Mode selectivity in ion–molecule reactions of NH3+

Abstract.

Ammonia ions (NH₃ ⁺) are prepared in two internal states of nearly the same energy, a state with five quanta in the umbrella bending mode (E_int=0.60 eV) and another state with one quantum in the all-symmetric stretch and two quanta in the umbrella bending mode (E_int=0.63 eV). These ions are allowed to react with different neutral reagents, and the product ions are mass analyzed and detected. For each reaction, the product branching ratios are measured as a function of center-of-mass collision energy. Whereas reactions with D₂O, D₂, and CD₄ are found to be uninfluenced by the state preparation of the NH₃ ⁺ reagent, reactions with ND₃, partially deuterated methylamine (CD₃NH₂), and tetrahydrofuran (c-(CH₂)₄O)show varying degrees of mode selectivity, the reaction with tetrahydrofuran to the least extent. We suggest that mode selectivity in these ion–molecule reactions should be a general feature when the charge transfer channel is energetically open, the geometry of the reagent ion differs markedly from the corresponding neutral, and different reaction pathways compete for product production.