Hai-Chou Chang

Structural Organization in Aqueous Solutions of 1-Butyl-3-methylimidazolium Halides: A High-Pressure Infrared Spectroscopic Study on Ionic Liquids

High-pressure infrared spectroscopy was applied to study the hydrogen-bonding structures of 1-butyl-3-methylimidazolium halides/D2O mixtures. No drastic changes were observed in the concentration dependence of the alkyl C-H band frequency at high concentration of 1-butyl-3-methylimidazolium chloride. Nevertheless, the alkyl C-H exhibits an increase in frequency upon dilution at low concentration. These observations may indicate a clustering of the alkyl groups at high concentration and the formation of a certain water structure around alkyl C-H groups in the water-rich region. The imidazolium C-H band at ca. 3051 cm(-1) displays a monotonic blue-shift in frequency as the sample was diluted at high concentration of 1-butyl-3-methylimidazolium chloride. That is, water can be added to change the structural organization of 1-butyl-3-methylimidazolium chloride in the ionic liquid-rich composition region by introducing water-imidazolium C-H interactions. Analyzing the pressure dependence of the imidazolium C-H stretches yielded anomalous nonmonotonic pressure-induced frequency shifts. This result may reflect the strengthening of C-H-O interactions between imidazolium C-H groups and the water clusters. Density functional theory calculations also revealed that the characteristic bonded C2-H vibration may be shifted via the modification of C2-H-Cl- associations.

STUDYING PROTONATED ION HYDRATION BY INFRARED SPECTROSCOPY OF SIZE-SELECTED NH4+(H2O)N CLUSTERS IN A FREE JET EXPANSION

Abstract This paper investigates heterogeneous water nucleation on ammonium ions in a supersonic jet using vibrational predissociation spectroscopy. For NH 4 + (H 2 O) n , it is revealed that nonhydrogen bonded NH stretching of NH 4 + is resonant at 3300–3400 cm −1 , which is fairly independent of the cluster size of n = 2–4. The frequencies, however, shift strongly from ∼2900 cm −1 of n = 2 to ∼3100 cm −1 of n = 4 for hydrogen bonded NH stretches. Hydration at the second shell starting with n = 5 has little effect on the ion core vibrations. Similar to the NH oscillations, nonhydrogen bonded and hydrogen bonded OH stretches are found for the solvent molecules at 3600–3800 and 3300–3600 cm −1 , respectively. Simultaneous observations of these four distinct stretches allow us to identify a number of structural isomers (both cyclic and noncyclic) at n = 4–6. Of particular interest is that the clusters form an energetically favored four-membered ring at n = 5 that is evidenced by its characteristic bonded OH stretching absorptions at ∼3550 cm −1 . This ring shaped structure remains at n ≥ 5, where a sharp feature that can be ascribed to the nonhydrogen bonded OH stretching of three-coordinated H 2 O emerges at ∼3690 cm −1 . All the results, both frequencies and assignments, are in close agreement with recent ab initio calculations. The association of the present observations with heterogeneous water nucleation in the gas phase and the infrared spectroscopy of neutral water clusters and crystalline ice is discussed.

Laboratory Investigation of Hydrogenated Diamond Surfaces: Implications for the Formation and Size of Interstellar Nanodiamonds

The formation and size of interstellar diamonds have been investigated by infrared spectroscopy in the laboratory. Employing hot-filament chemical vapor deposition (CVD) to synthesize the interstellar analogs, we successfully reproduced the infrared emission bands of nanodiamonds around HD 97048 and Elias 1. Analysis of the infrared absorption spectra of synthetic diamond crystallites (25-700 nm) from a commercial source reveals a strong size-dependent effect. The 3.53 μm feature emerges only for particles larger than 25 nm. Our experiments suggest that the carriers of the anomalous infrared emission bands at 3.43 and 3.53 μm could be nanodiamonds that are larger than 25 nm and are formed by a CVD-like process.

A High-Pressure Infrared Spectroscopic Study on the Interaction of Ionic Liquids with PEO-PPO-PEO Block Copolymers and 1,4-Dioxane

We have investigated the effect of pressure on imidazolium C-H---O interactions in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMI(+)TFSA(-))/L64 and EMI(+)TFSA(-)/1,4-dioxane mixtures. The addition of Pluronic L64 to EMI(+)TFSA(-) leads to appreciable changes in band frequencies and shapes of the imidazolium C-H stretching bands. A possible explanation is the formation of C-H---O interactions between imidazolium C-H groups and oxygen atoms of polyethylene oxides (PEOs). In other words, L64 can be added to change the relative contribution of the isolated and associated components of EMI(+)TFSA(-). In contrast to L64, the oxygen atoms of 1,4-dioxane cannot perturb the local structures of imidazolium C-H groups of EMI(+)TFSA(-) and the association configuration is still favored in the presence of 1,4-dioxane. As the pressure is elevated, 1,4-dioxane molecules tend to associate with themselves and TFSA(-) interacts with EMI(+) to form associated configurations. Our results suggest the formation of association between EMI(+) cation and L64 and the complexes are stable up to the pressure of 2.5 GPa.