Abstract
Room temperature ionic liquids (RTILs) represent a promising class of chemically tunable, low vapor pressure solvents with myriad kinetic applications that depend sensitively on the nature of gas-molecule interactions at the liquid surface. This paper reports on rovibronically inelastic dynamics at the gas-RTIL interface, colliding supersonically cooled hyperthermal molecular beams of NO (Π1/22, N = 0) from 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (or [Cnmim][Tf2N]) and probing the scattered NO molecules via laser induced fluorescence (LIF) from the A(2Σ) state. Specifically, inelastic energy transfer into NO rovibrational and electronic degrees of freedom is explored as a function of RTIL alkyl chain length (n), incident collision energy (Einc) and surface temperature (Ts). At low collision energies (Einc = 2.7(9) kcal/mol), the scattered NO molecules exhibit a rotational temperature (Trot) systematically colder than Ts for all chain lengths, which signals the presence of non-equilibrium dynamics in the desorption channel. At high collision energies (Einc = 20(2) kcal/mol), microscopic branching into trapping/desorption (TD) and impulsive scattering (IS) pathways is clearly evident, with the TD fraction (α) exhibiting a step-like increase between short (n = 2, 4) and long (n = 8, 12, 16) alkyl chains consistent with theoretical predictions. For all hydrocarbon chain lengths and RTIL temperature conditions, NO rotational excitation in the IS channel yields hyperthermal albeit Boltzmann-like distributions well described by a “temperature” (TIS = 900 -1200 K) that decreases systematically with increasing n. Non-adiabatic, collision induced hopping between ground and excited spin-orbit states is found to be independent of RTIL alkyl chain length and yet increase with collision energy. The scattering data confirm previous experimental reports of an enhanced presence of the alkyl tail at the gas-RTIL interface with increasing n, as well as provide support for theoretical predictions of an alkyl length dependent shift between chains oriented parallel vs. perpendicular to the surface normal.
Highlights
Room temperature ionic liquids (RTILs) are molten salts with melting temperatures < 100 ◦C which represent a novel class of green solvents with high electrical/thermal conductivity,[1] low vapor pressures,[2] and high thermal stability.[3]
The present study significantly extends our previous efforts[46,47] to elucidate equilibrium/nonequilibrium collision dynamics of radicals at the gas-RTIL interface, and how these dynamics are affected by conformation changes at the surface
Sample laser induced fluorescence (LIF) spectra (Einc = 20(2) kcal/mol) for both i) incident and ii) scattered NO molecules are found in the top panel of Fig. 1, which highlight the extreme differences between incident (1K) and outgoing quantum state distributions
Summary
Room temperature ionic liquids (RTILs) are molten salts with melting temperatures < 100 ◦C which represent a novel class of green solvents with high electrical/thermal conductivity,[1] low vapor pressures,[2] and high thermal stability.[3] These novel liquids consist purely of ions and have many valuable applications in catalytic reactions,[4,5,6] Li-ion batteries,[7,8] fuel cells,[9,10] CO2 capture,[11,12,13,14] and solar cells.[15,16] Of particular practical importance is that interfacial and bulk properties of RTILs. 105207-2 A.
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