Effects of collision energy and vibrational excitation of CH3+ cations on its reactivity with hydrocarbons: But-2-yne CH3CCCH3 as reagent partner

Abstract

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH + CHCCCH (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of CH, stemming from H loss from the addition complex. This is the only observed condensation product with the formation of new C-C bonds, and it might represent a viable pathway for the synthesis of complex organic species in astronomical environments and laboratory plasmas.

Study of the Reactivity of CH3COOH+• and COOH+ Ions with CH3NH2: Evidence of the Formation of New Peptide-like C(O)–N Bonds

“…It is worth mentioning that the cross section related to the m / z 31 ion, associated with the charge transfer, shows a significant decrease with collision energy (with a slope close to the E –1/2 trend, drawn in Figure as a guide). This is surprising, as the usual behavior of charge transfer, due to its long-range interaction, is relatively insensitive to the collision energy as has been previously observed in our apparatus. − To reconcile these observations, our hypothesis is that the m / z 31 ion is a poorly stable product of charge transfer, which dissociates to form the m / z 30 ion. Indeed, the dissociative charge transfer of CH 3 NH 2 +• is known to occur for an internal energy of 1.5 eV .…”

Roadmap on dynamics of molecules and clusters in the gas phase

“…Understanding how the initial quantum states of the reactants or their structures (i.e., isomers) drive the reactivity is also relevant to correctly model complex media such as plasmas or planetary ionospheres in which excited species and isomers are present. Photo-excitation with lasers or synchrotron radiation associated with various experimental setups (crossed beam, GIB or 3D trap) and ion detection techniques is a method of choice to induce specific population of reactants and to control the internal energy of either the neutral (see, for instance, [87] where vibrational excitation of the neutral partner is achieved using IR laser) or the charged species [93][94][95][96][97]. Quantum state selectivity of atomic and molecular cations can be obtained using threshold photoelectron-photoion coincidence (PEPICO) techniques, in which the time coincidence of ions with electrons having near zero kinetic energy allows to prepare vibrationally and electronically state-selected ions.…”

“…4 and 5 for synchrotron radiation tools for spectroscopic and dynamical processes) and are used, for instance, at the VUV beamline of SOLEIL synchrotron facility, to study state-selected reactivity in a GIB apparatus on which absolute reaction cross sections are measured, as a function of both the ion excitation and the collision energy [93]. Recent developments also allow to generate state selected molecular ions from beams of clusters or radicals [94], thus widening the range of possible studies, in particular to characterize the role of micro-solvation and isomer-specific reactivity [95].…”

Quantum dynamics of vibration–vibration energy transfer for vibrationally excited HF colliding with H2

“…Collision‐induced energy transfer process plays an important role in the non‐thermodynamic equilibrium systems, such as outer atmosphere, interstellar medium, and combustion . Because of the wide distribution of the hydrogen fluoride (HF) and hydrogen (H 2 ) molecules in a lot of non‐thermodynamic equilibrium systems, the inelastic collision between HF and H 2 has drawn considerable attentions .…”