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NH 4 + Association and Proton Transfer Reactions With a Series of Organic Molecules.

ABSTRACT: In this study, we present reactions of NH 4 + with a series of analytes (A): acetone (C₃H₆O), methyl vinyl ketone (C₄H₆O), methyl ethyl ketone (C₄H₈O), and eight monoterpene isomers (C₁₀H₁₆) using a Selective Reagent Ionization Time-of-Flight Mass Spectrometer (SRI-ToF-MS). We studied the ion-molecule reactions at collision energies of 55 and 80 meV. The ketones, having a substantially lower proton affinity than NH₃, produce only cluster ions NH 4 + (A) in detectable amounts at 55 meV. At 80 meV, no cluster ions were detected meaning that these adduct ions are formed by strongly temperature dependent association reactions. Bond energies of cluster ions and proton affinities for most monoterpenes are not known and were estimated by high level quantum chemical calculations. The calculations reveal monoterpene proton affinities, which range from slightly smaller to substantially higher than the proton affinity of NH₃. Proton affinities and cluster bond energies allow to group the monoterpenes as a function of the enthalpy for the dissociation reaction N H 4 + A ? A H + + N H 3 . We find that this enthalpy can be used to predict the NH 4 + (A) cluster ion yield. The present study explains product ion formation involving NH 4 + ion chemistry. This is of importance for chemical ionization mass spectrometry (CIMS) utilizing NH 4 + as well as NH 4 + (H₂O) as reagent ions to quantitatively detect atmospherically important organic compounds in real-time.

SUBMITTER: Canaval E

PROVIDER: S-EPMC6457267 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature

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NH 4 + Association and Proton Transfer Reactions With a Series of Organic Molecules.

Canaval Eva E Hyttinen Noora N Schmidbauer Benjamin B Fischer Lukas L Hansel Armin A

Frontiers in chemistry 20190403

In this study, we present reactions of NH 4 + with a series of analytes (A): acetone (C3H6O), methyl vinyl ketone (C4H6O), methyl ethyl ketone (C4H8O), and eight monoterpene isomers (C10H16) using a Selective Reagent Ionization Time-of-Flight Mass Spectrometer (SRI-ToF-MS). We studied the ion-molecule reactions at collision energies of 55 and 80 meV. The ketones, having a substantially lower proton affinit ...[more]

PMID: 31001517

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Project description:Chemical equilibria of multimerizations in systems with small numbers of particles exhibit a behavior seemingly at odds with that observed macroscopically. In this paper, we apply the recently proposed expression of equilibrium constant for binding, which includes cross-correlations in reactants' concentrations, to write an equilibrium constant for the formation of clusters larger than two (e.g., trimer, tetramer, and pentamer) as series of two-body reactions. Results obtained by molecular dynamics simulations demonstrate that the value of this expression is constant for all concentrations and system sizes, as well as at an onset of a phase transition to an aggregated state, where densities in the system change discontinuously. In contrast, the value of the commonly utilized expression of equilibrium constant, which ignores correlations, is not constant and its variations can reach few orders of magnitude. Considering different paths for the same multimer formation, with elementary reactions of any order, yields different expressions for the equilibrium constant, yet, with exactly the same value. This is also true for routes with essentially zero probability to occur. Existence of different expressions for the same equilibrium constant imposes equalities between averages of correlated, along with uncorrelated, concentrations of participating species. Moreover, a relation between an average particle number and relative fluctuations derived for two-body reactions is found to be obeyed here as well despite couplings to additional equilibrium reactions in the system. Analyses of transfer reactions, where association and dissociation events take place on both sides of the chemical equation, further indicate the necessity to include cross-correlations in the expression of the equilibrium constant. However, in this case, the magnitudes of discrepancies of the uncorrelated expression are smaller, likely because of partial cancellation of correlations, which exist on both the reactant and product sides.

Dataset Information

NH 4 + Association and Proton Transfer Reactions With a Series of Organic Molecules.

Publications

NH 4 + Association and Proton Transfer Reactions With a Series of Organic Molecules.

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