Vibrationally state-selected reactions of ammonia ions. II. NH+3(v)+CH4

Abstract

The effects of vibrational excitation of the ammonia ν2 inversion mode on the reaction of NH+3(X̃,v=0 to 9) with methane have been measured in a tandem quadrupole mass spectrometer over the center-of-mass collision energy range 1.5 to 10 eV. The hydrogen abstraction channel is enhanced by nearly a factor of 2 at nine quanta of vibrational energy relative to the v=0 level of the ion. Added vibration in the ammonia ion umbrella-bending mode facilitates the transition to the NH+4 product ion geometry. Protonated methylamine is formed at lower kinetic energies by attack of the ion at the methane carbon center, but with increasing vibrational excitation of the ammonia ion, the protonated methylamine decomposes by 1,2-elimination of molecular hydrogen and by C–N bond scission.

Rotational and Vibrational Energy Effects on Ion−Molecule Reactivity As Studied by the VT-SIFDT Technique

“…For example, C−H stretching facilitates hydrogen abstraction in the reaction of O - with CH 4 , and C−S stretching promotes S + transfer in the reaction of OCS + with C 2 H 2 . Zare and co-workers have used geometric arguments to explain reactions involving umbrella mode excitation in NH 3 + . , In contrast, our work on nucleophilic displacement reactions of CH 3 X shows that vibrations along the supposed reaction coordinate do not affect the kinetics measurably. This is also observed in the reaction of NH 3 + with ND 3 .…”

“…18 Zare and coworkers have used geometric arguments to explain reactions involving umbrella mode excitation in NH 3 + . 21,22 In contrast, our work on nucleophilic displacement reactions of CH 3 X shows that vibrations along the supposed reaction coordinate do not affect the kinetics measurably. This is also observed in the reaction of NH 3 + with ND 3 .…”

“…Most other techniques involve photoionization to vibrationally state select the ion. Multiphoton ionization (MPI) has been used by a number of groups. − The most comprehensive study involving MPI has been the recent work of Anderson and co-workers on the [C 2 H 2 :OCS] + system, where they examined the effects of nine different vibrational levels of the two ions as well as studying kinetic energy dependences and differential scattering of the products and photoelectron photoion coincidence (PEPICO). , Both of these techniques are well-suited to study vibrational effects but have difficulty with rotational state selection.…”

Reaction of state-selected ammonia ions with methane

“…This observation is in agreement with the previous investigation of Conaway, Ebata, and Zare. 6 A product channel showing similar collision energy dependence has been seen in the reaction of ammonia ion with both D 2 O 3 and D 2 . 13 In the former case the mechanism giving rise to the NH 2 D ϩ product was determined to be a twostep process: collision-induced dissociation of the ammonia ion resulting in the formation of a ͓NH 2 D 2 O͔ ϩ complex which would decay to the formal isotope exchange product.…”

“…The only state-selected study to date is that of Conaway, Ebata, and Zare. 6 They measured the reactivity of NH 3 ϩ ϩCH 4 as a function of both collision energy and ammonia 2 bending. The observed ionic products include the following: NH 4 ϩ from direct atom abstraction, CH 3 ϩ , CH 3 NH 3 ϩ , and CH 2 NH 2 ϩ .…”