Ion imaging study of reaction dynamics in the N+ + CH4 system

Abstract

The velocity map ion imaging method is applied to the ion-molecule reactions of N(+) with CH(4). The velocity space images are collected at collision energies of 0.5 and 1.8 eV, providing both product kinetic energy and angular distributions for the reaction products CH(4)(+), CH(3)(+), and HCNH(+). The charge transfer process is energy resonant and occurs by long-range electron transfer that results in minimal deflection of the products. The formation of the most abundant product, CH(3)(+), proceeds by dissociative charge transfer rather than hydride transfer, as reported in earlier publications. The formation of HCNH(+) by C-N bond formation appears to proceed by two different routes. The triplet state intermediates CH(3)NH(+) and CH(2)NH(2)(+) that are formed as N(+)((3)P) approaches CH(4) may undergo sequential loss of two hydrogen atoms to form ground state HCNH(+) products on a spin-allowed pathway. However, the kinetic energy distributions for formation of HCNH(+) extend past the thermochemical limit to form HCNH(+) + 2H, implying that HCNH(+) may also be formed in concert with molecular hydrogen, and requiring that intersystem crossing to the singlet manifold must occur in a significant (~25%) fraction of reactive collisions. We also report GAUSSIAN G2 calculations of the energies and structures of important singlet and triplet [CNH(4)(+)] complexes that serve as precursors to product formation.

The Selective Role of Long‐Range Forces in the Stereodynamics of Ion–Molecule Reactions: The He++Methyl Formate Case From Guided‐Ion‐Beam Experiments

“…[24] Ion-molecule reactions that have been studied in such detail are still limited, relativelys parse andi nm ostc ases confined to simple systemsw ith as mall number of atoms (for recente xamples, see refs. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]). …”

Imaging ion-molecule reactions: Charge transfer and halide transfer reactions of O+ with CH3Cl, CH3Br, and CH3I

“…As noted in our other publications [20,32,33], the kinetic energy distributions in charge transfer mirror the distributions of internal states consistent with the Franck-Condon factors connecting the neutral reactant and its ion [34,35]. The low energy photoelectron spectra of methyl halides provide some insight on this point: the spectra generally show a weakly structured band near threshold, consistent with Franck-Condon activity in the CÀ ÀX stretch of the methyl halide [36].The product angular distributions obtained by integrating the velocity space distributions over recoil speed may be calculated according to Eq.…”

Ion imaging study of dissociative charge transfer in the N2+ + CH4 system

“…1 Because of the high solar flux incident on the outermost layer of Titan's atmosphere, ion-molecule reaction schemes initiated by the primary photoions N 2 + and N + in reactions with methane have been postulated to be critical in establishing equilibrium concentrations of many chemical species detected by the Cassini mission to Saturn. 2,3 In a previous publication, 4 we reported on the dynamics of the reactions of N + with CH 4 , discussing the products of charge transfer and C-N bond formation to produce HCNH + . The relative values of the recombination energy of N + and the ionization potential of CH 4 make charge transfer to form CH 4 + exoergic, and approximately 75% of those products may dissociate to form CH 3 + .…”

“…2,3 In a previous publication, 4 we reported on the dynamics of the reactions of N + with CH 4 , discussing the products of charge transfer and C-N bond formation to produce HCNH + . The relative values of the recombination energy of N + and the ionization potential of CH 4 make charge transfer to form CH 4 + exoergic, and approximately 75% of those products may dissociate to form CH 3 + . In contrast, the larger recombination energy of N 2 + compared to N + (15.58 vs. 12.61 eV) and the large N-N bond strength in N 2 + cause the reactions of the molecular ion to result exclusively in dissociative ionization of CH 4 , and prohibit exoergic C-N bond formation.…”

“…The reactions of N 2 + and CH 4 have been the subject of several investigations, ranging from determination of reaction rates and product branching ratios to studies of energy disposal under single collision conditions. Thermal rate measurements have been carried out by several investigators in the specific context of exploring the ion chemistry of Titan both at room 5-7 and at very low temperatures, 8 and recently, Gichuhi and Suits 9 have employed rovibrationally state-selected N 2 + ions in conjunction with ion imaging methods to examine the branching ratio for the products of dissociative charge transfer.…”