Computational and experimental studies on stabilities, reactions and reaction rates of cations and ion-dipole complexes

Abstract

In this thesis, ion stability, ion-molecule reactions and reaction rates are studied using mass spectrometry and molecular modelling. In Chapter 2 the effect of functional group substitution on neutral and ionised ketene are studied. Electron-donating substituents show a stabilising positive induction effect on the ketene ion, while electron acceptor substituents destabilise it by a negative induction effect. There exists also resonance stabilisation of the product ion, caused by electron donation from the substituent to the product ion. This stabilisation is in some cases so strong that the existence of a covalently bound substituted ketene ion is not possible. The stabilisation effects on proton-bound complexes are studied in Chapter 3. An existing linear correlation method to estimate the stability of the formed complexes is extended by inclusion of a dipole moment factor in the method. This improves the accuracy of the stability estimates in general for a range of complexes. Especially in many cases where there is a strong ion-dipole interaction contributing to the stability of the complex, the estimations are improved. Chapter 4 focuses on the dissociation reactions of protonated oxalic acid. It is found from computations that the lowest-energy reaction proceeds to the dissociation into protonated water, carbon monoxide, and carbon dioxide. This happens via a unique ter-body complex. This ter-body complex is also argumented to be the reason for the main peak in a metastable ion mass spectrum of protonated oxalic acid. Reaction paths of the minor dissociation products were also obtained. The possibility of pyrimidine formation in interstellar dust clouds from an acrylonitrile radical cation dimer is discussed in Chapter 5. Metastable ion mass spectrometric experiments on acrylonitrile exhibit a small amount of pyrimidine formation, which results from a covalently bound adduct cation of neutral and ionized acrylonitrile. From experimental and computational studies this is found to be only a minor product, while the major product is self-protonation via proton-bound complexes. A related reaction of acrylonitrile with hydrogen cyanide is studied in Chapter 6 using solely computations. Pyrimidine formation is deemed possible, in terms of energy, although kinetic studies show the adduct formation to be very slow in comparison to proton-transport catalysis reactions. The latter reactions are identified as the most favourable reactions. Protonated hydrogen cyanide formation is found to be a possible minor process as well. Chapter 7 is focused on qualitative studies using semi-classical trajectory calculations on the dissociation rate of an ion-dipole complex with a deep potential well preceding the dissociation. To reduce required computational time, a qualitative interpolation method is developed to predict the behaviour of trajectories in cases where many vibrational modes are excited. It is shown that, on average, only one-tenth of the channels open to dissociation lead directly to dissociation products for moderate excess energies (less than 10 kcal mol-1). The rest of the open channels lead either to a very slowly progressing dissociation, or to semi-periodic behaviour of the trajectory.