Rotational dependence of the proton-transfer reaction HBr+ + CO2 → HOCO+ + Br. II. Comparison of HBr+ (2Π3/2) and HBr+ (2Π1/2)

Abstract

The effects of reactant ion rotational excitation on the exothermic proton-transfer reactions of HBr(+)((2)Π(1/2)) and DBr(+)((2)Π(1/2)), respectively, with CO(2) were studied in a guided ion beam apparatus. Cross sections are presented for collision energies in the center of mass system E(c.m.) in the range of 0.23 to 1.90 eV. The HBr(+)/DBr(+) ions were prepared in a state-selective manner by resonance enhanced multiphoton ionization. The mean rotational energy was varied from 3.4 to 46.8 meV for HBr(+)((2)Π(1/2)) and from 1.8 to 40.9 meV for DBr(+)((2)Π(1/2)). Both reactions studied are inhibited by collision energy, as expected for exothermic reactions. For all collision energies considered, the cross section decreases with increasing rotational energy of the ion, but the degree of the rotational dependence differs depending on the collision energy. For E(c.m.) = 0.31 eV, the cross sections of the deuteron transfer are significantly larger than those of the proton transfer. For higher E(c.m.) they differ very little. The current results for the exothermic proton transfer are systematically compared to previously published data for the endothermic proton transfer starting from HBr(+)((2)Π(3/2)) [L. Paetow et al., J. Chem. Phys. 132, 174305 (2010)]. Additional new data regarding the latter reaction are presented to further confirm the conclusions. The dependences on rotational excitation found cannot be explained by the corresponding change in the total energy of the system. For both the endothermic and the exothermic reaction, the cross section is maximized for the smallest rotational energy, at least well above the threshold.

Self-reactions in the HCl+ (DCl+) + HCl system: a state-selective investigation of the role of rotation

“…12,13 The apparatus consists of three zones, (i) the ion source, (ii) the reaction zone and (iii) the analysis zone. 12,13 The apparatus consists of three zones, (i) the ion source, (ii) the reaction zone and (iii) the analysis zone.…”

“…3,12,13 We are not aware of another example of the cross section running through a minimum as a function of E rot . We suggest that such a minimum could originate from the relative rotational velocities of two reacting moieties.…”

“…E.g., for the charge transfer reaction Kr + + HCl -Kr + HCl + , the cross section decreases with increasing rotational temperature of the neutral HCl. 12,13 That particular reaction system offered the rather unique possibility to switch a reaction from an endothermic version to an exothermic version by either addressing the HBr + ions in the spin-orbit ground state, 2 P 3/2 , or in the excited spin-orbit state, 2 P 1/2 . [3][4][5] Interestingly the analogous reaction Ar + + HCl leading to either charge transfer or hydrogen atom abstraction does not exhibit a similar rotational temperature dependence of the rate constant.…”

Potential energy surfaces for the HBr+ + CO2 → Br + HOCO+ reaction in the HBr+ 2Π3/2 and 2Π1/2 spin-orbit states

“…There is considerable experimental interest in the chemical dynamics of ion-molecule reactions with the reactants in specific quantum states. [1][2][3][4][5][6][7] Viggiano and co-workers 1 have investigated the influence of vibrational and rotational energies on the rates of ion-molecule reactions. For exothermic reactions, they found that rotational energy has a negligible influence on the reaction efficiency, while rotational energy increases the efficiency for endothermic reactions.…”

“…In recent research, Paetow et al 6,7 used a guided ion beam apparatus to measure the rate constants for HBr + and DBr + , in the 2 Π 3/2 and 2 Π 1/2 spin-orbit (SO) states, reacting with CO 2 to form Br + HOCO + /DOCO + . The mean rotational energy of the HBr + and DBr + ions was varied and it was found that the rate constant decreased with increase in rotational energy.…”