Isomers frequently exhibit different physical and chemical properties, which can make accurate modeling of their reactivity highly challenging but can also be exploited as a method for isomer identification. In both cases, this requires accurate experimental data recorded at an isomer-specific level. The reactions of the DCOH•+ and H2CO•+ ions with isobutane [CH(CH3)3] are investigated experimentally in the 0.07–10 eV collision energy range using a guided-ion beam tandem mass spectrometer coupled to a tunable vacuum ultraviolet source. The reaction of the DCOH•+ ion is dominated by dissociative proton transfer processes, while the reactivity of H2CO•+ is predominantly driven by dissociative charge transfer processes. The kinematics are also significantly different, with the overall reactivity of the DCOH•+ isomer following an approximately Langevin trend, while the reactivity of H2CO•+ is distinctly non-Langevin. In addition, for the H2CO•+ isomer, sharp changes in reactivity are observed at photon energies corresponding to Rydberg autoionization, leading to vibrational excitation of the C–O stretching mode. The findings highlight the inadequacy of modeling molecular ions as point charges and evidence the need for isomer-specific reactivity data.

The list of detected molecules in space has grown rapidly in recent years. Identified compounds now include both nitrogen and oxygen bearing species and detection is being made in a variety of different regions. Isomers have also become increasingly interesting, as their interconversion often is hindered by an energy barrier unlikely to be overcome given the energy-scarce conditions in space, they readily act as different species. In order to describe the chemical environments in space, model calculations are being performed, heavily relying on detailed knowledge about the abundances and reactivity of these species. However, the abundances and formation pathways of many compounds still remain largely unknown.

This thesis presents two experimental studies: i) the possible selective generation of the [CH₃N]⁺ isomers and their reactivity with hydrocarbons methane, ethene, ethyne, propene, propadiene and propyne and the alcohol methanol, and ii) the infrared-predissociation spectroscopy of the [CH₃N]⁺ and [C₃H₃]⁺ isomers (and the deuterated isotopologues of the latter). Both isomeric pairs have been proposed to exist on Titan as well as to act as key intermediates in the synthesis of heavier compounds, potentially including aerosols, tholins and amino acids. Theoretical calculations accompanies the above experiments, presenting potential energy surfaces of the reactions and predicted vibrational band positions. 

The results show that the reactions of [CH₃N]⁺ isomers with the hydrocarbons and the alcohol do lead to heavier compounds in exoergic pathways feasible to occur in space and on Titan. Products identified by our calculations includes protonated acrylonitrile, hydrogen cyanide and protonated pyrrole. The reactivity is also isomer dependent, highlighting the importance of isomer consideration in computational modeling of chemical networks in space. The obtained IRPD spectrum allows for identification of these species and pave the way for detection of them with infrared telescopes in space.

The gas phase reactivity of the radical cation isomers H₂CNH˙+ (methanimine) and HCNH₂˙+ (aminomethylene) with propene (CH₃CHCH₂) has been investigated by measuring absolute reactive cross sections and product branching ratios, under single collision conditions, as a function of collision energy (in the range ∼0.07–11.80 eV) using guided ion beam mass spectrometry coupled with VUV photoionization for selective isomer generation. Experimental results have been merged with theoretical calculations to elucidate reaction pathways and structures of products. The H₂CNH˙+ isomer is over a factor two more reactive than HCNH₂˙+. A major channel from both isomers is production of protonated methanimine CH₂NH₂+via hydrogen-atom transfer reaction but, while H₂CNH˙+ additionally gives charge and proton transfer products, the HCNH₂˙+ isomer leads instead to protonated vinylimine CH₂CHCHNH₂+, produced alongside CH₃˙ radicals. The reactions have astrochemical implications in the build up of chemical complexity in both the interstellar medium and the hydrocarbon-rich atmospheres of planets and satellites.

Interest in the observation and characterization of organic isomers in astronomical environments has grown rapidly with an increase in the sensitivity of detection techniques. Accurate modeling and interpretation of these environments require experimental isomer-specific reactivity and spectroscopic measurements. Given the abundance of formaldehyde (H2CO) in various astrophysical objects, the properties and reactivities of its cation isomers H2CO•+ and HCOH•+ are of significant interest. However, for the hydroxymethylene radical cation HCOH•+ (and its isotopologue DCOH•+), detailed reactivity studies have been limited by the lack of suitable experimental methods to generate this isomer with high purity. Here, potential approaches to the isomer-selective generation of HCOH•+ and DCOH•+ are characterized through differential reactivity measurements. While the dissociative photoionization of cyclopropanol (c-CH2CH2CHOH) is determined to be unsuitable, the dissociation of methanol-d3 (CD3OH) allows for the formation of DCOH•+ with a fractional abundance of >99% at photon energies below 14.8 eV. These results will allow future spectroscopic and reactivity measurements of HCOH•+/DCOH•+ to be conducted, laying the groundwork for future detection and incorporation into models of the interstellar medium.

The rotational spectra of the 1:1 complex formed by acrolein and methanol and its deuterated isotopologues have been analyzed. Two stable conformations in which two hydrogen bonds between the two moieties are formed were detected. The rotational lines show a hyperfine structure due to the methyl group internal rotation in the complex and the V3 barriers hindering the motion were determined as 2.629(5) kJ mol−1 and 2.722(5) kJ mol−1 for the two conformations, respectively. Quantum mechanical calculations at the MP2/aug-cc-pVTZ level and comprehensive analysis of the intermolecular interactions, utilizing NCI and SAPT approaches, highlight the driving forces of the interactions and allow the determination of the binding energies of complex formation.

Studying larger nucleophiles in bimolecular nucleophilic substitution (S_N2) reactions bridges the gap from simple model systems to those relevant to organic chemistry. Therefore, we investigated the reaction dynamics between the methoxy anion (CH₃O⁻) and iodomethane (CH₃I) in our crossed-beam setup combined with velocity map imaging at the four collision energies 0.4, 0.7, 1.2, and 1.6 eV. We find the two ionic products I⁻ and CH₂I⁻, which can be attributed to the S_N2 and proton transfer channels, respectively. The proton transfer channel progresses in a previously observed fashion from indirect to direct scattering with increasing collision energy. Interestingly, the S_N2 channel exhibits direct dynamics already at low collision energies. Both the direct stripping, leading to forward scattering, and the direct rebound mechanism, leading to backward scattering into high angles, are observed.

Ion chemistry has become increasingly important in the evolution of the chemical inventory of extraterrestrial environments. Isomers of ions have also come to play an important role as, in many instances, the cold environments in the interstellar medium and high layers of planet and satellite atmospheres do not supply enough energy to overcome isomerization barriers and the isomers effectively act as separate molecules. 

In this licentiate thesis, several studies of the [CH₃N]⁺ isomers are presented. Reactivity studies of the two isomers, the methanimine radical cation (H₂CNH⁺) and aminomethylene (HCNH₂⁺) with hydrocarbons C₂H₄, C₂H₂ and CH₄, and IRPD spectroscopy of both species have been performed. Complimentary ab initio calculations aid in the determination of formation pathways of observed product channels and in the assignment of the vibrational bands seen in the IRPD spectrum.

The results show that reaction pathways of the two isomers generally involve adduct formation followed by hydrogen ejection where the product or pathway is dependent on the ingoing reactant isomer. The IRPD spectrum allows identification of the different isomers via vibrational transitions. Isomer generation by electron ionization favours methanimine cation production with an abundance of 70% while with VUV photoionization it is possible to selectively produce isomers. It is concluded that isomerism must be considered when investigating the chemical environment of interstellar objects.

The vibrational transitions and the relative abundances of the two isomeric ions H₂CNH⁺ and H₂NCH⁺ generated through electron impact ionisation have been investigated in a noble gas tagging experiment. It could be shown that both species were formed with an abundance of 70 and 30% for H₂NCH⁺ and H₂CNH⁺, respectively. The obtained vibrational bands of the two species have been assigned to vibrational transitions through comparison with the results of ab initio calculations. These computations also predict both species to be moderately polar. The present investigations show that both isomers should be included in chemical model calculations of dark interstellar clouds, protoplanetary disks, star-forming regions as well as planetary atmospheres.

Experimental and theoretical studies are presented on the reactivity of the radical cation isomers H₂CNH^+• (methanimine) and HCNH₂^+• (aminomethylene) with ethyne (C₂H₂). Selective isomer generation is performed via dissociative photoionization of suitable neutral precursors as well as via direct photoionization of methanimine. Reactive cross sections (in absolute scales) and product branching ratios are measured as a function of photon and collision energies. Differences between isomers’ reactivity are discussed in light of ab-initio calculations of reaction mechanisms. The major channels, for both isomers, are due to H atom elimination from covalently bound adducts to give [C₃NH₄]⁺. Theoretical calculations show that while for the reaction of HCNH₂^+• with acetylene any of the three lowest energy [C₃NH₄]⁺ isomers can form via barrierless and exothermic pathways, for the H₂CNH^+• reagent the only barrierless pathway is the one leading to the production of protonated vinyl cyanide (CH₂CHCNH⁺), a prototypical branched nitrile species that has been proposed as a likely intermediate in star forming regions and in the atmosphere of Titan. The astrochemical implications of the results are briefly addressed.

Experimental and theoretical studies are presented on the reactivity of H2CNH'+ (methanimine) and HCNH2'+ (aminomethylene) with ethene (C2H4). Selective isomer generation is performed via dissociative photoionization of suitable neutral precursors and reactive cross sections and branching ratios are measured as a function of photon and collision energies. Differences between isomers' reactivity are discussed in light of ab-initio calculations on reaction mechanisms. The main products, for both isomers, are H-elimination, most likely occurring from covalently bound adducts (giving c-CH2CH2CHNH+/CH2NHCHCH2+) and H' atom transfer to yield H2CNH2+. The astrochemical implications of the results are briefly addressed.