Use of a Flowing Afterglow SIFT Apparatus To Study the Reactions of Ions with Organic Radicals

This paper describes a new technique for the experimental study of ions with organic radicals. A preliminary report has been published in J. Chem. Phys. 2004, 120, 3531, and we wish to communicate a complete description of this experiment. Clean, intense flows of hydrocarbon radicals/diradicals can be produced by a heated supersonic nozzle and are delivered to a flowing afterglow selected ion flow tube (FA-SIFT) instrument. The reactions of a simple radical, allyl (CH 2 CHCH 2 ), and a diradical, ortho- benzyne ( o -C 6 H 4 ), with hydronium (H 3 O + ) and hydroxide (HO - ) ions were studied at thermal energy. We have studied the following reactions: CH 2 CHCH 2 + H 3 O + → C 3 H 6 + + H 2 O, CH 2 CHCH 2 + HO - → no product ions, o -C 6 H 4 + H 3 O + → C 6 H 5 + + H 2 O, and o -C 6 H 4 + HO - → C 6 H 3 - + H 2 O. We find that proton transfer reactions with H 3 O + occur rapidly (the bimolecular rate constant, k II, is roughly 10 -9 cm 3 s -1 ). However, an unexpected result was obtained for o- C 6 H 4 + HO -; the exothermic proton abstraction is significantly slower than the collision rate by nearly an order of magnitude ( k II is approximately 10 -10 cm 3 s -1 ). We rationalize this by postulating a competing associative detachment. No charged products have been observed for CH 2 CHCH 2 + HO -, presumably because of similar detachment pathways.