Flow Tube Studies of Benzene Charge Transfer Reactions from 250 to 1400 K

Kinetics for the Reactions of Ar+, O2+, and NO+ with Isoprene (2-Methyl-1,3-butadiene) as a Function of Temperature (300-500 K)

Rate constants and product branching fractions were measured for reactions of Ar+, O2+, and NO+ with isoprene (2-methyl-1,3-butadiene C5H8) as a function of temperature. The rate constants are large (∼2 × 10-9 cm3 s-1) and increase with temperature, exceeding the ion-dipole/induced dipole capture rate. Adding a hard sphere term to the collision rate provides a more useful upper limit and predicts the positive temperature dependences. Previous kinetic energy-dependent rate constants show a similar trend. NO+ reacts only by non-dissociative charge transfer. The more energetic O2+ reaction has products formed through both non-dissociative and dissociative charge transfer, or possibly through an H atom transfer. The very energetic Ar+ has essentially only dissociative products; assumption of statistical behavior in the dissociation reasonably reproduces the product branching fractions.

Influence of Reduced Field Strength on Product Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

Classical ion mobility spectrometers (IMS) operated at ambient pressure, often use atmospheric pressure chemical ionization (APCI) sources to ionize organic compounds. In APCI, reactant ions ionize neutral analyte molecules via gas-phase ion-molecule reactions. The positively charged reactant ions in purified, dry air are H₃O⁺, NO⁺, and O₂^+•. However, the hydration of reactant ions in classical IMS operated at ambient pressure renders ionization of certain analytes difficult. In contrast to classical IMS operated at ambient pressure, High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are operated at a decreased pressure of 10-40 mbar, allowing operation at high reduced electric field strengths of up to 120 Td. At such high reduced field strengths, ions reach high effective temperatures causing collision-induced cluster dissociation of the hydrated gas-phase ions, allowing ionization of nonpolar and low proton affinity analytes. The reactant ion population, consisting of H₃O⁺(H₂O)_n, NO⁺(H₂O)_m, and O₂^+•(H₂O)_p with an individual abundance that strongly depends on the reduced field strength, differs from the reactant ion population in IMS operated at ambient pressure, which affects the ionization of analyte molecules. In this work, we investigate the influence of reduced field strength on the product ion formation of aromatic hydrocarbons used as model substances. A HiKE-IMS-MS coupling was used to identify the detected ion species. The results show that the analytes form parent cations via charge transfer with NO⁺(H₂O)_m and O₂^+•(H₂O)_p depending on ionization energy and protonated parent molecules via proton transfer and ligand switching with H₃O⁺(H₂O)_n mainly depending on proton affinity.

Formation of Pyrylium from Aromatic Systems with a Helium:Oxygen Flowing Atmospheric Pressure Afterglow (FAPA) Plasma Source

The effects of oxygen addition on a helium-based flowing atmospheric pressure afterglow (FAPA) ionization source are explored. Small amounts of oxygen doped into the helium discharge gas resulted in an increase in abundance of protonated water clusters by at least three times. A corresponding increase in protonated analyte signal was also observed for small polar analytes, such as methanol and acetone. Meanwhile, most other reagent ions (e.g., O₂^+·, NO⁺, etc.) significantly decrease in abundance with even 0.1% v/v oxygen in the discharge gas. Interestingly, when analytes that contained aromatic constituents were subjected to a He:O₂-FAPA, a unique (M + 3)⁺ ion resulted, while molecular or protonated molecular ions were rarely detected. Exact-mass measurements revealed that these (M + 3)⁺ ions correspond to (M - CH + O)⁺, with the most likely structure being pyrylium. Presence of pyrylium-based ions was further confirmed by tandem mass spectrometry of the (M + 3)⁺ ion compared with that of a commercially available salt. Lastly, rapid and efficient production of pyrylium in the gas phase was used to convert benzene into pyridine. Though this pyrylium-formation reaction has not been shown before, the reaction is rapid and efficient. Potential reactant species, which could lead to pyrylium formation, were determined from reagent-ion mass spectra. Thermodynamic evaluation of reaction pathways was aided by calculation of the formation enthalpy for pyrylium, which was found to be 689.8 kJ/mol. Based on these results, we propose that this reaction is initiated by ionized ozone (O₃^+·), proceeds similarly to ozonolysis, and results in the neutral loss of the stable CHO₂^· radical. Graphical Abstract ᅟ.

A novel technique for measurement of thermal rate constants and temperature dependences of dissociative recombination: CO2(+), CF3(+), N2O(+), C7H8(+), C7H7(+), C6H6(+), C6H5(+), C5H6(+), C4H4(+), and C3H3(+)

A novel technique using a flowing afterglow-Langmuir probe apparatus for measurement of temperature dependences of rate constants for dissociative recombination (DR) is presented. Low (~10(11) cm(-3)) concentrations of a neutral precursor are added to a noble gas∕electron afterglow plasma thermalized at 300-500 K. Charge exchange yields one or many cation species, each of which may undergo DR. Relative ion concentrations are monitored at a fixed reaction time while the initial plasma density is varied between 10(9) and 10(10) cm(-3). Modeling of the decrease in concentration of each cation relative to the non-recombining noble gas cation yields the rate constant for DR. The technique is applied to several species (O2(+), CO2(+), CF3(+), N2O(+)) with previously determined 300 K values, showing excellent agreement. The measurements of those species are extended to 500 K, with good agreement to literature values where they exist. Measurements are also made for a range of CnHm(+) (C7H7(+), C7H8(+), C5H6(+), C4H4(+), C6H5(+), C3H3(+), and C6H6(+)) derived from benzene and toluene neutral precursors. CnHm(+) DR rate constants vary from 8-12 × 10(-7) cm(3) s(-1) at 300 K with temperature dependences of approximately T(-0.7). Where prior measurements exist these results are in agreement, with the exception of C3H3(+) where the present results disagree with a previously reported flat temperature dependence.

Kinetics of Sulfur Oxide, Sulfur Fluoride, and Sulfur Oxyfluoride Anions with Atomic Species at 298 and 500 K

The rate constants and product-ion branching ratios for the reactions of sulfur dioxide (SO2-), sulfur fluoride (SFn-), and sulfur oxyfluoride anions (SOxFy-) with H, H2, N, N2, NO, and O have been measured in a selected-ion flow tube (SIFT). H atoms were generated through a microwave discharge on a H2/He mixture, whereas O atoms were created via N atoms titrated with NO, where the N had been created by a microwave discharge on N2. None of the ions reacted with H2, N2 or NO; thus, the rate constants are <1 x 10(-12) cm3 s-1. SOxFy- ions react with H by only fluorine-atom abstraction to form HF at 298 and 500 K. Successive F-atom removal does not occur at either temperature, and the rate constants show no temperature dependence over this limited range. SO2- and F- undergo associative detachment with H to form a neutral molecule and an electron. Theoretical calculations of the structures and energetics of HSO2- isomers were performed and showed that structural differences between the ionic and neutral HSO2 species can account for at least part of the reactivity limitations in the SO2- + H reaction. All of the SOxFy- ions react with O; however, only SO2- reacts with both N and O. SOxFy- reactions with N (SO2- excluded) have a rate constant limit of <1 x 10(-11) cm3 s-1. The rate constants for the SOxFy- reactions with H and O are < or =25% of the collision rate constant, as seen previously in the reactions of these ions with O3, consistent with a kinetic bottleneck limiting the reactivity. The only exceptions are the reactions of SO2- with N and O, which are much more efficient. Three pathways were observed with O atoms: F-atom exchange in the reactant ion, F- exchange in the reactant ion, and charge transfer to the O atom. No associative detachment was observed in the N- and O-atom reactions.

Bond-formation versuselectron transfer: C–C-coupling reactions of hydrocarbon dications with benzene

The bimolecular reactions of several hydrocarbon dications C(m)H(n)(2+) (m = 6-10, n = 4-9) with neutral benzene are investigated by tandem mass spectrometry using a multipole instrument. Not surprisingly, the major reaction of C(m)H(n)(2+) with benzene corresponds to electron transfer from the neutral arene to the dication resulting in the pair of monocationic products C(m)H(n)(+) + C(6)H(6)(+). In addition, also dissociative electron transfer takes place, whereas proton transfer from the C(m)H(n)(2+) dication to neutral benzene is almost negligible. Interestingly, the excess energy liberated upon electron transfer from the neutral arene to the C(m)H(n)(2+) dication is not equally partitioned in the monocationic products in that the cations arising from the dicationic precursor have a higher internal energy content than the monocations formed from the neutral reaction partner. In addition to the reactions leading to monocationic product ions, bond-forming reactions with maintenance of the two-fold charge are observed, which lead to a condensation of the C(m)H(n)(2+) dications with neutral benzene under formation of intermediate C(m+6)H(n+6)(2+) species and then undergo subsequent losses of molecular hydrogen or neutral acetylene. This reaction complements a recently proposed dicationic route for the formation of polycyclic aromatic hydrocarbons under extreme conditions such as they exist in interstellar environments.

Phenol Production in Benzene/Air Plasmas at Atmospheric Pressure. Role of Radical and Ionic Routes

Benzene can be efficiently converted into phenol when it is treated by either corona or dielectric barrier discharge (DBD) plasmas operating at atmospheric pressure in air or mixtures of N(2) and O(2). Phenol produced by corona discharge in an atmospheric pressure chemical ionization source (APCI) has been detected as the corresponding radical cation C(6)H(5)OH(+*) at m/z 94 by an ion trap mass spectrometer. On the other hand, phenol has been observed also as neutral product by gas chromatography-mass spectrometry analysis (GC-MS) after treatment in a DBD plasma. Experiments aimed at shading light on the elementary processes responsible for benzene oxidation were carried out (i) by changing the composition of the gas in the corona discharge source; (ii) by using isotopically labeled reagents; and (iii) by investigating some relevant ion-molecule reactions (i.e. C(6)H(6)(+*) + O(2), C(6)H(5)(+) + O(2)) via selected guided ion beam measurements and with the help of ab initio calculations. The results of our approach show that ionic mechanisms do not play a significant role in phenol production, which can be better explained by radical reactions resulting in oxygen addition to the benzene ring followed by 1,2 H transfer.

Two-Channel Dissociation of Chemically and Thermally Activated n-Butylbenzene Cations (C10H14+)

The charge-transfer reaction O(2)(+) + n-butylbenzene (C(10)H(14)) --> O(2) + C(10)H(14)(+) was studied in a turbulent ion flow tube at temperatures between 423 and 548 K and pressures between 15 and 250 Torr in the buffer gases He and N(2). Under chemical activation conditions stabilization vs dissociation ratios S/D of vibrationally highly excited C(10)H(14)(+)* as well as branching ratios of the fragments C(7)H(7)(+) (m/z = 91) vs C(7)H(8)(+) (m/z = 92) of the dissociation of C(10)H(14)(+)* were measured. Under thermal activation conditions, the rate constant of the dominating dissociation channel 92 was measured at 498 and 523 K. Employing information on the specific rate constants k(E) of the two channels 91 and 92 and on collisional energy transfer rates from the literature, the measured S/D curves and branching ratios 91/92 could be modeled well. It is demonstrated that the charge transfer occurs approximately equally through resonant transfer and complex-forming transfer. The thermal dissociation experiments provide a high precision value of the energy barrier for the channel 92, being 1.14 (+/-0.02) eV.

Reactions of POxCly- Ions with H and H2 from 298 to 500 K

Rate constants and product branching ratios for POxCly- ions reacting with H and H2 were measured in a selected ion flow tube (SIFT) from 298 to 500 K. PO2Cl-, PO2Cl2-, POCl2-, and POCl3- were all unreactive with H2, having a rate constant with an upper limit of <5 x 10(-12) cm3 s(-1). PO2Cl2- did not react with H atoms either, having a similar rate constant limit of <5 x 10(-12) cm3 s(-1). The rate constants for PO2Cl-, POCl2-, and POCl3- reacting with H showed no temperature dependence over the limited range of 298-500 K and were approximately 10-20% of the collision rate constant. Cl abstraction by H to form HCl was the predominant product channel for PO2Cl-, POCl2-, and POCl3-, with a small amount of Cl- observed from POCl2- + H. Reactions of O2 and O3 with the POCl- products ions from the reaction of POCl2- + H were observed to yield predominantly PO3- and PO2-, respectively. POCl- reacted with O2 and O3 with rate constants of 8.9 +/- 1.1 x 10(-11) and 5.2 +/- 3.3 x 10(-10) cm3 s(-1), respectively. No associative electron detachment in the reactions with H atoms was observed with any of the reactant ions; however, detachment was observed with a PO- secondary product ion at high H atom concentrations. Results of new G3 theoretical calculations of optimized geometries and energies for the products observed are discussed.

Ion-molecule rate constants and branching ratios for the reaction of N3++O2 from 120 to 1400 K

The kinetics of the reaction of N(3) (+) with O(2) has been studied from 120 to 1400 K using both a selected ion flow tube and high-temperature flowing afterglow. The rate constant decreases from 120 K to approximately 1200 K and then increases slightly up to the maximum temperature studied, 1400 K. The rate constant compares well to most of the previous measurements in the overlapping temperature range. Comparing the results to drift tube data shows that there is not a large difference between increasing the translational energy available for reaction and increasing the internal energy of the reactants over much of the range, i.e., all types of energies drive the reactivity equally. The reaction produces both NO(+) and NO(2) (+), the latter of which is shown to be the higher energy NOO(+) linear isomer. The ratio of NOO(+) to NO(+) decreases from a value of over 2 at 120 K to less than 0.01 at 1400 K because of dissociation of NOO(+) at the higher temperatures. This ratio decreases exponentially with increasing temperature. High-level theoretical calculations have also been performed to compliment the data. Calculations using multi-reference configuration interaction theory at the MRCISD(Q)/cc-pVTZ level of theory show that singlet NOO(+) is linear and is 4.5 eV higher in energy than ONO(+). A barrier of 0.9 eV prevents dissociation into NO(+) and O((1)D); however, a crossing to a triplet surface connects to NO(+) and O((3)P) products. A singlet and a triplet potential energy surface leading to products have been determined using coupled cluster theory at the CCSD(T)/aug-cc-pVQZ level on structures optimized at the Becke3-Lee, Yang, and Parr (B3LYP)/aug-cc-pVTZ level of theory. The experimental results and reaction mechanism are evaluated using these surfaces.

Reactions of N+, N2+, and N3+ with NO from 300 to 1400 K

Rate constants have been measured from 300 to 1400 K in a selected ion flow tube (SIFT) and a high temperature flowing afterglow for the reactions of N+, N2+ and N3+ with NO. In all of the systems, the rate constants are substantially less than the collision rate constant. Comparing the high temperature results to kinetics studies as a function of translational energy show that all types of energy (translational, rotational, and vibrational) affect the reactivity approximately equally for all three ions. Branching ratios have also been measured at 300 and 500 K in a SIFT for the N+ and N3+ reactions. An increase in the N2+ product at the expense of NO+ nondissociative charge transfer product occurs at 500 K with N+. The branching ratios for the reaction of N3+ with NO have also been measured in the SIFT, showing that only nondissociative charge transfer giving NO+ occurs up to 500 K. The current results are discussed in the context of the many previous studies of these ions in the literature.