Infrared Predissociation Research Articles

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

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.

Vibrational spectroscopy of size-selected neutral and cationic clusters combined with vacuum-ultraviolet one-photon ionization detection

Various vibrational spectroscopic techniques combined with vacuum-ultraviolet one-photon ionization mass spectrometry have recently been developed for jet-cooled molecules and clusters. These techniques open general applications of size-selected infrared and also Raman spectroscopies to neutral clusters by overcoming difficulties in the conventional vibrational spectroscopic methods for jet-cooled clusters. Their spectroscopic principles have been demonstrated by investigations for neutral clusters of simple protic molecules without an ultraviolet chromophore, which is necessary for the conventional vibrational spectroscopic methods of gas phase clusters based on the fluorescence or multiphoton ionization detection. In addition, the vacuum-ultraviolet photoionization detection scheme enables us to perform infrared predissociation spectroscopy of unstable cluster cations such as unprotonated cluster cations of protic molecules. In this Perspective, the spectroscopic principles and their applications for neutral and cationic clusters are reviewed.

Infrared spectroscopy of size-selected neutral clusters combined with vacuum-ultraviolet-photoionization mass spectrometry

Infrared (IR) spectroscopy of mass-selected neutral molecular clusters in supersonic beams is newly developed by combining a vacuum ultraviolet (VUV) photoionization. In this spectroscopy, the ion intensity of the mass-selected species is used as a measure of the population of the corresponding neutral cluster species. The IR excitation of the neutral species may cause the vibrational predissociation leading to the population reduction, which appears as a dip spectrum of the photoionization signal. This method can be called VUV-ionization detected IR predissociation spectroscopy (VUV-ID-IRPDS). The method is applied to the IR spectra of ammonia dimer and trimer.