Association reactions at low pressure. I. Collision-stabilized association below 1 micron

Abstract

Rate coefficients for collision-stabilized association of a protonated ion AH + and its parent molecule A to form the proton bound dimer A 2H + were measured in an ion cyclotron resonance (ICR) cell at room temperature for the systems where A  dimethyl ether and acetonitrile. Third-order rate coefficients are reported for the dimethyl ether system with the third bodies (CH 3) 2O, Ar, N 2, Ne, He, and for the acetonitrile system with the third bodies CH 3CN, N 2, Ne, and He. A selected ion flow tube (SIFT) study on these same systems provides evidence that the rate coefficient, k f, for (A 2H +)* formation may be less than the collision rate, k c. The lifetimes of the collision complex (A 2H +)* are ≥ 0.5 × 10 −6 s. A consequence of k f < k c is that derivation of the average lifetime of (A 2H +)* with respect to unimolecular decomposition from the third-order rate coefficient may generally lead to values that are too small.