Ionic liquids (ILs) are salts with an extremely low melting point. Substantial efforts have been made to address their low melting point from the enthalpic standpoint (i.e. interionic interactions). However,...

The rotational dynamics of carbon monoxide (CO) in ionic liquids (ILs) was investigated by nuclear magnetic resonance (NMR) relaxation measurements and molecular dynamics (MD) simulations. NMR spin−lattice relaxation time measurements were performed for 17 O-enriched CO in 10 ILs (four imidazolium-cation-based, four phosphonium-cation-based, and two ammonium-cation-based ILs, all paired with the bis(trifluorosulfonylmethane)imide anion). In combination with previously reported data for five ILs and viscosity data, our results indicated that the obtained rotational relaxation times (τ 2R ) were much smaller than those predicted using the Stokes−Einstein− Debye (SED) theory. For the same viscosity/temperature values, the τ 2R −1 value increased linearly with increasing carbon number of the alkyl group in the cation. The deviation from the SED equation was due to the insensitivity of τ 2R to the carbon number, even though a higher carbon number generally leads to higher viscosity values for ILs. To investigate the unique rotational properties of CO in the ILs, MD simulations were performed on five representative ILs (two imidazolium, two phosphonium, and one ammonium) containing CO solutes. From rotational correlation function analyses, the CO rotation mainly occurred in a free rotation-like manner within 1 ps, which explained the relative insensitivity of CO rotation to viscosity. In the subsequent time scale (>1 ps), the minor component of the CO rotation was discriminated among different ILs. It was strongly suggested that, because CO preferably locates in the outer part of the alkyl groups in the cation, the slow CO rotation is correlated with the outer alkyl dynamics, which are decoupled from the whole cation rotation.

Translational diffusion coefficients of two transient species in 1-alkyl-3-methylimidazolium-based ionic liquids (Cnmim+) with different alkyl chain lengths (n = 4, 8, 12) and the bis(trifluoromethanesulfonyl)imide anion were measured by transient grating spectroscopy. The systems of interest are the benzophenone ketyl radical (BPK) produced by the hydrogen-abstraction reaction of benzophenone (BP) and the zwitterionic merocyanine dye (MC) produced by the photoisomerization of 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran (SP). The diffusion coefficients of BPK and MC closely obey the Stokes-Einstein (SE) relation in the ionic liquids regardless of the alkyl chain lengths of the cations, whereas the diffusion coefficients of BP are larger than those predicted by the SE equation. The temperature dependences of the translational diffusion coefficients of BP, BPK, and MC reveal that the diffusional activation energies of BPK and MC are larger than those of BP in all solvents. The origin of this difference is discussed in terms of intermolecular interactions within the ionic-liquid solvents.

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