Ultrafast solvation dynamics of nitrous oxide in alkylmethylimidazolium ionic liquids

Abstract

Nitrous oxide is a potent greenhouse gas and the leading ozone depleting pollutant. 1-alkyl-3-methylimidazolium bistriflimide based ionic liquids are effective at trapping N2O as well as CO2. Despite being similar to CO2 in many ways, N2O has a permanent dipole moment that allows it to dissolve in both nonpolar and polar solvents. Fourier transform infrared spectroscopy, polarization-controlled two-dimensional infrared spectroscopy, and ultrafast pump–probe spectroscopy of the asymmetric stretch (ν3) band of N2O characterize its solvation structure and dynamics in a series of alkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids ([CnC1Im][Tf2N], n = 2–12). The asymmetric stretch redshifts by 4 cm−1 as the imidazolium alkyl chain increases from 2 to 12 carbons. Vibrational relaxation occurs on a 71–94 ps timescale, decreasing with chain length. Vibrational energy redistribution rates are independent of chain length (14 ps). Rotational correlation times increase with chain length from 20 to 37 ps. Reorientational dynamics are modeled as wobbling in a cone and compared to hydrodynamic and quasihydrodynamic models of rotational diffusion. The polarization-weighted frequency fluctuation correlation functions (PW-FFCFs) decay due to both reorientation-induced spectral diffusion and structural spectral diffusion (SSD). The PW-FFCF is modeled as wobbling in a cone with a first-order Stark effect. The experiments support the solvation of N2O in the charge enhanced domains of the ionic liquids, similar to CO2.