Vibrational Excitation Hindering an Ion-Molecule Reaction: The c−C3H2+<

Abstract

Experiments within a cryogenic 22-pole ion trap have revealed an interesting reaction dynamic phenomenon, where rovibrational excitation of an ionic molecule slows down a reaction with a neutral partner. This is demonstrated for the low-temperature hydrogen abstraction reaction c-, where excitation of the ion into the ν 7 antisymmetric C-H stretching mode decreased the reaction rate coefficient toward the products c-Supported by high-level quantum-chemical calculations, this observation is explained by the reaction proceeding through a c-C 3 H 2 þ − H 2 collision complex in the entrance channel, in which the hydrogen molecule is loosely bound to the hydrogen atom of the c-C 3 H 2 þ ion. This discovery enables high-resolution vibrational action spectroscopy for c-C 3 H 2 þ and other molecular ions with similar reaction pathways. Moreover, a detailed kinetic model relating the extent of the observed product depletion signal to the rate coefficients of inelastic collisions reveals that rotational relaxation of the vibrationally excited ions is significantly faster than the rovibrational relaxation, allowing for a large fraction of the ions to be vibrationally excited. This result provides fundamental insight into the mechanism for an important class of chemical reactions, and is capable of probing the inelastic collisional dynamics of molecular ions.

Deviation of the rate of the reaction from Langevin behaviour below 1 K, branching ratios for the and product channels, and product-kinetic-energy distributions

“…Correcting the product-ion signals for these losses leads to the branching ratios depicted in the lower panel of Figure 4. Apart from a very small fluctuation, which we attribute to a weak variation of the background signal that is not entirely corrected by the background subtraction, the branching ratios extracted from the measurements performed up to τ = 15 µs consistently yield a value of 0.341 (15) for η. The D-atom-transfer channel is therefore approximately twice less likely than the proton-transfer channel at low collision energies.…”

“…Accurate data on their rates, on branching ratios for competing product channels, and on the quantum-state distribution of the reaction products are relevant for the modelling of chemical processes in plasmas, planetary atmospheres [4,5] and interstellar clouds [6][7][8][9][10][11][12]. Measurements of thermal reaction rates and collision-energy-dependent reaction cross sections have been carried out down to about 10 K for numerous ion-molecule reactions using guidedion beam techniques [13][14][15] and in supersonic beams expanding out of Laval nozzles (CRESU technique for 'Cinétique de Réaction en Ecoulement Supersonique Uniforme' [16]). The study of ion-molecule reactions at temperatures below about 10 K or low collision energies has long been hampered by heating effects of the reactant ions by stray electric fields in the reaction volume.…”

Roadmap on dynamics of molecules and clusters in the gas phase

“…18) or Fourier-transform ion cyclotron resonance (FT-ICR) is conventional tools to evaluate ionmolecule reactivity. Multipolar ion traps such as the 22-pole trap have become, in its cryogenically cooled version, a standard tool for the measurement of rate constants of low-temperature astrochemically relevant reactions of cations and anions [85,86]. The quest to gain insights into fundamental aspects of chemical reactions has stimulated the development of molecular beam-based devices: Crossed-beams apparatuses remain forefront techniques to measure product differential scattering cross sections at variable collision energies that can be reduced by adopting lower crossing angles or merging supersonic beams onto a single axis (merged beams, see below).…”

“…with simple molecules (e.g., ortho and para H 2 ) as well as H/D atoms from room temperature down to about 10 K (for a recent example, see [85]). By coupling 22-pole traps with IR lasers, the effect of vibrational excitation of the molecular ion on reactivity at low temperatures is possible, revealing unexpected behaviors such as the decrease in the reaction rate due to ro-vibrational excitation [86].…”

Foreword: Prof. Gauss Festschrift

“…Based on either single-determinantal [4][5][6][7][8] or multi-determinantal 9-11 reference functions, Jürgen's developments in coupled cluster theory have further enabled his ventures into computational spectroscopy [12][13][14][15][16] , relativistic quantum chemistry 17,18 , and computational thermochemistry 19 . Most recently, Jürgen has immersed himself into the study of near-exact approximations to the electronic Schrödinger equation, 20 and of the finer details of molecular reactivity, 21,22 to name just a few topics. In addition, Jürgen is one of the principal authors of the CFOUR quantum chemical program package, whose origins go back to his and John F. Stanton's stay at the University of Florida with Rod Bartlett in the early 1990s.…”