Termolecular ion-molecule reactions in Titan's atmosphere. II: The structure of the association adducts of HCNH+ with C2H2 and C2H4

The ion-molecule reactivity of the products formed in the association react ions of HCNH+ with C2H2 (C3H4N+) and C2H4 (C3H6N+) has been investigated to provide information on the structures of the adducts thus formed. The C3H4 N+ and C3H6N+ adducts were formed in the reaction flow tube of a flowing af terglow sourced-selected ion flow tube (FA-SIFT) and their reactivity with a neutral molecular "probe" examined. The reactivity of possible known stru ctural isomers for the C3H4N+ and C3H6N+ ions was investigated in both the FA-SIFT and an ion cyclotron resonance spectrometer (ICR). Ab initio invest igations of the potential energy surfaces for both structures at the G2(MP2 ) level have also been performed and structures corresponding to local mini ma on both surfaces have been identified and evaluated. The results of thes e experimental and theoretical studies show that at room temperature, the C 3H4N+ adduct ion contains two isomers; a less reactive one that is likely t o be a four-membered cyclic covalent isomer (similar to 70%) and a faster r eacting component that is probably an electrostatic complex (similar to 30% ). The C3H6N+ adduct ion formed from HCNH+ + C2H4 at room temperature is a single isomer that is likely to be the four-membered covalently bound cycli c CH2CH2CHNH+ species. (J Am Soc Mass Spectrom 2001, 12, 557-564) (C) 2001 American Society for Mass Spectrometry.