The Raman and infrared spectra of a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ([C 2-4 MIM]PF 6 ) ionic liquids have been recorded and analyzed using density functional theory (DFT) and RHF methods at the 6-311+G(2d,p) computational level. The DFT calculations reproduce the vibrational spectra of 1-ethyl-3-methyl imidazolium hexafluorophosphate [EMIM]PF 6 , 1-propyl-3-methyl imidazolium hexafluorophosphate [PMIM]PF 6 , and 1-butyl-3-methyl imidazolium hexafluorophosphate [BMIM]PF 6 using correction factors of 0.964-0.967 with correlation coefficients R 2 of 0.999. The vibrational spectra calculated at the RHF/6-311+G-(2d,p) level require a correction factor of 0.89 and a correlation coefficient R 2 of 0.999 using the fully optimized structures. The 1-alkyl-3-methyl hexafluorophosphate ionic liquids have common Raman C-H stretching frequencies that may serve as possible probes in studies of ionic liquid interactions. The DFT (B3LYP) and RHF gas-phase molecular structures of the [C 2-4 MIM]PF 6 ion pairs indicate hydrogen bonding interactions between the fluorine atoms of the PF 6anion and the C2 hydrogen on the imidazolium ring. Additional interactions are observed between PF 6and the H atoms on the adjacent alkyl side chains.

This paper presents an exploratory study of the binding interactions of xenon with the surface of several different proteins in the solution and solid states using both conventional and hyperpolarized 129 Xe NMR. The generation of hyperpolarized 129 Xe by spin exchange optical pumping affords an enhancement by 3-4 orders of magnitude of its NMR signal. As a result, it is possible to observe Xe directly bound to the surface of micromolar quantities of lyophilized protein.The highly sensitive nature of the 129 Xe line shape and chemical shift are used as indicators for the conditions most likely to yield maximal dipolar contact between 129 Xe nuclei and nuclear spins situated on the protein. This is an intermediate step toward achieving the ultimate goal of NMR enhancement of the binding-site nuclei by polarization transfer from hyperpolarized 129 Xe. The hyperpolarized 129 Xe spectra resulting from exposure of four different proteins in the lyophilized, powdered form have been examined for evidence of binding. Each of the proteins, namely, metmyoglobin, methemoglobin, hen egg white lysozyme, and soybean lipoxygenase, yielded a distinctly different NMR line shape. With the exception of lysozyme, the proteins all possess a paramagnetic iron center which can be expected to rapidly relax the 129 Xe and produce a net shift in its resonance position if the noble gas atom occupies specific binding sites near the iron. At temperatures from 223 to 183 K, NMR signals were observed in the 0-40 ppm chemical shift range, relative to Xe in the gas phase. The signals broadened and shifted downfield as the temperature was reduced, indicating that Xe is exchanging between the gas phase and internal or external binding sites of the proteins. Additionally, conventional 129 Xe NMR studies of metmyoglobin and lipoxygenase in the solution state are presented. The temperature dependence of the chemical shift and line shape indicate exchange of Xe between adsorption sites on lipoxygenase and Xe in the solvent on the slow to intermediate exchange time scale. The NMR results are compared with N 2 , Xe, and CH 4 gas adsorption isotherms. It is found that lipoxygenase is unique among the proteins studied in possessing a relatively high affinity for gas molecules, and in addition, demonstrating the most clearly resolved adsorbed 129 Xe NMR peak in the lyophilized state.

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