Anthony J. Midey

Temperature Dependences for the Reactions of O- and O2- with O2(a1Δg) from 200 to 700 K

Rate constants and product ion distributions for the O- and O2- reactions with O2(a 1Deltag) were measured as a function of temperature from 200 to 700 K. The measurements were made in a selected ion flow tube (SIFT) using a newly calibrated O2(a 1Deltag) emission detection scheme with a chemical singlet oxygen generator. The rate constant for the O2- reaction is approximately 7 x 10(-10) cm3 s-1 at all temperatures, approaching the Langevin collision rate constant. Electron detachment was the only product observed with O2-. The O- reaction shows a positive temperature dependence in the rate constant from 200 to 700 K. The product branching ratios show that almost all of the products at 200 K are electron detachment, with an increasing contribution from the slightly endothermic charge-transfer channel up to 700 K, accounting for 75% of the products at that temperature. The increase in the overall rate constant can be attributed to this increase in the contribution the endothermic channel. The charge-transfer product channel rate constant follows the Arrhenius form, and the detachment product channel rate constant is essentially independent of temperature with a value of approximately 6.1 x 10(-11) cm3 s-1.

Rate constants and branching ratios for the reactions of various positive ions with naphthalene from 300 to 1400 K

Abstract Temperature dependent rate constants and branching ratios are reported for the reactions of a variety of ions with recombination energies ranging from 9.26 eV (NO + ) to 21.56 eV (Ne + ) with naphthalene. For most ions, the measurements are made between 300 and 370 K in a variable temperature-selected ion flow tube (VT-SIFT). For the reactions of Ar + and N 2 + , data have also been measured between 300 and 500 K in the selected ion flow tube. In addition, for the reactions of O 2 + and N 2 + , data have been obtained between 500 and 1400 K in a high temperature flowing afterglow (HTFA). These are among the first determinations of branching ratios for ion–molecule reactions measured over 700 K. All reactions are found to proceed at the Langevin collision rate for all temperatures studied. The reactions proceed by nondissociative and dissociative charge transfer except for the reaction involving F + where some of the reactivity is attributed to chemical channels. No dissociative charge transfer is observed for ions with recombination energies equal to or less than that for N 2 + at room temperature. At higher temperatures in the N 2 + reaction and for ions with higher recombination energies (F + , Ne + ), naphthalene cation dissociation is observed, implying a threshold over 16 eV. This value is substantially higher than the known thermodynamic threshold because of kinetic shifts and quenching of the excited state of C 10 H 8 + by collisions with the helium buffer gas. The observed product thresholds and branching ratios are presented within the context of previous work and the implications for combustion chemistry are discussed.

Rate constants for the reaction of O2+ with NO from 300 to 1400 K

The rate constants for the charge transfer reaction of O2+ with NO have been measured from 300 to 1400 K using a high temperature flowing afterglow. The current results agree well with the previous flowing afterglow studies made at temperatures up to 900 K. The rate constants have no significant temperature dependence over the entire temperature range. The rate constants are in good agreement with the previous flow drift tube measurements at the same translational temperatures. Any dependence of the rate constant on internal energy is therefore small.

High-performance ion mobility spectrometry with direct electrospray ionization (ESI-HPIMS) for the detection of additives and contaminants in food.

High-performance ion mobility spectrometry (HPIMS) with an electrospray ionization (ESI) source detected a series of food contaminants and additive compounds identified as critical to monitoring the safety of food samples. These compounds included twelve phthalate plasticizers, legal and illegal food and cosmetic dyes, and artificial sweeteners that were all denoted as detection priorities. HPIMS separated and detected the range of compounds with a resolving power better than 60 in both positive and negative ion modes, comparable to the commonly used high-performance liquid chromatography (HPLC) methods, but with most acquisition times under a minute. The reduced mobilities, K0, have been determined, as have the linear response ranges for ESI-HPIMS, which are 1.5-2 orders of magnitude for concentrations down to sub-ng μL(-1) levels. At least one unique mobility peak was seen for two subsets of the phthalates grouped by the country where they were banned. Furthermore, ESI-HPIMS successfully detected low nanogram levels of a phthalate at up to 30 times lower concentration than international detection levels in both a cola matrix and a soy-based bubble tea beverage using only a simplified sample treatment. A newly developed direct ESI source (Directspray) was combined with HPIMS to detect food-grade dyes and industrial dye adulterants, as well as the sweeteners sodium saccharin and sodium cyclamate, with the same good performance as with the phthalates. However, the Directspray method eliminated sources of carryover and decreased the time between sample runs. Limits-of-detection (LOD) for the analyte standards were estimated to be sub-ng μL(-1) levels without extensive sample handling or preparation.

Absolute rate coefficients for the reactions of O2−+N(S3∕24) and O2−+O(P3) at 298 K in a selected-ion flow tube instrument

The absolute rate coefficients at 298 K for the reactions of O(2) (-) + N((4)S(3/2)) and O(2) (-) + O((3)P) have been determined in a selected-ion flow tube instrument. O atoms are generated by the quantitative titration of N atoms with NO, where the N atoms are produced by microwave discharge on N(2). The experimental procedure allows for the determination of rate constants for the reaction of the reactant ion with N((4)S(3/2)) and O((3)P). The rate coefficient for O(2) (-) + N is found to be 2.3x10(-10)+/-40% cm(3) molecule(-1) s(-1), a factor of 2 slower than previously determined. In addition, it was found that the reaction proceeds by two different reaction channels to give (1) NO(2)+e(-) and (2) O(-)+NO. The second channel was not reported in the previous study and accounts for ca. 35% of the reaction. An overall rate coefficient of 3.9 x 10(-10) cm(3) molecule(-1) s(-1) was determined for O(2) (-) + O, which is slightly faster than previously reported. Branching ratios for this reaction were determined to be <55%O(3) + e(-) and >45%O(-) + O(2).

RATE CONSTANTS FOR THE REACTION OF AR+ WITH O2 AND CO AS A FUNCTION OF TEMPERATURE FROM 300 TO 1400 K: DERIVATION OF ROTATIONAL AND VIBRATIONAL ENERGY EFFECTS

Rate constants for the charge-transfer reactions of Ar+ with O2 and CO have been measured in a high temperature flowing afterglow from 300 to 1400 K. Comparisons between our results and the previous flow drift tube studies of Dotan and Lindinger at 300 K illustrate the effects of internal excitation on the reactivity. The rate constants measured for both systems agree favorably with the drift tube results from 300 to 900 K. Rotational and translational energy decreases charge transfer equally, consistent with previous experiments, indicating a long-lived collision complex forms during the reaction. The flowing afterglow rate data deviate from the drift tube results above 900 K as a result of populating vibrationally excited states of the neutral reagents. Charge transfer from the thermally populated spin-orbit excited state of Ar+ with O2 and CO only slightly enhances the rate constants at 1400 K. Populating the v″>0 levels reduces the threshold for accessing excited state products, and the rate constants...

Determination of the CO3− bond strength via the resonant two-photon photodissociation threshold: Electronic and vibrational spectroscopy of CO3−∙Arn

We use a two-laser pump-probe technique coupled with messenger atom tagging to determine the bond energy of O(-) to CO(2) in the CO(3) (-) ion, a prevalent species in the upper atmosphere. In this technique, the argon-tagged ion is first electronically excited using a visible laser, then irradiated with a tunable near-infrared beam across the CO(2)...O(-) dissociation threshold while O(-) products are monitored. This method yields a bond energy of 2.79+/-0.05 eV, which is about 0.5 eV higher than previously reported. Combining this with the well-known heats of formation of O(-) and CO(2), 105.6 and -393.1 kJmol, respectively [Thermodynamic Properties of Individual Substances, edited by L. V. Gurvich, I. V. Veyts, and C. B. Alcock (Hemisphere, New York, 1989), Vol. 1 and CODATA Thermodynamic Tables, edited by O. Garvin, V. B. Parker, and J. H. J. White (Hemisphere, New York, 1987)], yields the CO(3) (-) heat of formation: DeltaH(0) (0)=-556.7+/-4.8 kJmol. The one-photon (i.e., linear) infrared and electronic spectra of CO(3) (-) are also presented and compared to those obtained previously. The one-photon electronic spectrum is nearly identical to two-photon spectra, implying that argon does not significantly perturb the ion or its symmetry. The infrared spectrum is drastically different than that obtained in an argon matrix, however, indicating that the ion is likely distorted in the matrix environment.

Kinetics of ion-molecule reactions with dimethyl methylphosphonate at 298 K for chemical ionization mass spectrometry detection of GX.

Kinetics studies of a variety of positive and negative ions reacting with the GX surrogate, dimethyl methylphosphonate (DMMP), were performed. All protonated species reacted rapidly, that is, at the collision limit. The protonated reactant ions created from neutrals with proton affinities (PAs) less than or equal to the PA for ammonia reacted exclusively by nondissociative proton transfer. Hydrated H(3)O(+) ions also reacted rapidly by proton transfer, with 25% of the products from the second hydrate, H(3)O(+)(H(2)O)(2), forming the hydrated form of protonated DMMP. Both methylamine and triethylamine reacted exclusively by clustering. NO(+) also clustered with DMMP at about 70% of the collision rate constant. O(+) and O(2)(+) formed a variety of products in reactions with DMMP, with O(2)(+) forming the nondissociative charge transfer product about 50% of the time. On the other hand, many negative ions were less reactive, particularly, SF(5)(-), SF(6)(-), CO(3)(-), and NO(3)(-). However, F(-), O(-), and O(2)(-) all reacted rapidly to generate m/z = 109 amu anions (PO(3)C(2)H(6)(-)). In addition, product ions with m/z = 122 amu from H(2)(+) loss to form H(2)O were the dominant ions produced in the O(-) reaction. NO(2)(-) underwent a slow association reaction with DMMP at 0.4 Torr. G3(MP2) calculations of the ion energetics properties of DMMP, sarin, and soman were also performed. The calculated ionization potentials, proton affinities, and fluoride affinities were consistent with the trends in the measured kinetics and product ion branching ratios. The experimental results coupled with the calculated ion energetics helped to predict which ion chemistry would be most useful for trace detection of the actual chemical agents.

Survey of the Reactivity of O2(a 1Δg) with Negative Ions

The reactivity of O(2)(a (1)Delta(g)) was studied with a series of anions, including (-)CH(2)CN, (-)CH(2)NO(2), (-)CH(2)C(O)H, CH(3)C(O)CH(2)(-), C(2)H(5)O(-), (CH(3))(2)CHO(-), CF(3)CH(2)O(-), CF(3)(-), HC(2)(-), HCCO(-), HC(O)O(-), CH(3)C(O)O(-), CH(3)OC(O)CH(2)(-), and HS(-). Reaction rate constants and product ion branching ratios were measured. All of the carbanions react through a common pathway to produce their major products. O(2)(a) adds across a bond at the site of the negative charge, resulting in the cleavage of this bond and the O=O bond. Oxyanions react through a hydride transfer to produce their major products. Proton transfer within these product ion-dipole complexes can occur, where the final branching ratios reflect the basicity of the resulting anions. Several of these anions (CF(3)(-), HC(2)(-), CH(3)OC(O)CH(2)(-)) were also found to undergo several sequential reactions within a single encounter. These three basic types of mechanisms are supported by calculations; a potential energy diagram for each type of reaction has been calculated. Additionally, six of these reactions had been qualitatively studied before; our results are in agreement with previous data.

Ion chemistry of VX surrogates and ion energetics properties of VX: new suggestions for VX chemical ionization mass spectrometry detection.

Room temperature rate constants and product ion branching ratios have been measured for the reactions of numerous positive and negative ions with VX chemical warfare agent surrogates representing the amine (triethylamine) and organophosphonate (diethyl methythiomethylphosphonate (DEMTMP)) portions of VX. The measurements have been supplemented by theoretical calculations of the proton affinity, fluoride affinity, and ionization potential of VX and the simulants. The results show that many proton transfer reactions are rapid and that the proton affinity of VX is near the top of the scale. Many proton transfer agents should detect VX selectively and sensitively in chemical ionization mass spectrometers. Charge transfer with NO(+) should also be sensitive and selective since the ionization potential of VX is small. The surrogate studies confirm these trends. Limits of detection for commercial and research grade CIMS instruments are estimated at 80 pptv and 5 ppqv, respectively.