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

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

Styrene and ethylbenzene absorption in ionic liquids : comparing DFT affinity calculations with experimental data

Styrene is a widely used bulk chemical produced by dehydrogenation of ethylbenzene (EB). Purification of styrene to contain <100 ppm EB is not cost-effective by conventional separation methods. One separation method is extractive distillation with an ionic liquid (IL) as a binding agent for one of the components, thereby lowering the vapour pressure of this component. In this study, using quantum density functional theory (DFT), we have simulated 22 IL anion–cation pairs, styrene and EB affinities to them, and ion-pair dimer affinities of the ILs. These are compared with experimental liquid–liquid equilibrium studies of M.T.G. Jongmans, B. Schuur, and A.B. de Haan, Ind. Eng. Chem. Res. 50 (2011), pp. 10800–10810. It is shown that experimental selectivity and distribution coefficients of styrene and EB in the ILs are related to computed gas phase anion–cation stabilisation energies and ion-pair–ion-pair dimer affinities. The inverse of molar volume is found to strongly correlate with the selectivity. The computational results also qualitatively correlate with molar volume, and consequently, it is possible to use DFT calculations as a qualitative prediction tool in screening of ILs for this separation process. This tool does not account for effects caused by long alkyl chains, as the length does not seem to affect dimer stabilisation energy beyond ethyl grou

The acrylonitrile dimer ion: A study of its dissociation via self-catalysis, self-protonation and cyclization into the pyrimidine radical cation

Large energy barriers prohibit the rearrangement of solitary acrylonitrile ions, CH2CHCN+, into their more stable hydrogen-shift isomers CH2CCNH+ or CHCH–CNH+. This prompted us to examine if these isomerizations occur by self-catalysis in acrylonitrile dimer ions. Such ions, generated by chemical ionization experiments of acrylonitrile with an excess of carbon dioxide, undergo five dissociations in the μs time frame, as witnessed by peaks at m/z 53, 54, 79, 80 and 105 in their metastable ion mass spectrum. Collision experiments on these product ions, deuterium labeling, and a detailed computational analysis using the CBS-QB3 model chemistry lead to the following conclusions: (i) the m/z 54 ions are ions CH2CHCNH+ generated by self-protonation in ion–dipole stabilized hydrogen-bridged dimer ions [CH2CHCNH–C(CN)CH2]+ and [CH2CHCNH–C(H)C(H)CN]+; the proton shifts in these ions are associated with a small reverse barrier; (ii) dissociation of the H-bridged ions into CH2CCNH+ or CHCH–CNH+ by self-catalysis is energetically feasible but kinetically improbable: experiment shows that the m/z 53 ions are CH2CHCN+ ions, generated by back dissociation; (iii) the peaks at m/z 79, 80 and 105 correspond with the losses of HCN, C2H2 and H, respectively. The calculations indicate that these ions are generated from dimer ions that have adopted the (much more stable) covalently bound “head-to-tail” structure [CH2CHCN–C(H2)C(H)CN]+; experiments indicate that the m/z 79 (C5H5N) and m/z 105 (C6H6N2) ions have linear structures but the m/z 80 (C4H4N2) ions consist of ionized pyrimidine in admixture with its stable pyrimidine-2-ylidene isomer. Acrylonitrile is a confirmed species in interstellar space and our study provides experimental and computational evidence that its dimer radical cation yields the ionized prebiotic pyrimidine molecule

Dissociation of protonated oxalic acid [HOOC-C(OH)2]+ into H30++CO+CO2: an experimental and CBS-QB3 computational study

The predominant dissociation process observed for metastable protonated oxalic acid ions HOOC-C(OH)2+ (generated by self-protonation) leads to H3O++CO+CO2. We have traced the mechanism of this intriguing reaction using the CBS-QB3 model chemistry. Our calculations show that a unique ter-body complex, O=C=O ...H3O+...CO, plays a key role in the rearrangement process. This complex can also dissociate to the proton bound dimers [H2O...H...O=C=O]+ and [H2O...H...CO]+ which are minor processes observed in the metastable ion mass spectrum. A further minor process leads to the proton bound dimer O=C=O...H+...CO which is formed by water extrusion from the ter-body complex. Argments are provided that the ter-body complex is also generated in the ion source by the collision encounter between neutral and ionized oxalic acid