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In 1953, an accidental discovery by Melvin Calvin and co-workers provided the first example of a solid (the α-polymorph of choline chloride) showing hypersensitivity to ionizing radiation: under certain conditions, the radiolytic yield of decomposition approached 5 × 10(4) per 100 eV (which is 4 orders of magnitude greater than usual values), suggesting an uncommonly efficient radiation-induced chain reaction. Twenty years later, the still-accepted mechanism for this rare condition was suggested by Martyn Symons, but no validation for this mechanism has been supplied. Meanwhile, ionic liquids and deep eutectic mixtures that are based on choline, betainium, and other derivitized natural amino compounds are presently finding an increasing number of applications as diluents in nuclear separations, where the constituent ions are exposed to ionizing radiation that is emitted by decaying radionuclides. Thus, the systems that are compositionally similar to radiation hypersensitive solids are being considered for use in high radiation fields, where this property is particularly undesirable! In Part 5 of this series on organic cations, we revisit the phenomenon of radiation hypersensitivity and explore mechanistic aspects of radiation-induced reactions involving this class of task-specific, biocompatible, functionalized cations, both in ionic liquids and in reference crystalline compounds. We demonstrate that Symons' mechanism needs certain revisions and rethinking, and suggest its modification. Our reconsideration suggests that there cannot be conditions leading to hypersensitivity in ionic liquids.

We have investigated the ultrafast molecular dynamics of five pyrrolidinium cation room temperature ionic liquids using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids studied are N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P14+/NTf2-), N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bromide P1EOE+, and N-ethoxyethyl-N-methylpyrrolidinium dicyanoamide P1EOE+/DCA-). For comparing dynamics among the five ionic liquids, we categorize the ionic liquids into two groups. One group of liquids comprises the three pyrrolidinium cations P14+, P1EOM+, and P1EOE+ paired with the NTf2- anion. The other group of liquids consists of the P1EOE+ cation paired with each of the three anions NTf2-, Br-, and DCA-. The overdamped relaxation for time scales longer than 2 ps has been fit by a triexponential function for each of the five pyrrolidinium ionic liquids. The fast ( approximately 2 ps) and intermediate (approximately 20 ps) relaxation time constants vary little among these five ionic liquids. However, the slow relaxation time constant correlates with the viscosity. Thus, the Kerr spectra in the range from 0 to 750 cm(-1) are quite similar for the group of three pyrrolidinium ionic liquids paired with the NTf2- anion. The intermolecular vibrational line shapes between 0 and 150 cm(-1) are fit to a multimode Brownian oscillator model; adequate fits required at least three modes to be included in the line shape.