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In general, intermolecular distancesin condensed phases at theangstrom scale are difficult to measure. We were able to do so byusing the vibrational energy transfer method, an ultrafast vibrationalanalogue of Förster resonance energy transfer. The distancesamong SCN–anions in KSCN crystals and ion clustersof KSCN aqueous solutions were determined with the method. In thecrystalline samples, the closest anion distance was determined tobe 3.9 ± 0.3 Å, consistent with the XRD result. In the 1.8and 1 M KSCN aqueous solutions, the anion distances in the ion clusterswere determined to be 4.4 ± 0.4 Å. The clustered anion distancesin aqueous solutions are very similar to the closest anion distancein the KSCN crystal but significantly shorter than the average aniondistance (0.94–1.17 nm) in the aqueous solutions if ion clusteringdid not occur. The result suggests that ions in the strong electrolyteaqueous solutions can form clusters inside of which they have directcontact with each other. [ABSTRACT FROM AUTHOR]

Energy-gap-dependentvibrational-energy transfers among the nitrilestretches of KSCN/KS13CN/KS13C15Nin D2O, DMF, and formamide liquid solutions at room temperaturewere measured by the vibrational-energy-exchange method. The energytransfers are slower with a larger energy donor/acceptor gap, independentof the calculated instantaneous normal mode (“phonons”in liquids) densities or the terahertz absorption spectra. The energy-gapdependences of the nonresonant energy transfers cannot be describedby phonon compensation mechanisms with the assumption that phononsare the instantaneous normal modes of the liquids. Instead, the experimentalenergy-gap dependences can be quantitatively reproduced by the dephasingmechanism. A simple theoretical derivation shows that the fast molecularmotions in liquids randomize the modulations on the energy donor andacceptor by phonons and diminish the phonon compensation efficiencyon energy transfer. Estimations based on the theoretical derivationssuggest that, for most nonresonant intermolecular vibrational-energytransfers in liquids with energy gaps smaller than the thermal energy,the dephasing mechanism dominates the energy-transfer process. [ABSTRACT FROM AUTHOR]