Scattering Dynamics of Hyperthermal Oxygen Atoms on Ionic Liquid Surfaces: [emim][NTf₂] and [C₁₂mim][NTf₂]

Abstract

Collisions of hyperthermal oxygen atoms, with an average laboratory-frame translational energy of 520 kJ mol−1, on continuously refreshed ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([emim][NTf2]) and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C12mim][NTf2]), were studied with the use of a beam-surface scattering technique. Time-of-flight and angular distributions of inelastically scattered O and reactively scattered OH and H2O were collected for various angles of incidence with the use of a rotatable mass spectrometer detector. For both O and OH, two distinct scattering processes were identified, which can be empirically categorized as thermal and nonthermal. Nonthermal scattering is more probable for both O and OH products. The observation of OH confirms that at least some reactive sites, presumably alkyl groups, must be exposed at the surface. The ionic liquid with the longer alkyl chain, [C12mim][NTf2], is substantially more reactive than the liquid with the shorter alkyl chain, [emim][NTf2], and proportionately much more so than would be predicted simply from stoichiometry based on the number of abstractable hydrogen atoms. Molecular dynamics models of these surfaces shed light on this change in reactivity. The scattering behavior of O is distinctly different from that of OH. However, no such differences between inelastic and reactive scattering dynamics have been seen in previous work on pure hydrocarbon liquids, in particular, the benchmark, partially branched hydrocarbon, squalane (C30H62). The comparison between inelastic and reactive scattering dynamics indicates that inelastic scattering from the ionic liquid surfaces takes place predominantly at nonreactive sites that are effectively stiffer than the reactive alkyl chains, with a higher proportion of collisions sampling such sites for [emim][NTf2] than for [C12mim][NTf2].

Inelastic Scattering of Radicals at the Gas–Ionic Liquid Interface: Probing Surface Dynamics of BMIM–Cl, BMIM–BF4, and BMIM–Tf2N by Rovibronic Scattering of NO [2Π1/2(0.5)]

“…In particular, the oxygen scattering studies have highlighted the presence of anions in the interfacial region. 26,49 The current results from NO + RTIL collisional scattering, which should be similarly sensitive to the composition of the very topmost layers of the liquid, strongly support and confirm the above expectations that anions are present at the interface. This is immediately clear from the fact that the choice of RTIL counteranion has a strong effect on both the rotational and spin−orbit electronic distributions of the scattered NO products.…”

Determining the composition of the vacuum–liquid interface in ionic-liquid mixtures

“…14,[16][17][18] Complementing the established techniques, we have developed an alternative approach based on reactive-atom scattering (RAS), combined either with mass spectrometric (RAS-MS) or laser-induced fluorescence (RAS-LIF) detection. [18][19][20][21][22][23][24][25][26] The essence of this method, of which the RAS-LIF variant is the one we use here, is that projectiles interrogate the liquid surface via selective reaction with specific functional groups, producing a gas-phase product which is detected quantitatively. The dynamical attributes of these products confirm that they originate in the extreme outer layers of the liquid.…”

Dynamic QM/MM: A Hybrid Approach to Simulating Gas-Liquid Interactions

“…1 3 ] surface we found that the anion is more abundant at the surface than the cation and that the ethyl-chains that lie on the surface tend to stick up towards the vacuum [37]. However, we found that there exists a slightly different surface topology for [emim][NTf 2 ], which includes a larger anion that keeps the ethyl chain from protruding out of the surface [36]. In contrast, the surface of [C 12 mim][NTf 2 ] has long hydrocarbon chains sticking up into the vacuum, causing a noticeable difference in the reaction profile that occurs upon gaseous atom collision.…”

“…This means that there are readily accessible H atoms sticking up out of the surface for H abstraction or H elimination reactions to occur before the incident O crosses the surface threshold (the point where the density is equal to half the bulk density). Our study was done in tandem with Minton and coworker's similar experimental study [36] 3 ] surface causing minor changes in the local chemistry as shown in Table 1. Nearly all of the incident Ar atoms scatter from the surface (a probability of 0.91) with a small fraction trapped at the end of the simulation time of 7.3 ps.…”

“…Our F þ squalane studies [29] have been used to interpret experiments done by Nesbitt and coworkers at thermal collision energies [33][34][35], showing that there is a component of the HF product vibrational and rotational distributions which involves escape of HF from the surface with little relaxation. In more recent work we have considered the collisions of atomic oxygen (and also hyperthermal argon) with the surfaces of ionic liquids [36,37], providing detailed information about a class of chemical reactions that has not been considered previously, and enabling the interpretation of experiments by Minton, McKendrick, and collaborators.…”