Electron Attachment Rate Constants Research Articles

Variable Electron and Neutral Density Attachment Mass Spectrometry: Temperature-Dependent Kinetics of Electron Attachment to PSCl3 and PSCl2 and Mutual Neutralization of PSCl2− and PSCl− with Ar+

We describe the VENDAMS (variable electron and neutral density attachment mass spectrometry) technique to measure the rate constants of various processes occurring as primary, secondary, and higher order chemistry in a flowing afterglow at high charge densities over a temperature range of 300 to 550 K. In particular, we report measurements of rate constants of ion-ion mutual neutralization and electron attachment to radical species, processes which have proven difficult to study through other means. The product negative ion abundances from the addition of PSCl(3) to an Ar(+)/e(-) plasma have been measured as a function of initial electron densities between 1 × 10(8) and 4 × 10(10) cm(-3). Data at lower electron densities yield branching ratios of the primary electron attachment to PSCl(3); determination of the reactions and rate constants occurring at low electron densities then allows for determination of the greater number of reactions and rate constants contributing at higher electron densities. Reaction rate constants and branching ratios of electron attachment to PSCl(2) are reported; this is the first measurement of electron attachment to a radical as a function of temperature. The data show an unusual negative temperature dependence; however, a zero or even slightly positive dependence is within the uncertainty. Measured electron attachment rate constants are 1.4 × 10(-7), 1.1 × 10(-7), and 9.1 × 10(-8) ± 40% cm(3) s(-1) at 300, 400, and 550 K, respectively; the dominant product channel is PSCl + Cl(-) (95, 87, and 77% at 300, 400, and 550 K), and the minor channel is PSCl(-) + Cl. Ion-ion mutual neutralization rate constants of both PSCl(-) and PSCl(2)(-) with Ar(+) are reported over the investigated temperature range; rate constants at 300 K are 4.9 × 10(-8) ± 20% cm(3) s(-1) and 4.5 × 10(-8) ± 15% cm(3) s(-1) and show temperature dependences of T(-0.5±0.3) and T(-0.9±0.3), respectively.

Electron attachment to halomethanes at high temperature: CH2Cl2, CF2Cl2, CH3Cl, and CF3Cl attachment rate constants up to 1100 K

We have used a high-temperature flowing-afterglow Langmuir-probe apparatus to measure rate constants for electron attachment to halomethanes which attach electrons very inefficiently at room temperature, yielding Cl(-) ion product. We studied CH(2)Cl(2) (495-973 K), CF(2)Cl(2) (291-1105 K), and CF(3)Cl (524-1004 K) and include our recent measurement for CH(3)Cl (700-1100 K) in the discussion of the electron attachment results. The measured attachment rate constants show Arrhenius behavior in the temperature ranges examined, from which estimates of rate constants at 300 K may be made: CH(2)Cl(2) (1.8x10(-13) cm(3) s(-1)), CH(3)Cl (1.1x10(-17) cm(3) s(-1)), and CF(3)Cl (4.2x10(-14) cm(3) s(-1)), all of which are difficult to measure directly. In the case of CF(2)Cl(2), the room temperature rate constant was sufficiently large to be measured (1.6x10(-9) cm(3) s(-1)). The Arrhenius plots yield activation energies for the attachment reactions: 390+/-50 meV (CH(2)Cl(2)), 124+/-20 meV (CF(2)Cl(2)), 670+/-70 meV (CH(3)Cl), and 406+/-50 meV (CF(3)Cl). Comparisons are made with existing data where available. G3 calculations were carried out to obtain reaction energetics. They show that the parent anions of CH(2)Cl(2) CF(2)Cl(2), CH(3)Cl, and CF(3)Cl are stable, though CH(3)Cl(-) exists only as an electrostatically bound complex.

Quasifree electron attachment to carcinogens

The quasifree electron attachment rate constants kk of recognized carcinogens and noncarcinogens have been measured in nonpolar liquids by a pulse-conductivity technique. In our initial study of the kk in cyclohexane, we reported that the kk of 37 of 42 carcinogens are diffusion controlled, whereas the kk of 30 of 34 noncarcinogens are less than diffusion controlled [Cancer Biochem. Biophys. 5, 103 (1981)]. Since kk is strongly dependent upon the electron mobility uu values of kk of the same carcinogens and noncarcinogens were measured in liquid isooctane [uu (isooctane) = 5.2 as compared to uu (cyclohexane) = 0.22 cm2/V · s at 21°C] to determine the effect of the solvent on the kk-carcinogenicity correlation. In isooctane, the kk of 29 of 35 carcinogens tested are diffusion controlled, and 23 of 27 noncarcinogens are less than diffusion controlled. These results yield a sensitivity and a specificity, which are two criteria used to evaluate screening tests, of 83 and 86%, respectively. In cyclohexane the sensitivity and specificity are both 88%. These results will be compared with the sensitivity and specificity of the Ames test for the same chemicals, and implications of kk-carcinogenicity correlation to designing a carcinogen prescreen test that is based on kk will be discussed.

Electron attachment toCl2from 300 to 1100 K: Experiment and theory

Rate constants for dissociative electron attachment to ${\text{Cl}}_{2}$ have been measured from 300 to 1100 K in a high-temperature flowing-afterglow Langmuir-probe apparatus. $R$-matrix calculations have been carried out which compare well with the present measurements with possible deviation at the highest temperatures. The attachment rate constants do not show Arrhenius behavior. The temperature dependence of the calculated rate constants for successive vibrational levels provides insight as to this behavior. While the lowest vibrational level of ${\text{Cl}}_{2}$ dominates attachment at low temperatures, the rate constant is not flat with temperature because of the $p$-wave character of the attachment process. The non-Arrhenius behavior is due to a conflict between the increase in attachment cross section with vibrational level (temperature) and the decline in the cross section with electron energy above 50 meV.

A new instrument for thermal electron attachment at high temperature: NF3 and CH3Cl attachment rate constants up to 1100 K

A new high temperature flowing afterglow Langmuir probe (HT-FALP) apparatus is described. A movable Langmuir probe and a four-needle reactant gas inlet were fitted to an existing high temperature flowing afterglow apparatus. The instrument is suitable for study of electron attachment from 300-1200 K, the upper limit set to avoid softening of the quartz flow tube. We present results for two reactions over extended ranges: NF(3) (300-900 K) and CH(3)Cl (600-1100 K). Electron attachment rate constants for NF(3) had been measured earlier using our conventional FALP apparatus. Those measurements were repeated with the FALP and then extended to 900 K with the HT-FALP. CH(3)Cl attaches electrons too weakly to study with the low temperature FALP but reaches a value of approximately 10(-9) cm(3) s(-1) at 1100 K. F(-) is produced in NF(3) attachment at all temperatures and Cl(-) in CH(3)Cl attachment, as determined by a quadrupole mass spectrometer at the end of the flow tube. Future modifications to increase the plasma density should allow study of electron-ion recombination at high temperatures.

Electron attachment to SF6 under well defined conditions: comparison of statistical modeling results to experiments

Experiments were carried out using a flowing-afterglow Langmuir-probe apparatus to measure rate constants for electron attachment to SF6 and thermal detachment from SF6−. In a recent series of papers, these results were combined with new and existing data on nondissociative and dissociative attachment to SF6 and compared to statistical modeling of the various processes involved in the stabilization of the ionic products of attachment. This paper gives a summary of those findings. The major conclusions are: (a) only the ground electronic state of SF6− needs to be invoked to explain available data; (b) the electron affinity of SF6 is higher than previously thought, namely, EA(SF6) = 1.20 (± 0.05) eV; (c) the endothermicity of the dissociative electron attachment reaction that yields SF5− is 0.41 eV (± 0.05) eV at 0 K; (d) combining these two numbers gives the bond energy D0o(F—SF5−) = 1.61 (± 0.05) eV.

Thermal electron capture by some halopropanes

Thermal electron attachment rate constants for CF 3CHClCH 3, CF 2ClCFClCF 3 and CBrF 2CH 2CH 2Br have been measured with electron swarm method. Corresponding rate constants are equal to 7.6×10 −11, 5.5×10 −9 and 1.5×10 −8 cm 3 molecule −1 s −1, respectively. The dissociative electron attachment (DEA) spectra for nine haloalkanes have been determined using negative ion mass spectrometry. The correlation between rate constants, position of the DEA peaks and vertical attachment energy (VAE) available in literature has been demonstrated.

Using Gas-Phase Molecular Descriptors to Predict Dechlorination Rates of Chloroalkanes by Zerovalent Iron

Reductive dehalogenation of chlorinated compounds is the most important process occurring within the zerovalent iron (Fe0) barrier. The relative reaction rates of individual halocarbons with Fe0 can vary considerably. This variability has been the stimulus for using various chemical descriptors for a priori predictions of transformation rates via linear free-energy relationships (LFERs). Our objective was to determine the efficacy of four molecular descriptors to describe the transformation rates of three chloromethanes, three chloroethanes, and six chloropropanes by Fe0. This was accomplished by generating an internally consistent set of rate constants under controlled environmental conditions (16 degrees C, anaerobic) and regressing the surface-area normalized rate constants (k(SA)) against (i) energy of the lowest unoccupied molecular orbital (E(LUMO)); (ii) vertical attachment energies (VAE); (iii) thermal electron attachment rate constants; and (iv) the molar response from a commercial electron capture detector (ECD). Results showed good correlations between k(SA)'s and all four descriptors (r2: 0.72-1.0), but a separate trend line was required for the chloromethanes and the chloro- ethanes/ propanes. Given the availability and ease with which ECD response can be obtained, this physical measurement may provide a practical means of determining relative rates of reactivity of various halocarbons in permeable reactive iron barriers.

Thermal electron capture by some chlorobromopropanes

Thermal electron attachment rate constants for CH2ClCHBrCH3 and CH2ClCH2CH2Br have been measured using electron swarm method. Corresponding rate constants are equal to 3.5×10-10 and 2.5×10-10 cm3 molec-1 s-1, respectively. Parallely, negative ion mass spectra of these compounds as well as CH2FCH2Br, CH2ClCH2Br, CH2BrCH2Br and CF3CHClBr has been measured with negative ion mass spectrometry method. The rate constants have been compared with the negative ion mass spectra.

Rates of Electron Attachment to Aluminum Oxides in Air-Xenon Mixtures

This paper reports results from measurements of the electron concentration in wakes behind aluminum spherical models flying at velocities of 2–5 km/sec in air-xenon mixtures at a total pressure in the ballistic track of 20–100 torr. The heating of the models moving in the ballistic track and aluminum ablation from their surfaces were calculated. The rate constant of electron attachment to microparticles is found, and it is shown that it depends on the fraction of xenon in the mixture. The dependence of the coefficient of electron accommodation to the microparticle surface on the xenon concentration in the mixture is obtained and explained from a physical point of view.

Electron attachment and detachment: Electron affinities of isomers of trifluoromethylbenzonitrile

Rate constants for electron attachment to the three isomers of trifluoromethylbenzonitrile [(CF(3))(CN)C(6)H(4), or TFMBN] were measured over the temperature range of 303-463 K in a 133-Pa He buffer gas, using a flowing-afterglow Langmuir-probe apparatus. At 303 K, the measured attachment rate constants are 9.0 x 10(-8) (o-TFMBN), 5.5 x 10(-8) (m-TFMBN), and 8.9 x 10(-8) cm(3) s(-1) (p-TFMBN), estimated accurate to +/-25%. The attachment process formed only the parent anion in all three cases. Thermal electron detachment was observed for all three anion isomers, and rate constants for this reverse process were also measured. From the attachment and detachment results, the electron affinities of the three isomers of TFMBN were determined to be 0.70(o-TFMBN), 0.67(m-TFMBN), and 0.83 eV (p-TFMBN), all +/-0.05 eV. G3(MP2) [Gaussian-3 calculations with reduced Møller-Plesset orders (MP2)] calculations were carried out for the neutrals and anions. Electron affinities derived from these calculations are in good agreement with the experimental values.

Electron attachment to C 60 in nonpolar solvents

Rate constants for electron attachment to C 60 were measured by pulse radiolysis in several nonpolar solvents. The rates for this highly exoergic reaction are fast, comparable to the rates found for the most efficient acceptors, such as SF 6. The linear dependence of the rate on electron mobility for liquids in which the mobility is low, indicates the reaction is diffusion limited, but the rate drops below the diffusion rate for liquids in which the mobility is high.

Thermal Electron Attachment to Chloroalkanes and Herschbach Ionic Morse Potentials for Carbon Tetrachloride (CCl4)

New rate constants for thermal electron attachment to chloroalkanes were measured in a recent swarm ionization cell study (Barszczewska, W.; Kopyra, J.; Wnorowska, J; Szamrej, I J. Phys. Chem. A 2003, 107, 11427). Electron affinities of haloakanes were calculated and used to construct one ionic potential energy curve to model the electron attachment process. Six of the 11 summary items presented should be revisited based on the following results and published complementary studies. The adiabatic electron affinity of CCl4 is calculated using the MNDO CURES-EC approach to be 2.2 eV in agreement with experiment. The Feynman, Lesk, Herschbach, Wentworth hypotheses place a lower limit of zero on the long-range electron affinity, the adiabatic electron affinity, and the anion dissociation energy in a given bond dimension. Eight ionic Morse potential energy curves for CCl4(−) are calculated using the Herschbach classifications and experimental or predicted metrics. The antibonding, a, and bonding, b, curves are ...

Electron attachment and detachment and the electron affinity of cyclo-C4F8.

New measurements have been made of rate constants for electron attachment to c-C(4)F(8) (octafluorocyclobutane) and thermal electron detachment from the parent anion, c-C(4)F(8) (-), over the temperature range 298-400 K in 133 Pa of He gas in a flowing-afterglow Langmuir-probe apparatus. From these data the electron affinity for c-C(4)F(8) was determined, EA(c-C(4)F(8))=0.63+/-0.05 eV. The motivation was to resolve a discrepancy between our earlier EA estimate and a higher value (EA=1.05+/-0.10 eV) reported from a recent experiment of Hiraoka et al. [J. Chem. Phys. 116, 7574 (2002)]. The electron attachment rate constant is 9.3+/-3.0x10(-9) cm(3) s(-1) at 298 K. The electron detachment rate constant is negligible at room temperature but climbs to 1945+/-680 s(-1) at 400 K. G3(MP2) calculations were carried out for the neutral (D(2d), (1)A(1)) and anion (D(4h), (2)A(2u)) and yielded EA(c-C(4)F(8) (-))=0.595 eV. Bond energies were also calculated for loss of F from c-C(4)F(8) and loss of F or F(-) from c-C(4)F(8) (-). From these, dissociative electron attachment is found to be endothermic by at least 1.55 eV.

Electron attachment to and anion reactions with SF5Cl: electron-swarm and selected ion flow tube studies

The first investigation of non-thermal electron attachment to SF5Cl in a swarm environment is reported here. An electron swarm mass spectrometer, the drift tube of which operates in an atmospheric pressure of N2 or Ar, has been used to obtain the data. Experimental attachment rate coefficients, ka, and the anion products resulting from electron attachment are reported. ka is observed to decrease as the mean electron energy increases. The estimated thermal (300K) electron attachment rate constant is kth≈(1.3±0.4)×10−8cm3s−1, which is significantly less than the expected s-wave capture rate. The mass spectra obtained show a number of anions resulting from electron attachment, namely F2−, SF4−, SF5− and SF5Cl−, with SF5− and SF5Cl− being the only anions observed as the mean electron energy approaches zero. Anion reactions with SF5Cl made it difficult to accurately assign branching ratios, and therefore only a qualitative description of the results will be presented here. Anion reactions with SF5Cl are of general interest and make interesting comparisons with anion reactions involving SF6. In this paper, we also present the results of a selected ion flow tube study investigating the 300K reactions of SF5Cl with a number of anions; O2−, SF6−, OH−, CF3−, F−, Cl− and SF5−.

Electron attachment and detachment: C6F6

Electron attachment to C6F6 is especially interesting because of the large change in symmetry between the neutral (D6h) and anion (C2v). We have made measurements of rate constants for electron attachment to C6F6 and thermal electron detachment from the parent anion, C6F6−, over the temperature range 297–400K, in 133Pa of He gas. A flowing-afterglow Langmuir probe (FALP) apparatus was used for this work. At 298K, the electron attachment rate constant is ka=8.6±3.0×10−8cm3s−1, and the detachment rate constant kd is approximately 35s−1. As the temperature increases kd increases rapidly, to about 3000s−1 at 400K. The attachment/detachment equilibrium implies that the electron affinity of C6F6 is 0.53±0.05eV. Density functional calculations were carried out in order to obtain thermal quantities needed to convert the equilibrium constant ka/kd into EA(C6F6). G3(MP2) calculations yielded an electron affinity of 0.454eV. The fluoride affinity of C6F6 was calculated to be 1.26eV at 298K using this same method. We expect the G3(MP2) results to be good within 0.1eV.

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.

Low-Energy Electron Attachment by Chloroalkanes

Thermal electron attachment rate constants for 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, 1- and 2-chloropropanes, and 1,1-, 1,2-, 2,2-, and 1,3-dichloropropanes have been measured using an electron swarm method. It has been found that all the investigated compounds attach electrons only in a two-body process. Corresponding rate constants are equal to 1.4 × 10-10, 3.2 × 10-8, 2.7 × 10-13, 3.8 × 10-12, 5.7 × 10-11, 8.1 × 10-12, 6.3 × 10-12, and 1.2 × 10-11 cm3 molecule-1 s-1, respectively. The dependence of the electron capture rate constants on the electronic polarizability of the accepting center of the molecule and the vertical attachment energy has been demonstrated.

Electron attachment and detachment: cyclo-C 4F 4Cl 2

Electron attachment to 1,2-dichlorotetrafluorocyclobutene (c-C 4F 4Cl 2) and electron detachment from the parent anion (c-C 4F 4Cl 2 −) have been studied over a temperature range from 295 to 556 K in a flowing-afterglow Langmuir-probe (FALP) apparatus. Electron attachment to c-C 4F 4Cl 2 over this temperature range yields only the parent anion in a helium buffer gas at 80–160 Pa pressure. The electron attachment rate constant was found to be independent of temperature over the range studied, at a value of (2.4±0.8)×10 −7 cm 3 s −1, that is, essentially upon every collision. The rate constant for electron detachment from c-C 4F 4Cl 2 − is strongly temperature dependent, being imperceptible at room temperature and climbing to a value of 3100±1200 s −1 at 511 K. The equilibrium constant obtained from these data yields the electron affinity (EA) of c-C 4F 4Cl 2 of 0.87±0.08 eV. G3(MP2) and density functional theory (DFT) calculations were carried out for the C 4F 4Cl 2 neutral, anion, and fragments in order to aid in interpreting the experiments. Both the G3(MP2) and DFT calculations yield values for EA(c-C 4F 4Cl 2), 0.79 and 0.93 eV, respectively, that are in good agreement with the experimental value. The calculations also show that there are no open exothermic dissociative electron attachment channels for c-C 4F 4Cl 2. Further, we present structures, atomic charges, dipole moments, heat capacities, and polarizabilities for these molecules. The results are contrasted with previous results on 1,2-dichlorooctafluorocyclohexene (c-C 6F 8Cl 2) and 1,2-dichlorohexafluorocyclopentene (c-C 5F 6Cl 2), both of which possess larger EAs.

Electron Interactions With BCl3

In this paper we review and assess the cross sections for collisions of low-energy electrons with boron trichloride (BCl3). The only available experimental cross section data are for partial and total ionization and electron attachment, and the electron attachment cross sections are uncertain. Calculated values are available for the total elastic, differential elastic, and momentum transfer cross sections, and derived cross sections have been published for vibrational excitation and dissociation. Other than some rather uncertain data on electron attachment rate constants and some measurements of electron drift velocities in BCl3/Ar and BCl3/He mixtures, there are no measurements of the electron attachment, ionization, or transport coefficients for this gas. Analysis of the experimental data on the electron affinity, electron attachment, and electron scattering, enabled identification of negative ion states of BCl3 at about −0.3, 1.0, 2.8, 5.2, 7.6, and 9.0 eV. Because the existing electron collision data are few and uncertain, relevant data are provided for photon impact on BCl3..