. From measurements performed by pulsed electron-beam high pressure mass spectrometry, equilibrium constants and free energies for the association reactions, M − + SiF 4 ⇌ M − (SiF 4 ), at 150 °C are reported for each of the molecular anions, M − . It is shown that the strengths of these Lewis acid–base interactions of SiF 4 are much greater than ion–dipole interactions previously reported between these molecular anions and several common solvent molecules of relatively high dipole moment, including methanol, acetonitrile, dimethylformamide, and dimethlysulfoxide. The strengths of the Lewis acid–base interactions of SiF 4 with molecular anions show a strong inverse dependence on the electron affinity of the parent molecule and on the availability of a specific Lewis base site on the molecular anion that can be closely approached by the central Si atom of SiF 4 . It is also shown that strong interactions between SiF 4 and the molecular anions derived from compounds of very low electron affinity can be gainfully used for the trace detection of such compounds by electron capture mass spectrometry.