Dehydrohalogenation and Dehydration Reactions of i-C₃H₇Br and i-C₃H₇OH by Sodium Ions Studied by Guided Ion Beam Techniques and Quantum Chemical Methods

Abstract

Dehydrohalogenation and dehydration reactions of gas-phase i-C3H7Br and i-C3H7OH molecules induced by collision with Na(+), all participants being in their electronic ground state, were studied experimentally in our laboratory using a radiofrequency-guided ion beam apparatus and covering the 0.10-10.00 eV center of mass (CM) energy range. In Na(+) + i-C3H7Br collisions the formation of [C3H6-Na](+) and [HBr-Na](+) by dehydrohalogenation was observed and quantified, as well as that of the ion-molecule adduct [Na-i-C3H7Br](+) together with its decomposition products C3H7(+) and NaBr. In Na(+) + i-C3H7OH collisions the dehydration product [H2O-Na](+) was also found, while [C3H6-Na](+) was hardly detected. Moreover, the [Na-i-C3H7OH](+) adduct formation as well as its decomposition into C3H7(+) and NaOH were also quantified. For all these processes, absolute reaction cross sections were measured as a function of the CM collision energy. From measured excitation functions, rate constants for the formation of [C3H6-Na](+), [HBr-Na](+), and [H2O-Na](+) at 303 K were obtained. Complementing the experiments, exhaustive ab initio structure calculations at the MP2 level of theory were performed, giving information on the most relevant features of the potential energy surfaces (PESs) where the dehydrohalogenation, dehydration, and decomposition reactions take place adiabatically for both collision systems. On these PESs different stationary points associated with potential energy minima and transition state barriers were characterized, and their connectivity was ensured using the intrinsic-reaction-coordinate method. The main topology features of the ab initio calculated PESs allowed a qualitative interpretation of the experimental data also exposing the role of the sodium ion as a catalyst in elimination reactions.

The reactivity of cyclopropyl cyanide in titan's atmosphere: a possible pre-biotic mechanism

“…In fact, at 4 and 6 eV nominal CE the ion-molecule collision complex can successfully overcome the barrier separating adduct (1) and T1 structures. In previous studies on reactive ionmolecule processes involving non-covalent adducts 58,59 , it was found that their formation was relevant only at low energies. Hence, for nominal CEs higher than the potential energy barrier associated to P1 structure formation, non-covalent adduct contribution ought to be regarded as negligible, so that only P1 is expected to contribute to the measured signal at m/z 135.…”

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 Na + + i-C 3 H 7 OH, the exothermic character of reactions (1) and (2) is smaller than in the case of the lithium ion, with the latter being faster at some 9.6 × 10 9 cm 3 s 1 molec 1 , while is proved impossible to measure the excitation function for the more endothermic reaction (1) experimentally. 50 For this system the transition state potential energy barrier increases. 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.…”

“…For M + + i-C 3 H 7 Br, the reaction endothermicity increases from lithium to potassium ions, being reaction cross sections for Li + and Na + in the range of a few Å 2 (although something larger in the case of Na + ) 50 while those for K + are in the tenths Å 2 range. 22 For M + + i-C 3 H 7 OH a similar behavior is also found, showing large cross section values in collisions with Li + and Na + (in the high 10 Å 2 range for the former and of a few 10 2 Å 2 range for the latter) while for K + , cross section values are in the 10 2 Å 2 range.…”

“…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.…”

“…The reactivity in both channels was also enough to allow an estimation of the corresponding rate constants (5.1 × 10 13 cm 3 s 1 molec 1 for reaction (1) and 2.6 × 10 13 cm 3 s 1 molec 1 for (2), as given in Ref. 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.…”