To provide insights on the molecular growth mechanisms leading to large carbon and nitrogen-bearing species in Titan’s ionosphere, laboratory measurements of ion-neutral reactions were conducted and the results are presented in the following doctoral dissertation. The product ions of the C₂H₂N⁺ reactions with saturated and unsaturated neutral hydrocarbons have been investigated using Guided Ion Beam Mass Spectrometer (GIB-MS). The data have been used to reveal the dependence of product ion intensity on pressure and also with respect to collision energy variations to derive useful insights on the nature of chemical reactions leading to the observed species. Complementary theoretical calculations have been also carried out to assist in interpretation of these measurements and to elucidate the reaction schemes. This study provides comprehensive insights on the formation of long chain and heavy carbon-nitrogen bearing species through exothermic and barrierless ion-neutral reactions which are likely to play a significant role in the formation routes of heavy positive ions of interest, which have been detected in Titan’s ionosphere by Cassini’s Ion Neutral Mass Spectrometer (INMS).

In this study we report theoretical and experimental evidence for the formation of ionic products by the ion-neutral reaction of C2H2N+ with C2H6. Our investigations consist of laboratory measurements using a guided ion beam mass spectrometer together with complementary ab initio quantum chemical computations, at the MP2/6-311++G(d,p) level of theory, in order to elucidate the energetics and geometries of the intermediates and transition states that are involved in the production of the observed product ions. This study also provides insights on the isomeric nature of the observed product ions, their formation pathways together with collision energy and pressure dependences. The experimental data agrees well with the predictions of the ab initio calculations. Despite data provides evidence for the occurrence of C2H5+ as the most salient product ion, the production of CH3+, C2H3+, C3H5+, C3H7+ and C2H4N+ is also evident. A reaction scheme was proposed to elucidate the mechanisms responsible for the formation of the observed product ions. These processes might be intermediate steps in the generation of long chain carbon and nitrogen-bearing compounds in Titan's ionosphere, the interstellar medium or circumstellar envelopes.

The current study was undertaken to probe the ionic products of the ion-neutral reaction of C2H2N+ with CH4 using guided ion beam mass spectrometry (GIB-MS) in which the CH3+, C2H3+, HCNH+, C2H5+, C2H3N+ and C3H4N+ ions are identified as products. Theoretical calculations were performed to suggest reaction pathways leading to the detected products. These processes might be of relevance for the generation of long chain carbon-nitrogen bearing compounds in Titan's atmosphere, the interstellar medium or circumstellar envelopes.

The ion-neutral reactions of the C₂H₂N⁺ cation with C₂H₂ have been investigated using a Guided Ion Beam Mass Spectrometer (GIB-MS). The following ionic products were observed: CH₃⁺, C₂H₂⁺, C₂H₃⁺, HNC⁺/HCN⁺, HCNH⁺, C₃H⁺, C₂N⁺, C₃H₃⁺, HCCN⁺ and C₄H₂N⁺. Theoretical calculations have been carried out to propose reaction pathways leading to the observed products. These processes are of relevance for the generation of long chain nitrogen-containing species and they may be of interest for the chemistry of Titan’s ionosphere or circumstellar envelopes.

In order to facilitate its detection in astronomical observations, the measurement of the rotational spectrum of acrylic acid has been extended to 397 GHz using a free space cell at room temperature. 295 new lines were assigned to the s-cis conformer and 286 new lines to the s-trans conformer of acrylic acid. Using the determined experimental parameters, the predictions of the rotational transition frequencies up to 900 GHz and their intensities were obtained at temperatures of 100 and 200 K and at room temperature. Based on these predictions, a search of the most intense transitions of acrylic acid in star-forming regions was performed using published data from the HERSCHEL Science Archive. No lines were found but the possibility of observing rotational transitions of acrylic acid in astronomical objects is discussed.

To provide some insights on the formation of long-chain carbon-nitrogen bearing ions in Titan’s atmosphere, laboratory measurements of ion-neutral reactions were conducted and results are presented in the following licentiate thesis. The product ions of the C₂H₂N⁺ reaction with saturated and unsaturated neutral hydrocarbons have been studied using guided ion beam mass spectrometer (GIB-MS). The data have been used to infer the product ion signal intensity dependences on pressure and collision energies to obtain information on the reactions contributing to these signals. Theoretical calculations have been carried out to interpret these measurements and to elucidate the reaction pathways leading to the observed product ions. This study provides understanding on the formation of long-chain carbon-nitrogen bearing ions through exothermic and barrierless ion-neutral reactions which are of interest as possible formation routes to heavy positive ions detected in Titan’s atmosphere by Cassini’s ion neutral mass spectrometer (INMS).

The weakly bound 1:1 complex between acrylonitrile (CH2=-CHCN) and water has been characterized spectroscopically in the millimeter wave range (59.6-74.4 GHz) using a Free Jet Absorption Millimeter Wave spectrometer. Precise values of the rotational and quartic centrifugal distortion constants have been obtained from the measured frequencies of the normal and isotopically substituted water moiety (DOH, DOD, (HOH)-O-18). Structural parameters have been estimated from the rotational constants and their differences among isotopologues: the complex has a planar structure with the two subunits held together by a O-H center dot center dot center dot N (2.331(3) angstrom) and a C- H center dot center dot center dot O (2.508(4) angstrom) interaction. The ab initio intermolecular binding energy, obtained at the counterpoise corrected MP2/aug-cc-pVTZ level of calculation, is D-e = 24.4 kJ mol(-1)

The gas phase reactivity of C₂H₂N⁺ with C₂H₄ is studied using a combined experimental and theoretical approach through guided ion beam mass spectrometry (GIB-MS) and ab initio calculations. Results indicate the formation of C₃H₅⁺, C₂H₄⁺, C₄H₅N⁺ and C₃H₃⁺ as major products, in addition to minor generation of C₂H₃⁺, C₂H₄N⁺, C₄H₄N⁺, C₃H₂N⁺, and CH₃⁺. Theoretical calculations show that reaction proceeds via formation and subsequent rearrangements of a [C₄H₆N]⁺ adduct. In the quest to probe its singlet electronic state hypersurface, the most relevant stationary points along the reaction pathway leading to the various products are located and optimized. The results of our calculations at the MP2/6-311++G(d,p) level were found to be in agreement with our experimental findings, as well as with the derived enthalpies of reaction for the designated reaction channels. The present study provides substantial new insights on the formation of complex organics in Titan’s ionosphere detected by the Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft. These pathways might also be of relevance for the generation of long chain carbon-nitrogen bearing compounds in low-temperature environments e.g. the interstellar medium or circumstellar envelopes.