Reactions of substituted benzene anions with N and O atoms: Chemistry in Titan’s upper atmosphere and the interstellar medium

Highly Efficient Compositional and Compound Specific Isotopic Analysis of Volatile Primary Amines and Ammonia in the Murchison Meteorite Using SPME On-Fiber Derivatization: Optimization for Bennu Sample Analyses

RATIONALE

Extraterrestrial amines and ammonia are critical ingredients for the formation of astrobiologically important compounds such as amino acids and nucleobases. However, conventional methods for analyzing the composition and isotopic ratios of volatile amines suffer from lengthy derivatization and purification procedures, high sample mass consumption, and chromatographic interferences from derivatization reagents and non-target compounds.

METHODS

Here we demonstrate a highly efficient method to analyze the composition and compound specific isotopic ratios of C1 to C6 amines as well as ammonia based on solid phase micro-extraction (SPME) on-fiber derivatization. 2,3,4,5,6-pentafluorobenzyl chloroformate (PFBCF) adsorbed on a solid phase SPME fiber is subsequently exposed to the headspace of the water extract of the Murchison meteorite to selectively extract, derivatize and concentrate volatile amines and ammonia. PFBCF does not directly contact the aqueous solution containing other soluble organics.

RESULTS

An aliquot of volatile amines and ammonia in the headspace are selectively derivatized on the SPME fiber and subsequently thermally desorbed onto the GC injector for analysis. Only the amounts of amines required for either compositional or isotopic analysis are derivatized and consumed in the process, preserving the bulk fraction of amines and ammonia for other analyses, and the process does not affect other volatile compound classes. Carbon and hydrogen isotopic ratios of amines are obtained by isotopic mass balance.

CONCLUSIONS

The exceptional selectivity and sensitivity of SPME on-fiber derivatization of volatile amines in carbonaceous chondrite extracts allow minimization of sample consumption. Carbon and hydrogen isotopic values of individual amines in the Murchison meteorite are consistent with their extraterrestrial origin, with a substantial fraction inherited from interstellar molecular clouds. SPME on-fiber derivatization is well suited for analyzing extraterrestrial materials, especially precious asteroid return samples.

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

Direct dynamics in a proton transfer reaction of isomer product competition. Insight into the suppressed formation of the isoformyl cation

Proton transfer between HOCO⁺ and CO produces the formyl cation HCO⁺ and isoformyl cation HOC⁺ isomers initiating multiple astrochemical reaction networks. Here, the direct chemical dynamics simulations are performed to uncover the underlying atomistic dynamics of the above reaction. The simulations reproduce the measured product energy and scattering angle distributions and reveal that the reaction proceeds predominantly through a direct stripping mechanism which results in the prominent forward scattering observed in experiments. The reaction dynamics show propensity for the HCO⁺ product even at a collision energy larger than the threshold for HOC⁺ formation. This is a consequence of the larger opacity and impact parameter range for HCO⁺. In accordance with the revealed direct mechanistic feature, the reaction can be controlled by orienting the reactants into a reactive H-C orientation that also favors HCO⁺ formation. Considering the lack of equilibrated reactant complexes and the on the fly migration of the proton, the CO₂-catalyzed isomerization is assumed to have insignificant impact on the isomer ratios. This work provides insights of dynamical effects besides energetics into the interesting finding of strongly suppressed formation of the metastable isoformyl cation for related proton transfer reactions in the measurements.

Potential energy surfaces for high-energy N + O2 collisions

Potential energy surfaces for high-energy collisions between an oxygen molecule and a nitrogen atom are useful for modeling chemical dynamics in shock waves. In the present work, we present doublet, quartet, and sextet potential energy surfaces that are suitable for studying collisions of O₂(³Σ_g ^-) with N(⁴S) in the electronically adiabatic approximation. Two sets of surfaces are developed, one using neural networks (NNs) with permutationally invariant polynomials (PIPs) and one with the least-squares many-body (MB) method, where a two-body part is an accurate diatomic potential and the three-body part is expressed with connected PIPs in mixed-exponential-Gaussian bond order variables (MEGs). We find, using the same dataset for both fits, that the fitting performance of the PIP-NN method is significantly better than that of the MB-PIP-MEG method, even though the MB-PIP-MEG fit uses a higher-order PIP than those used in previous MB-PIP-MEG fits of related systems (such as N₄ and N₂O₂). However, the evaluation of the PIP-NN fit in trajectory calculations requires about 5 times more computer time than is required for the MB-PIP-MEG fit.

Kinetics of CO+ and CO2+ with N and O atoms

We have measured reaction rate constants for CO⁺ and CO₂⁺ reacting with N and O atoms using a selected ion flow tube apparatus equipped with a microwave discharge atom source. Experimental work was supplemented by molecular structure calculations. Calculated pathways show the sensitivity of kinetic barriers to theoretical methods and imply that high-level ab initio methods are required for accurate energetics. We report room-temperature rate constants of 1.0 ± 0.4 × 10^-11 cm³ s^-1 and 4.0 ± 1.6 × 10^-11 cm³ s^-1 for the reactions of CO⁺ with N and O atoms, respectively, and 8.0 ± 3.0 × 10^-12 cm³ s^-1 and 2.0 ± 0.8 × 10^-11 cm³ s^-1 for the reactions of CO₂⁺ with N and O atoms, respectively. The reaction of CO₂⁺ + O is observed to yield O₂⁺ exclusively. These values help resolve discrepancies in the literature and are important for modeling of the Martian atmosphere.

Reactivity of amino acid anions with nitrogen and oxygen atoms

Gas-phase reaction of deprotonated tyrosine with a ground state O atom generates five ionic products.