Jane M. Van Doren

A selected ion flow tube study of the reactions of H3O+, NO+, and O2+ with saturated and unsaturated aldehydes and subsequent hydration of the product ions

Abstract We have carried out a selected ion flow tube (SIFT) study of the reactions of H 3 O + , NO + , and O 2 + ions with several saturated and unsaturated aldehydes. This study is mainly directed toward providing the essential data for a projected SIFT mass spectrometry (SIFTMS) study of the volatile emissions from cooked meats, which always include aldehydes. Thus, it is necessary to know the rate coefficients and the product ions of the reactions of the above-mentioned ions, used as the precursor ions for SIFTMS analyses, with the aldehydes, if proper identification and quantification of the emitted species are to be achieved. The results of this study show that the reactions of H 3 O + with the aldehydes, M, result in the protonated molecules MH + and for the saturated aldehydes also in (M - OH) + ions resulting from the loss of a H 2 O molecule from the nascent MH + ion. The NO + reactions invariably proceed via the process of hydride ion, H − , transfer producing (M - H) + ions, but parallel minor association product ions NO + · M are observed for some of the unsaturated aldehyde reactions. The O 2 + reactions proceed by way of charge transfer producing nascent M + ions that partially dissociate producing fragment ions. Because water vapour is invariably present in real samples analysed by SIFTMS, the current experiments were also carried out following the introduction of humid laboratory air into the helium carrier gas of the SIFT. Thus, the reactions of the product ions that form hydrates were also studied as a prelude to future SIFTMS studies of the (humid) emissions from cooked meats.

Ion chemistry of ClONO2 involving NO3 − core ions: A detection scheme for ClONO2 in the atmosphere

Rate constants and product branching ratios for reactions involving atmospherically interesting ions and ClONO{sub 2} have been measured. H{sub 3}O{sup +} reacts rapidly with ClONO{sub 2}, but hydrates of H{sub 3}O{sup +} do not. This implies that ClONO{sub 2} does not play a central role in the positive ion chemistry of the atmosphere. CO{sub 3}{sup -} reacts with ClONO{sub 2} to form NO{sub 3}{sup -}. Both NO{sub 3}{sup -} (H{sub 2}O) react with ClONO{sub 2} to form NO{sub 3}{sup -} (ClONO{sub 2}). H{sub 2}O does not react with NO{sub 3}{sup -} (ClONO{sub 2}), which is essential to the proposed in situ measurement technique. NO{sub 3}{sup -} (ClONO{sub 2})does react with HNO{sub 3} and HCl, producing NO{sub 3}{sup -} (HNO{sub 3}) in both cases. The reaction of NO{sub 3}{sup -} (HCl) with ClONO{sub 2} also produces NO{sub 3}{sup -} (HNO{sub 3}). These latter two efficient reactions, which are discussed in detail in a separate publication, are analogous to the efficient neutral heterogeneous reaction of HCl with ClONO{sub 2} which is important in the chemistry of the Antarctic stratosphere. A detection scheme is presented for atmospheric ClONO{sub 2} based on the reactions studied and utilizing mass spectrometry. A ClONO{sub 2} detection limitmorexa0» of 10{sup 7} molecules cm{sup -3} is estimated based on the operating characteristics of current ion-molecule based mass spectrometric field instruments. 19 refs., 1 tab.«xa0less

Electron attachment to PCl3 and POCl3, 296–552 K

Rate constants for electron attachment to PCl3 and POCl3 have been measured over the temperature range 296–552 K in 135 Pa of helium gas, using a flowing-afterglow Langmuir-probe apparatus. Electron attachment to PCl3 is dissociative, producing only Cl− ions in this temperature range. The rate constant is 6.4±1.6×10−8u2009cm3u2009s−1 at 296 K and increases with temperature in a way that may be described by an activation energy of 43±10u2009meV. Electron attachment to POCl3 is a richer process in which a nondissociative channel (POCl3−) competes with two dissociative ones (POCl2− and Cl−). The rate constant for electron attachment to POCl3 is 1.8±0.4×10−7u2009cm3u2009s−1 at 296 K and is relatively temperature independent in our temperature range. POCl2− is the major product over the entire temperature range. Ab initio MP2 and MP4 calculations have been carried out on ground-state neutral and anionic PCln and POCln for n=1–3. The calculated adiabatic electron affinities agree with experimental estimates where available. The ca...

The formation and destruction of H3O

We report the first measurements of rate constants for formation and reaction of the hydrated‐hydride ion H3O−. We studied the Kleingeld–Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated‐hydride ion and carbon monoxide. The OD−+H2CO reaction is about 35% efficient at 298 K, with OD−/OH− exchange occurring in about half the reactions. H3O− was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6×10−12 cm3u2009s−1. We also studied a new reaction in which H3O− is formed: The association of OH− with H2 in a He carrier gas at low temperatures. The rate coefficient for this ternary reaction is 1×10−30 cm6u2009s−1 at 88 K. Rate coefficients and product branching fractions were determined for H3O− reactions with 19 neutral species at low temperatures (88–194 K) in an H2 carrier. The results of ion‐beam studies, negative‐ion photoelectron spectroscopy, and ion‐molecule react...

Experimental and theoretical investigation of electron attachment to SF5Cl

Thermal electron attachment to SF(5)Cl has been studied with the flowing afterglow Langmuir probe technique. The rate coefficient is moderate, 4.8(+/-1.2)x10(-8) cm(3) s(-1), and invariant with temperature over the temperature range of 300-550 K. The reaction is dissociative, forming mainly SF(5) (-)+Cl. Minor yields of Cl(-) and FCl(-) were also found. The yields of the minor channels increase slightly with temperature. Statistical unimolecular rate modeling is employed to elucidate the character of the dissociation pathways and to support the assumption that the dissociations involve the formation of metastable anionic SF(5)Cl(-).

Electron attachment to POCl3: Measurement and theoretical analysis of rate constants and branching ratios as a function of gas pressure and temperature, electron temperature, and electron energy

Two experimental techniques, electron swarm and electron beam, have been applied to the problem of electron attachment to POCl3, with results indicating that there is a competition between dissociation of the resonant POCl3-* state and collisional stabilization of the parent anion. In the electron beam experiment at zero electron energy, the fragment ion POCl2- is the dominant ion product of attachment (96%), under single-collision conditions. Small amounts (approximately 2% each) of POCl3- and Cl- were observed. POCl3- and POCl2- ion products were observed only at zero electron energy, but higher-energy resonances were recorded for POCl-, Cl-, and Cl2- ion products. In the electron swarm experiment, which was carried out in 0.4-7 Torr of He buffer gas, the parent anion branching ratio increased significantly with pressure and decreased with temperature. The electron attachment rate constant at 297 K was measured to be (2.5+/-0.6)x10(-7) cm3 s(-1), with ion products POCl2- (71%) and POCl3- (29%) in 1 Torr of He gas. The rate constant decreased as the electron temperature was increased above 1500 K. Theory is developed for (a) the unimolecular dissociation of the nascent POCl3-* and (b) a stepladder collisional stabilization mechanism using the average energy transferred per collision as a parameter. These ideas were then used to model the experimental data. The modeling showed that D0 o(Cl-POCl2-) and EA(POCl3) must be the same within +/-0.03 eV.

Observation of dihalide elimination upon electron attachment to oxalyl chloride and oxalyl bromide, 300-550 K.

Rate coefficients have been measured for electron attachment to oxalyl chloride [ClC(O)C(O)Cl] and oxalyl bromide [BrC(O)C(O)Br] in He gas at 133 Pa pressure over the temperature range of 300-550 K. With oxalyl chloride, the major ion product of attachment is Cl2(-) at all temperatures (66% at 300 K); its importance increases slightly as temperature increases. Two other product ions formed are Cl- (18% at 300 K) and the phosgene anion CCl2O- (16% at 300 K) and appear to arise from a common mechanism. With oxalyl bromide, the Br2(-) channel represents almost half of the ion product of attachment, independent of temperature. Br- accounts for the remainder. For oxalyl chloride, the attachment rate coefficient is small [(1.8 +/- 0.5) x 10(-8) cm3 s(-1) at 300 K], and increases with temperature. The attachment rate coefficient for oxalyl bromide [(1.3 +/- 0.4) x 10(-7) cm3 s(-1) at 300 K] is nearly collisional and increases only slightly with temperature. Stable parent anions C2Cl2O2(-) and C2Br2O2(-) and adduct anions Cl- (C2Cl2O2) and Br- (C2Br3O2) were observed but are not primary attachment products. G2 and G3 theories were applied to determine geometries of products and energetics of the electron attachment and ion-molecule reactions studied. Electron attachment to both oxalyl halide molecules leads to a shorter C-C bond and longer C-Cl bond in the anions formed. Trans and gauche conformers of the neutral and anionic oxalyl halide species have similar energies and are more stable than the cis conformer, which lies 100-200 meV higher in energy. For C2Cl2O2, C2Cl2O2(-), and C2Br2O2(-), the trans conformer is the most stable conformation. The calculations are ambiguous as to the oxalyl bromide geometry (trans or gauche), the result depending on the theoretical method and basis set. The cis conformers for C2Cl2O2 and C2Br2O2 are transition states. In contrast, the cis conformers of the anionic oxalyl halide molecules are stable, lying 131 meV above trans-C2Cl2O2(-) and 179 meV above trans-C2Br2O2(-). Chien et al. [J. Phys. Chem. A 103, 7918 (1999)] and Kim et al. [J. Chem. Phys. 122, 234313 (2005)] found that the potential energy surface for rotation about the C-C bond in C2Cl2O2 is extremely flat. Our computational data indicate that the analogous torsional surfaces for C2Br2O2, C2Cl2O2(-), and C2Br2O2(-) are similarly flat. The electron affinity of oxalyl chloride, oxalyl bromide, and phosgene were calculated to be 1.91 eV (G3), and 2.00 eV (G2), and 1.17 eV (G3), respectively.

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 .

CIONO−2: A strongly bound negative ion

The gas phase reaction of NO2− with ClONO2 has been investigated at 300 K using a selected ion flow tube. Reaction occurs efficiently, k=1.5 (±40%)×10−9 cm3u2009s−1. The major ionic products are NO3− (∼90%) and ClONO2− (∼10%). The cluster ion NO2− (ClONO2) is formed in <1% of the reactive collisions under our experimental conditions. The observation of ClONO2−, arising from charge transfer from NO2−, indicates that the electron affinity of ClONO2 is ≥2.1 eV. We have determined the apparent second‐order clustering rate coefficient for NO3−+ClONO2 to be 3.0 (±40%)×10−11 cm3u2009s−1 at 300 K in 0.5 Torr He.

G3 and density functional theory investigations of the structures and energies of SFnCl(n=0–5) and their anions

Computations of structures and total energies have been carried out for neutral and anionic SF(n)Cl (n=0-5), using the composite G3 method and density functional theory (DFT) at the B3LYP6-311+G(3df) level. The total energies and zero-point energies have been used here to derive electron affinities, bond dissociation energies, and heats of formation. In addition, vibrational frequencies, polarizabilities, and dipole moments are reported. Results are compared with earlier work for SF(m) (m=1-6) and demonstrate how the relatively weak S-Cl bond and reduced symmetry influence the properties of these molecules and anions. Comparisons are also made between G3 and DFT results for SF(n)Cl. Of particular interest is the alternating pattern of agreement between calculated electron affinity values with n. These calculations also provide critical energetic data needed to understand experimental measurements of electron attachment to SF(5)Cl [Van Doren et al., J. Chem. Phys. 128, 094309 (2008)] for which numerous ion products have been reported in the literature at low electron energy.