Increase of the Charge Transfer Rate Coefficients for NO+ and O2+• Reactions with Isoprene Molecules at Elevated Interaction Energies

Computational insights into phthalate ester-linked VOCs: A density functional theory (DFT)-based approach for chemical ionization mass spectrometry (CI-MS) analysis

Rationale

The presence of volatile organic compounds (VOCs), notably diethyl phthalate, dimethyl phthalate, di‐n‐butyl phthalate, di(2‐ethylhexyl) phthalate, and similar compounds in soft drinks, raises significant concerns due to their known or potential adverse health effects. Monitoring these compounds is imperative to comprehend their implications on human health and the overall quality of soft drinks. Chemical ionization mass spectrometry (CI‐MS) techniques emerge as powerful tools for VOC quantification in soft drinks, offering fast analysis times, high detection sensitivity, real‐time analysis capabilities, and versatility across various scientific fields.

Methods

Achieving absolute quantification of VOCs using proton transfer reaction mass spectrometry (PTR‐MS) presents challenges, with individual VOC calibration proving labor intensive. Theoretical approaches pioneered by Su and colleagues, including density functional theory (DFT), offer avenues for approximating VOC concentrations and understanding ion‐molecule reactions. Specifically, DFT method B3LYP/6–311++G(d, p) computes molecular parameters like dipole moment, polarizability, proton affinity, and ionization energy for large phthalate esters. Rate constants of ion‐molecule reactions are determined using the parametrized trajectory method under varying E/N and temperature conditions.

Results

The analysis of computed parameters across seven complex molecules reveals notable findings. Bis(2‐methoxyethyl) phthalate, for instance, exhibits a superior dipole moment, suggesting intensified electrostatic interactions with ions and heightened rate constants. The increased proton affinity observed in certain molecules renders them suitable for specific ionization methods. Furthermore, enthalpy change and free energy computations affirm the reactivity of ions with phthalate esters, with distinct variations noted in rate constants based on dipole moment and polarizability.

Conclusions

In conclusion, the parametrized trajectory method, coupled with computational analysis of molecular parameters, offers a means to compute rate constants for ion‐molecule reactions, enabling determination of VOC concentrations in soft drinks without external calibration standards in PTR‐MS analyses. The observed variations in rate constants with temperature and reagent ions align with collision theory principles and existing literature findings, underscoring the utility of these approaches in VOC identification and quantification using PTR‐MS.

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.

How to Use Ion-Molecule Reaction Data Previously Obtained in Helium at 300 K in the New Generation of Selected Ion Flow Tube Mass Spectrometry Instruments Operating in Nitrogen at 393 K

Selected ion flow tube mass spectrometry (SIFT-MS) instruments have significantly developed since this technique was introduced more than 20 years ago. Most studies of the ion-molecule reaction kinetics that are essential for accurate analyses of trace gases and vapors in air and breath were conducted in He carrier gas at 300 K, while the new SIFT-MS instruments (optimized to quantify concentrations down to parts per trillion by volume) operate with N₂ carrier gas at 393 K. Thus, we pose the question of how to reuse the data from the extensive body of previous literature using He at room temperature in the new instruments operating with N₂ carrier gas at elevated temperatures. Experimentally, we found the product ions to be qualitatively similar, although there were differences in the branching ratios, and some reaction rate coefficients were lower in the heated N₂ carrier gas. The differences in the reaction kinetics may be attributed to temperature, an electric field in the current flow tubes, and the change from He to N₂ carrier gas. These results highlight the importance of adopting an updated reaction kinetics library that accounts for the new instruments' specific conditions. In conclusion, almost all previous rate coefficients may be used after adjustment for higher temperatures, while some product ion branching ratios need to be updated.

Kinetics for the Reactions of H3O+(H2O)n=0-3 with Isoprene (2-Methyl-1,3-butadiene) as a Function of Temperature (300-500 K)

We report kinetics studies of H₃O⁺(H₂O)_n=0-3 with isoprene (2-methyl-1,3-butadiene, C₅H₈) as a function of temperature (300-500 K) measured using a flowing afterglow-selected ion flow tube. Results are supported by density functional (DFT) calculations at the B3LYP/def2-TZVP level. H₃O⁺ (n = 0) reacts with isoprene near the collision limit exclusively via proton transfer to form C₅H₉⁺. The first hydrate (n = 1) also reacts at the collision limit and only the proton transfer product is observed, although hydrated protonated isoprene may have been produced and dissociated thermally. Addition of a second water (n = 2) lowers the rate constant by about a factor of 10. The proton transfer of H₃O⁺(H₂O)₂ to isoprene is endothermic, but transfer of the water ligands lowers the thermicity and the likely process occurring is H₃O⁺(H₂O)₂ + C₅H₈ → C₅H₉⁺(H₂O)₂ + H₂O, followed by thermal dissociation of C₅H₉⁺(H₂O)₂. Statistical modeling indicates the amount of reactivity is consistent with the process being slightly endothermic, as is indicated by the DFT calculations. This reactivity was obscured in past experiments due to the presence of water in the reaction zone. The third hydrate is observed not to react and helps explain the past results for n = 2, as n = 2 and 3 were in equilibrium in that flow tube experiment. Very little dependence on temperature was found for the three species that did react. Finally, the C₅H₉⁺ proton transfer product further reacted with isoprene to produce mainly C₆H₉⁺ along with a small amount of clustering.

Theoretical Investigation of Charge Transfer from NO+ and O2+ Ions to Wine-Related Volatile Compounds for Mass Spectrometry

Density-functional theory (DFT) is used to obtain the molecular data essential for predicting the reaction kinetics of chemical-ionization-mass spectrometry (CI-MS), as applied in the analysis of volatile organic compounds (VOCs). We study charge-transfer reactions from NO⁺ and O₂⁺ reagent ions to VOCs related to cork-taint and off-flavor in wine. We evaluate the collision rate coefficients of ion-molecule reactions by means of collision-based models. Many NO⁺ and O₂⁺ reactions are known to proceed at or close to their respective collision rates. Factors affecting the collision reaction rates, including electric-dipole moment and polarizability, temperature, and electric field are addressed, targeting the conditions of standard CI-MS techniques. The molecular electric-dipole moment and polarizability are the basic ingredients for the calculation of collision reaction rates in ion-molecule collision-based models. Using quantum-mechanical calculations, we evaluate these quantities for the neutral VOCs. We also investigate the thermodynamic feasibility of the reactions by computing the enthalpy change in these charge-transfer reactions.

Ligand Switching Ion Chemistry: An SIFDT Case Study of the Primary and Secondary Reactions of Protonated Acetic Acid Hydrates with Acetone

A study was performed of the reactions of protonated acetic acid hydrates, CH₃COOHH⁺(H₂O)_n, with acetone molecules, CH₃COCH₃, using a selected ion flow-drift tube (SIFDT). The rationale for this study is that hydrated protonated organic molecules are major product ions in secondary electrospray ionization mass spectrometry (SESI-MS) and ion mobility spectrometry (IMS). Yet the formation and reactivity of these hydrates are only poorly understood, and kinetics data are only sparse. The existing SIFDT instrument in our laboratory was upgraded to include an octupole ion guide and a separate drift tube by which hydrated protonated ions can be selectively injected into the drift tube reactor and their reactions with molecules studied under controlled conditions. This case study shows that, in these hydrated ion reactions with acetone molecules, the dominant reaction process is ligand switching producing mostly proton-bound dimer ions (CH₃COCH₃)H⁺(CH₃COOH), with minor branching into (CH₃COCH₃)H⁺(H₂O). This switching reaction was observed to proceed at the collisional rate, while other studied hydrated ions reacted more slowly. An attempt is made to understand the reaction mechanisms and the structures of the reaction intermediate ions at the molecular level. Secondary switching reactions of the asymmetric proton-bound dimer ions lead to a formation of strongly bound symmetrical dimers (CH₃COCH₃)₂H⁺, the terminating ion in this ion chemistry. These results strongly suggest that, in SESI-MS and IMS, the presence of a polar compound, like acetone in exhaled breath, can suppress the analyte ions of low concentration compounds like acetic acid thus compromising their quantification.

Styrene radical cations for chemical ionization mass spectrometry analyses of monoterpene hydrocarbons

RATIONALE

Monoterpene hydrocarbons play an important role in the formation of secondary aerosol particles and in atmospheric chemistry. Thus, there is a demand to measure their individual concentrations in situ in real time. Currently, only the total concentration of monoterpenes C₁₀ H₁₆ can be determined by chemical ionization mass spectrometry techniques using reagent ions H₃ O⁺ , NO⁺ and (C₆ H₆ )_n ^+• without gas chromatographic separation.

METHODS

The styrene cation C₈ H₈ ^+• was investigated as a reagent for chemical ionization of monoterpenes. The modified selected ion flow drift tube, SIFDT, technique was used to characterize the differences in product ion distributions between α-phellandrene, α-pinene, γ-terpinene, β-pinene, ocimene, sabinene, 3-carene, (R)-limonene, camphene and myrcene.

RESULTS

The monoterpene molecular cation C₁₀ H₁₆ ^+• is the main product (about 90%) for all isomers except (R)-limonene and camphene with an efficient channel of C₈ H₈ ^+• C₁₀ H₁₆ adduct formation and γ-terpinene with unexpectedly significant product ions at m/z 134 and 135 corresponding to losses of H₂ and H.

CONCLUSIONS

Utilization of the styrene cation for the ionization of monoterpenes is beneficial due to the very low fragmentation of the product ions. Specific association product ions for camphene and (R)-limonene and fragment product ions for γ-terpinene allow them to be distinguished from other isomers that produce mostly the molecular cation.