Experimental studies of gas-phase ion-molecule reactions (1994)
AuthorsWilson, Paul Francisshow all
Development on both selected ion flow tube (SIFT) and ion cyclotron resonance (ICR) instruments is described. Modifications to the SIFT described here include; a new, off-axis ion source, and new hardware and programs to measure the neutral flow and the ion count using a personal computer. Mechanical, electrical, electronic, and programming modifications to the ICR instrument are described. Several well known ion-molecule reactions are used to calibrate, and monitor the performance of the ICR instrument. The reactions of t-C₄H₉Cl with a number of protonated bases, BH⁺, are reported. The reactions were studied using both the SIFT and the ICR. The branching ratio of the product channels is reported for each reaction. For some bases, the process, BH⁺ + t-C₄H₉Cl →’ t- C₄H₉⁺ + B + HCl appears to be fast, although it is significantly endothermic. The thermochemistry of the system is discussed, and it is suggested that either the tabulated thermochemical values are significantly wrong, or the reaction proceeds via formation of weakly bound complexes which dissociate on focussing in the down stream region of the SIFT. The chemistry of several srtuctural isomers of protonated ethyl cyanide, C₃H₆N⁺ is examined. Two reactions thought to be routes to interstellar synthesis of ethyl cyanide are shown to be unlikely to yield that ion upon dissociative recombination. The association of HCNH⁺ with C₂H₄ is shown to lead to the protonated ethyl isocyanide isomer. The association of CH₃#x207A; with CH₃CN is reasoned to lead to formation of the CH₃CNCH₃#x207A; structure. The isomerism observed is rationalised in terms of the potential surface for the system derived from both experimental observation, and several previous ab initio studies. The reactivity of the methoxymethyl cation with several oxygen and nitrogen bases is reported. The exothermic proton transfer channel is not observed, but competing methyl cation and CH⁺ transfer dominate. The reactivity in both the SIFT and the ICR is explained in terms of several factors. An activation barrier to proton transfer proceeds from ring closure to form the neutral product, oxirane. The SN2 methyl cation transfer process is sterically hindered and results proceeds via a tight transition state, whereas the alkyl transfer process has a greater density of states at the transition state. Where there is a labile hydrogen on the base, the alkyl transfer process dominates because of its' looser transition state. The association reactions of acrylonitrile are reported in both the ICR and SIFT instruments. The reaction of CH₂CHCN⁺ shows competition between proton transfer and association. Proton transfer dominates in the ICR and association dominates in the SIFT. The termolecular rate of formation of the proton bound dimer of acrylonitrile is measured at 1.2 x 10⁻²³cm¶ s⁻±. An RRKM study of the association of CH₃⁺and acetonitrile is reported. The collisional parameters of both helium and acetonitrile bath gases are estimated. The average downward energy transferred per collision, ‹ΔΕdown›, for helium is estimated as 300 cm-⁻±, and for acetonitrile as 950 cm-⁻±. The fall off of the association reaction with pressure is shown in comparison with experimental results. The ion-molecule reactions of acetylene have been studied, and the results confirm earlier work. The ions C₆H₅⁺, and C₆H₄⁺ are shown to exist as a mixture of two or more isomers of differing reactivity. One isomer reacts with unsaturated hydrocarbons at the collision rate while the other is unreactive. C₆H₄⁺ exists as a mixture of isomers when formed from sequential ion-molecule reactions of acetylene or electron impact or chemical ionisation on halobenzenes. C₆H₄⁺ exists as a mixture of two isomers when formed from sequential ion-molecule reactions of acetylene.