Study of electronic structure of unstable molecules by infrared spectroscopy

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Infrared spectra of unstable molecules, radical or hydrogen-bonded cluster, which exist only in the ultra-cold gas condition such as molecule in supersonic free jet, have been tried to observe. However, no infrared spectrum of radical and cation was observed by our method. Infrared spectra for hydrogen-bonded clusters of fluorinated pyridine and phenol with water were observed for the OH and CH stretching vibrations in a supersonic free jet. Structure of the clusters was determined based on the infrared spectra and ab-initio molecular orbital calculations. For 1:2 and 1:3 clusters, bridged structure is formed with water molecules, where first water bonds to the nitrogen atom in the pyridine ring and second and third water molecules interact with hydrogen atom at the 6-th position in the ring. The overtones of bending vibration of water molecule become stronger in the infrared spectra as the number of water molecule increase in the cluster. The obtained structure of cluster is supported by the ab-initio molecular orbital calculations.The infrared spectrum of 1:2 cluster between 2,6-difluorophenol and water was observed in the electronic excited state. The spectrum show that the hydrogen bond between oxygen atom of phenolic OH and water become weaker in the electronic excited state. The result well supports the interpretation estimated before.