A selected-ion flow tube study of some gas-phase ion-molecule reactions of potential relevance to the chemistry of dense interstellar clouds

摘要

Results are reported for the studies of several systems of ion-molecule reactions of potential relevance to the chemistry of interstellar clouds. Measurements were obtained using a selected-ion flow tube operated at room temperature (300 ± 5 K) and using helium buffer gas at a pressure of 0.30 ± 0.01 Torr. The proton affinities of C₄H₂ and C₂N₂ were determined by measurement of the rate coefficients for forward and reverse proton transfer reactions involving compounds of similar proton affinity. The results obtained were P A(C₂N₂) = 674 ± 4 kJ mol-¹ and PA(C₄H₂) = 741 ± 4 kJ mol-¹: this latter quantity is significantly below the literature value, based on an earlier measurement obtained from ICR bracketing. Isomerism of the ions C₂N⁺, C₃N⁺, CHN⁺ and CH₂N⁺ was investigated, using reactivity with various neutrals to distinguish between isomers. The ions CCN⁺/CNC⁺ and CCCN⁺/c-C₃N⁺ were distinguished on the basis of their reactivity with H₂: in both instances, the isomer featuring a terminal N atom reacted rapidly while the other isomer was unreactive. Identification of the isomers HCN⁺/HNC⁺ was complicated by the occurrence of tautomerisation of HCN⁺ to the more stable isomer HNC⁺ by the mechanism of 'forth and back' proton transfer which occurred with several neutral reagents: reaction with CF₄ was subsequently used to distinguish between these isomers, since HCN⁺ reacted rapidly with CF₄ while HNC⁺ was unreactive. The reactions of all of these isomeric systems were examined with several neutrals abundant in interstellar clouds. The ions HCNH⁺ and CNH₂⁺ could not be distinguished on the basis of reactivity with the neutrals surveyed: we cannot exclude the possibility that only one of these isomers, HCNH⁺, was formed using the ion producing methods used. The reactivity of several ions C₃HnN⁺ (n = 1 → 4) and C₃HnO⁺ (n = 0 → 3), with various neutrals, was investigated to ascertain the importance of these ions in the interstellar synthesis of acrylonitrile, tricarbon monoxide and propynal. Several ion-molecule reactions of CH₂CHCN were also studied to this end. The results indicate that C₃HnN⁺ (n > 0) and C₃HnO⁺ (n > 0) are unreactive with the most prominent cloud constituents H₂ and CO; thus dissociative recombination of these ions should represent a significant source of the target molecules. Several ion-molecule reactions of the types X⁺ + CH₂HCN, and C₃nN⁺ + X, produce ions which, on dissociative recombination, are expected to yield cyanopolyynes and cyclopropenylidene. Several reactions of the C₃HnO+ ions suggest pathways to higher-order polycarbon monoxides and dioxides. The reactivity of the molecular ions of C₂N₂, C₄N₂ and C₃0₂ have also been studied, to gauge the likely consequences of reactions of such ions within interstellar clouds. The thermochemistry of the reaction HCN⁺ + CF4₄ → CF₃⁺ + HF + CN is explored with regard to the proposal that this reaction may be 'entropy-driven'. The interstellar significance of a novel class of neutral-neutral reactions has been considered. The reactivity of the ions C₄Hn⁺ (n = 0 → 4), C₃HnN⁺ (n = 0 → 4), and C₃HnO⁺ (n = 0 → 3) with the neutrals H₂, CO, C₂H₂ and HCN is discussed in greater detail. Previous studies have determined that ions featuring linear carbon-chain skeletons are more reactive with H₂ and with CO if they feature 'bare' (non-hydrogenated) terminal carbon atoms: the present study suggests that ions with bare terminal C atoms are also more reactive than ions where the terminal atom is N or 0 rather than C. This observation may be explained by the degree of carbene character evident in such ions. These results are also discussed with reference to the predominance of very highly unsaturated linear molecules within interstellar clouds.