Experimental and ab initio studies of the reactive processes in gas phase i-C3H7Br and i-C3H7OH collisions with potassium ions

Abstract

Collisions between potassium ions and neutral i-C3H7Br and i-C3H7OH, all in their electronic ground state, have been studied in the 0.10-10.00 eV center of mass (CM) collision energy range, using the radiofrequency-guided ion beam technique. In K(+) + i-C3H7Br collisions KHBr(+) formation was observed and quantified, while the analogous KH2O(+) formation in K(+) + i-C3H7OH was hardly detected. Moreover, formation of the ion-molecule adducts and their decomposition leading to C3H7(+) and either KBr or KOH, respectively, have been observed. For all these processes, absolute cross-sections were measured as a function of the CM collision energy. Ab initio structure calculations at the MP2 level have given information about the potential energy surfaces (PESs) involved. In these, different stationary points have been characterized using the reaction coordinate method, their connectivity being ensured by using the intrinsic-reaction-coordinate method. From the measured excitation function for KHBr(+) formation the corresponding thermal rate constant at 303 K has been calculated. The topology of the calculated PESs allows an interpretation of the main features of the reaction dynamics of both systems, and in particular evidence the important role played by the potential energy wells in controlling the reactivity for the different reaction channels.

The role of Li+ ions in the gas phase dehydrohalogenation and dehydration reactions of i-C3H7Br and i-C3H7OH molecules studied by radiofrequency-guided ion beam techniques and ab initio methods

“…In the case of Na + + i-C 3 H 7 Br, reactions (1) and (2) are both endothermic, and the reaction pathway shows a small transition state potential energy barrier over reactants 50 but slightly below the asymptotic energies of both reaction products. However, in the case of K + + i-C 3 H 7 Br, both reactions (1) and (2) are also endothermic 22 but there is a high transition potential energy barrier over the asymptotic energy of both reaction products. Taking into account the ab initio information on the adiabatic PESs of all three systems, their reactivity can be mainly correlated with the height of the transition state potential energy barrier between reactants and products.…”

“…For K + + i-C 3 H 7 OH, reaction (1) is endothermic, while (2) is exothermic but the transition state potential energy barrier is so much higher than the asymptotic one of both reactions that it was not possible to measure their excitation functions. 22 As in the case of reactions with isopropyl bromide, the presence of the potential energy well (mainly due to ionmolecule interactions which stabilize the collision complex) allows measuring the adduct excitation functions in reactions with Na + and K + but not Li + . This is probably a consequence of the high reactivity of the [Li− −i-C 3 H 7 OH] + since, while being the most stable structure of the three adducts, it needs to overcome the lowest potential energy barrier in order to arrive at products (1) and (2).…”

“…50) although the reactivity of the Na + + i-C 3 H 7 Br system is two orders of magnitude lower than the Li + + i-C 3 H 7 Br one. As to K + + i-C 3 H 7 Br, quantum chemistry calculations 22 show that both reactions (1) and (2) are much more endothermic that in the case of Na + but now the PES profile along the reaction pathway has a potential energy barrier much higher than the reaction endothermicity, and it proved impossible to measure the reactive excitation function for reaction (1) while for (2) a very low value (5 × 10 27 cm 3 s 1 molec 1 ) 22 was found.…”

“…The main features of the PESs along the reaction path were obtained by ab initio electronic structure calculations, using a methodology which has given good results in previous works. 22 It functions by locating and characterizing stationary points at the Möller Plesset (MP2) [31][32][33][34] level using the 6-31G Pople basis set including polarization functions: p for each hydrogen atom and d for each heavy atom. Geometries and energies of the different stationary points localized along the reaction path were fully optimized as well as those of the reactants and products.…”

“…15 Using crossed beam techniques, Creasy and Farrar studied the dehydrohalogenation of some propyl chloride and propyl bromide isomers induced by collisions with low energy Li + , measuring branching ratios for different reaction channels 16 at fixed collision energies, and also the similarly induced t-butanol dehydration in the gas phase. 17 In our previous studies on ion-molecule dehydrohalogenation or dehydration reactions, [18][19][20][21][22] we reported excitation functions for different reaction channels including some for previously not-described reactions. [14][15][16][17] These studies were complemented by ab initio calculations with which it proved possible to characterize the stationary points associated with the different reaction channels in the ground singlet state potential energy surface (PES) on which the adiabatic reactive processes took place at low collision energies.…”