Selected reactions of ions and molecules in a flowing afterglow-selected ion flow/drift tube (FA-SIFDT).
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The work presented herein revolves around the design, installation, testing, and use of a new flowing afterglow-selected ion flow drift tube (FA-SIFDT) at the University of Canterbury. This is the latest in the series of FA and SIFT apparatus that have been installed at Canterbury. The second chapter contains a detailed description of the new instrument. Also present is a description of the characterisation of the new FA-SIFT in particular a comparison of two different types of venturi inlet, viz. an annulus and a hole type injector. The performance of both of these types of venturi inlet with respect to pumping efficiency, signal transmission, dissociation of weakly bound cluster ions, and the isomerisation of ions during injection. Despite the greater mechanical complexity of the annulus injector it is concluded that the annulus injector is slightly superior to the hole type injector, predominantly when the injection of ions needs to be at low energy. Next a new application of the SIFT, namely its use in the real time detection, identification and quantification of trace components in gas samples, is presented. This technique has only recently been developed by overseas researchers and its use at Canterbury has been facilitated by the new FA-SIFDT. The technique has been applied to several systems: the analysis of the changes in the largest trace components of human breath (ammonia, acetone, isoprene) during exercise; the monitoring of breath concentrations of organic solvents following exposure; and the headspace analysis of the gases emitted by soil following fertilisation with an artificial urine solution. The reactions of H3+, N2H+, and H3O+ with thirteen different hydrocarbons have been investigated. The rate coefficients and product distributions of these reactions were investigated in order to obtain a better understanding of exothermic proton transfer reactions. The H3+ was generated in two different manners in an effort to get accurate data about the products of proton transfer from ground state H3 . As expected, in most cases where the proton transfer was exothermic a rate coefficient just less than the collision rate was observed. The reactions became more dissociative as more energy was placed into the collision complex with the channels that predominated usually being between 100 and 200 kJ mol-1 exothermic. The proton affinity of cyanogen (C2N2) has been investigated using the FA-SIFDT. The new value has been determined using the equilibrium method with reference to the C2N2/C2H2 and CH3Cl/C2N2 cycles. The new value is 651±2 kJ mol-1 some 24 kJ mol-1 less than the previously tabulated value. The reactions of methylated cyanogen were also investigated with the intention of determining the methyl cation affinity of cyanogen. Instead it has been determined that the CH3+.C2N2 adduct is strongly bonded, a characteristic that has previously been observed for alkyl ion-cyanide functionality type adducts. Two classes of reaction relevant to the lower cosmic ray-induced ionosphere of Saturn's largest moon Titan have been investigated. The first class is the bimolecular reactions of the N3+ and N4+ ion species with a range of hydrocarbons likely to be present in Titan's atmosphere. These ionic species will be formed deep into Titan's atmosphere by the termolecular association of N+ and N2+ (the primary ions formed in Titan's atmosphere) with nitrogen. The association reactions of some of the terminal ions in Titan's atmosphere, viz. H3O+, HCNH+ and c-C3H3+, with methane, ethylene and acetylene in the presence of both helium and nitrogen bath gases. The termolecular reaction rate coefficients are greatly enhanced in the presence of a nitrogen carrier gas. Preliminary results of an investigation into the structure of the HCNH+.C2H4 and HCNH+.C2H2 adducts is also presented. The termolecular association of the CH2CHCN+ and CH2CHCNH+ ions derived from acrylonitrile with neutral acrylonitrile have been investigated over a wide range of pressure in both the SIFT and an ion cyclotron resonance (ICR) mass spectrometer. This has enabled the pressure dependence of these two reaction to be experimentally investigated and theoretically modelled. In the non-protonated case (CH2CHCN+) there is a competition between termolecular association and bimolecular reaction to give CH2CHCNH+. The reaction is observed to pressure saturate without a complete conversion to termolecular kinetics, an assumption made by the commonly used single-well model for association. A different, double-well, association mechanism is proposed for this reaction and modelled theoretically giving an acceptable fit. The CH2CHCNH+/CH2CHCN system shows only termolecular kinetics and is well modelled by a single-well model. Work begun by earlier researchers at Canterbury on the reactions of ionic species with neutral carbon atoms has been continued. However an effective method for forming neutral carbon atoms has still not been found. The ultraviolet photolysis of carbon suboxide (C3O2) was hoped to solve this problem, however the wavelength of photolysis used was too long to get any significant C atom production. A vacuum UV flash system may be required.