Infrared Spectroscopy of Jet-Cooled Tautomeric Dimer of 7-Azaindole: A Model System for the Ground-State Double Proton Transfer Reaction

To investigate the ground-state double proton transfer (GSDPT) reaction, we carried out a laser spectroscopic study on the tautomeric dimer of 7-azaindole in a supersonic jet. We have recorded an infrared (IR) spectrum of the tautomeric dimer in the S0 state. The NH band exhibits a broad and less-structured pattern. The band pattern is discussed on the basis of the hierarchical vibrational interaction mechanism. As a result, much higher density of state (DOS) at the NH stretch level is expected than that of the normal dimer. Such a high DOS should be related to the anharmonicity of the potential energy surface near the barrier of the GSDPT reaction. To get more information, an N-D deuteration effect is examined. In the present experiment, five deuterated dimers are identified by visible−visible or IR−visible population labeling spectroscopy. The IR band pattern of the NH-ND dimer is very different from that of the NH-NH dimer. Among several N-D deuteration effects, a change in a condition between the inter- or intramolecular vibrational energy flows due to the single N-D deuteration is considered to be important.