Eric P. Grimsrud

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.

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.

Thermal electron detachment rate constants for azulene− at atmospheric pressure by the photodetachment-modulated electron capture detector

Abstract The direct measurement of rate constants kd for the thermal electron detachment of a polyatomic negative ion has been previously reported on only one occasion—for the azulene negative ion (Az−) by pulsed high-pressure mass spectrometry (PHPMS) at 4 torr total buffer gas pressure over the temperature range 140–200°C. We report here measurements of kd for Az− at a pressure of 2 atm over the temperature range 130–190°C, using a recently developed technique called the photodetachment-modulated electron capture detector (PDM-ECD). This method is based on the competition which exists in the PDM-ECD for destruction of Az− by three processes: thermal electron detachment, photodetachment and ion-ion recombination. The kd values for Az− reported here are found to be somewhat lower, about one-third as great at a given temperature, than those of the previous study. These differences in the two sets of kd measurements suggest the existence of a small but significant pressure dependence which causes kd to be slightly lowered by a 400-fold increase in buffer gas pressure. It is also recognized, however, that a systematic error in the treatment of PDM-ECD data may exist, owing to limitations of current knowledge concerning the rates of ion—ion recombination reactions at near-atmospheric pressures. The complete UV-vis photodetachment spectrum of Az− is also reported here and is shown to bear very strong resemblance to the condensed phase absorption spectrum of Az−.

Electron capture chemistry of CBrCl3 at atmospheric pressure by mass spectrometry and the photodetachment-modulated electron capture detector

Abstract Measurements are reported of the electron capture (EC) chemistry of CBrCl3 in a buffer gas at atmospheric pressure over the temperature range 30–250°C. These measurements are made with an atmospheric pressure ionization mass spectrometer (APIMS) and with a recently developed atmospheric pressure transducer called the photodetachment-modulated EC detector (PDM-ECD). It is shown that the initial reaction of CBrCl3 with thermal electrons proceeds by dissociative EC to form primarily Br− and some Cl−. At low temperatures the neutral products formed in the initial reaction, CCl3 and CBrCl2, are also shown to undergo rapid EC to form additional amounts of Cl−. At high temperatures, however, there is no evidence for EC by CCl3 and CBrCl2, because of what is thought to be a faster wall reaction which effectively destroys these radical species. It is concluded that the stoichiometry of the reaction of electrons with CBrCl3 and with CCl4 in an EC detector (ECD) will be between 1.0 and 2.0 and will be strongly dependent on temperature. These results are compared with previous studies of the EC chemistry of CBrCl3 by APIMS and by the flowing afterglow/Langmuir probe (FALP) technique.

Hydration of halide and bromine negative ions in a supersonic jet expansion

Abstract The water cluster ions, X − (H 2 O) n , where X − = F − , Cl − , Br − , or I − and n = 1−24, are observed within a supersonic free jet expansion of moist argon buffer gas by an atmospheric pressure ionization mass spectrometer. The relative intensities of these ions suggest the existence of special stabilities for certain large cluster ions containing 10 or more water molecules. The magnitudes of these special stabilities increase with decreasing size of the core ion so that they are most distinct for clusters of F − and not detectable for those of I − . Special stabilities are also observed for relatively large water clusters of bromine negative ion, Br − 2 (H 2 O) n .

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.

Gas phase SN2 nucleophilic displacement reactions at atmospheric pressure by the photodetachment-modulated electron capture detector

Abstract Rate constants for the reactions of Cl − ion with 20 different alkyl bromides (RBr) have been measured at 125°C in an atmospheric pressure buffer gas by the photodetachment-modulated electron capture detector (PDM-ECD). By this method, the relative concentrations of the reactant ion, Cl − , and the product ion, Br − , are spectroscopically measured in a steady-state reaction volume with various concentration of purified RBr introduced to the ion source by gas chromatography. Most of the reactions reported here have not been previously studied and, therefore, the present data set provides additional insight into substituent effects in gas phase S N 2 reactions.