Infrared spectroscopy of gas-phase hydrated K +:18-crown-6 complexes: Evidence for high energy conformer trapping using the argon tagging method

Abstract

We report our findings on the K +(18-crown-6 ether)(H 2O) 1Ar 1–4 system using gas-phase infrared predissociation (IRPD) spectroscopy. With the argon tagging technique, we have been able to observe two different conformers, including a conformer that features a bidentate H 2O actively competing with K + for the preferred binding site inside the 18-crown-6 cavity. The detection of this conformer in our experiment was surprising since density functional theory (DFT) calculations predict it to be 55.6 kJ mol −1 higher in energy than the lower energy, traditional (K + bound)-type conformer. We have been able to selectively probe each conformer by varying both the loss channel monitored and the number of solvating argons. The bidentate conformer was present in the IRPD spectra only when monitoring the action spectrum leading to the loss of all ligands. This suggests that the bidentate conformer undergoes rearrangement following photoexcitation to the much more stable bare K +(18-crown-6) complex. We explored the barrier to such rearrangement using D 2O substitution in the experiment and with DFT calculations.