Gas-phase heteroaromatic substitutions. 5. Gas-phase protonation of five-membered heteroaromatic rings by hydro-t-helium(1+)-3He (3HeT+) ions. A model reaction for a theoretical approach to heteroaromatic reactivity

The reactions of the powerful Broensted acid, the 3HeT+ ion from the β decay of mol. tritium, with gaseous pyrrole, N-methylpyrrole, furan, and thiophene are reported. The tritiated heteroarom. substrate is the major product, and it is accompanied in the case of furan by very minor amts. (ca. 3%) of tritiated crotonaldehyde. The nature and the yields of the products and their intramol. tritium distribution suggest that the gas-phase protonation reaction on simple five-membered heteroaroms. is regulated by factors related to the electrostatic interactions established within the encounter pair. A close correspondence exists between the gas-phase protonation results and recent theor. and exptl. evidence concerning related substitution processes.